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Fact Sheet 1

Scientific Evidence for Olive Oil and its Effects on Lipid Metabolism

Author:
Prof. Dr. med. Gerd Assmann

Dr. troph. Ursel Wahrburg
The Institute of Arteriosclerosis Research at the University of Münster, Germany

Dyslipidaemia as a cardiovascular risk factor

In recent years new clinical recommendations for the assessment and treatment of lipid risk for coronary heart disease (CHD) have been developed by national and international expert and consensus groups as a consequence of a rapidly expanding knowledge basis. Lipid risk factors included in current clinical guidelines are total or low density lipids (LDL) cholesterol, high density lipids (HDL) cholesterol, and plasma triglycerides.

There is no doubt that elevated cholesterol levels due to high levels of LDL cholesterol play a causal role in atherosclerotic heart disease. Extensive observational epidemiological data from both between- and within-population studies relate elevations of total cholesterol or LDL cholesterol to increased CHD incidence. These data are corroborated by genetic and experimental evidence. Furthermore, a large body of interventional epidemiological data, from both primary and secondary prevention trials show a reduction in CHD events with a reduction in total or LDL cholesterol. In general, a 1% reduction in total cholesterol yields a 2-3 % reduction in CHD risk.

Numerous epidemiological studies have also established that HDL cholesterol is an independent and powerful predictor of CHD incidence. Until now, information on the effects of increasing HDL cholesterol levels in humans is limited, but suggests favourable effects on CHD risk.

The findings regarding a relation between plasma triglyceride level and CHD incidence are mixed. There is a consistent, strong association in case-control studies. The relation is confirmed in most prospective studies on univariate analysis, but on multivariate analysis of their data it weakens or disappears, in particular when HDL cholesterol, coagulation factors or indicators of abnormal glucose metabolism are taken into account.

In the last years evidence has emerged that elevated triglyceride levels confer a high risk for CHD if they occur in conjunction with low HDL cholesterol and elevated LDL cholesterol (so-called lipid triad) or if the LDL/HDL cholesterol ratio is high and triglyceride concentration is elevated. In addition, it has been shown that the conjunction of elevated triglycerides and low HDL cholesterol is often associated with increased insulin resistance, hyperinsulinaemia, higher glucose levels, hypertension, and central obesity. This constellation of risk factors constitutes a syndrome predisposing to atherosclerosis.

General cutpoints for dyslipidaemia

As a general rule, there is the finding that one single or isolated lipid value can not be classified as „normal“ or „elevated“. The lipid cutpoints shown in are not absolute. They should only be regarded as general advice for risk evaluation and therapy. All other risk factors should be also taken into account when assessing the cardiovascular risk. All therapeutic decisions should generally be based on the patient's overall risk profile. Furthermore, treatment goals for hyperlipidaemia also strongly depend on the global risk.

Dietary influences on serum lipids and lipoproteins

Decades of research work have clearly demonstrated that diet has a strong influence on serum levels of lipids and lipoproteins. Thus, diet is a cornerstone both in the prevention and treatment of lipid metabolism disorders and CHD.

The action of dietary saturated fatty acids (SFA) as a lipid class to raise total cholesterol levels is well established. The increase in total cholesterol induced by SFA is due mostly to an increase in LDL cholesterol. This increase in LDL cholesterol is accompanied by an elevation of LDL apolipoprotein (apo) B-100, without changes in the ratio between LDL cholesterol and apo B-100. Thus, the increase in LDL cholesterol is due to an increase in the number of LDL particles and not to changes in the cholesterol content of LDL particles. Diets high in fat and SFA also lead to an increase in HDL cholesterol and apo A-I.

This elevation in HDL cholesterol deserves some attention because it is said to antagonise the adverse effects of high LDL concentrations. However, low HDL cholesterol associated with low LDL cholesterol in populations consuming high-carbohydrate, low-fat diets does not increase the coronary risk, whereas populations with high LDL due to high saturated fatty acids (SFA) diets undoubtedly have a high coronary risk in spite of their higher HDL levels. Obviously the magnitude of LDL elevation outweighs the smaller relative increase in HDL cholesterol. Furthermore, the LDL/HDL cholesterol ratio was found to be higher on diets high in total fat and SFA than on low-fat or polyunsaturated fatty acid (PUFA)rich diets in nearly all studies.

Saturated dietary fat usually contains a mixture of SFA of different chain lengths. It has been demonstrated that the different SFA are not equally hypercholesterolaemic. The principal SFA in Western diets is palmitic acid (C16:0), followed by stearic acid (C18:0), myristic acid (C14:0) and lauric acid (C12:0). In a mixed Western diet lauric, myristic and palmitic acids together usually make up about 60-70% of all SFA. These three fatty acids are responsible for the cholesterol-raising effect of saturated fat.

Palmitic acid is the main SFA of animal fat, while myristic acid is abundant in butter fat, palm kernel oil and coconut oil. The latter two also contain very high proportions of lauric acid. It is still a matter of discussion which of the three fatty acids has the highest cholesterol-raising potential, but as a result from several well-controlled studies, the differences among lauric, myristic and palmitic acids appear modest. All three clearly raise LDL cholesterol.

The cholesterol-elevating effect of stearic acid, of which the highest content is found in cacoa butter, is much less than that of lauric, myristic and palmitic acids, and more closely approximates the effect of oleic acid. Recent trials, however, have reported a modest fall in HDL cholesterol in response to dietary stearic acid relative to dietary unsaturated fatty acids. Thus, stearic acid and oleic acid are equivalent in their effects on LDL cholesterol but might be somewhat different in their effects on HDL cholesterol concentrations.

Despite the recent findings about differences in the cholesterol-raising potential of the different SFA, the general recommendation to reduce the amount of SFA consumed in the Western diet is still valid, especially in practice. Most foods contain a mixture of different fatty acids, and, in addition, the cholesterol-elevating SFA are the major fatty acids of a typical Western diet.

In individuals who are not overweight, foods rich in SFA would have to be replaced with other types of food in order to maintain the energy balance in equilibrium. In the past, carbohydrate-rich foods were generally considered to be an ideal candidate for replacing SFA-rich foods. The second alternative as a replacement for saturated fat were polyunsaturated fatty acids (PUFA).

PUFA naturally occur in two major groups with distinct metabolic properties: -3 and -6 fatty acids. These terms indicate the position of the first double bond counting from the methyl end of the fatty acids. The major dietary PUFA is linoleic acid (C18:2, -6) which is predominant in most vegetable oils and in products derived from them. The dietary intake of the other -6-PUFA, -linolenic acid (C18:3,-6) and arachidonic acid (C20:4,-6), represents less than 2% of the total dietary fatty acids.

The -3 fatty acid -linolenic acid (C18:3,-3) is found in very small amounts in many vegetable oils, in higher proportions only in soybean oil, rapeseed oil and linseed oil, whereas the long-chain -3 fatty acids eicosapentaenoic acid (C20:5,-3) and docosahexaenoic acid (C22:6,-3) are contained in fats and oils of marine origin. Marine cold-water fish (herring, mackerel, salmon, tuna) and their products are the major dietary source of these fatty acids. With regard to their effects on lipid metabolism, -3 fatty acids primarily lower serum triglyceride. They have only minor effects on total, LDL and HDL cholesterol.

As a cholesterol-lowering alternative to SFA, only -6 PUFA are of importance. The substitution of SFA with linoleic acid leads to a marked fall in total cholesterol. This reduction is mainly due to a decrease in LDL cholesterol which is caused by a reduction in the number of LDL particles. Diets with high amounts of PUFA (>12-15% of energy) and a high P/S ratio (> 2) have been shown to also lower HDL cholesterol.

Trans fatty acids

In the majority of naturally occurring unsaturated fatty acids the double bonds are in cis configuration. Trans fatty acids, mainly the trans isomers of oleic acid (elaidic acid (C18:1,-9,trans) and vaccenic acid (C18:1,-7,trans)), are produced during hydrogenation, either in the rumen of cows or in oil-hardening factories. The change from cis to trans configuration of the double bond leads to a straightening of the molecule and to changes in the physical and biochemical properties of the fatty acid. Trans fatty acids have a higher melting point, leading to increased solidity of hydrogenated fats.

The effects of trans fatty acids on serum lipoproteins markedly differ from those of the natural cis isomers. Trans fatty acids have been shown to raise LDL cholesterol concentrations and to decrease HDL cholesterol. Furthermore, they elevate plasma concentrations of lipoprotein (a), an atherogenic lipoprotein that was hitherto thought impervious to dietary changes. However, many people eat no more than a few grams of trans fatty acids per day, and these quantities produce only small effects on serum lipoprotein concentration.

Olive oil and monounsaturated fatty acids

For many years, monounsaturated fatty acids (MUFA) were not given much attention. More recently, however, a large body of evidence has accumulated showing that MUFA might have some advantages over carbohydrate and PUFA as a substitute for SFA in Western diets.

The major MUFA in the diet is oleic acid (C18:1,-9). Oleic acid is the predominant fatty acid of olive oil Thus, in recent years scientific attention has been focused on the so-called Mediterranean diet and on olive oil as one of its most characteristic components. In the Mediterranean area MUFA usually provide more than 15% of energy (up to 27% in Crete), and they are primarily derived from olive oil. Simultaneously, coronary heart disease incidence, as well as hypercholesterolaemia are by far lower than in other European countries and in the U.S.

Many studies have compared the effects of MUFA and PUFA on plasma lipoproteins under different experimental conditions. Well designed and controlled studies are characterised by the following criteria: The dietary intervention trials were conducted with human subjects randomised to a high PUFA-diet and a high MUFA-diet. They consisted of at least two intervention periods which were similar in all respects except for the contents of MUFA and PUFA. Total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides were analysed as end-point data from the dietary intervention periods, and the studies included more than 10 participants on each of the diets to obtain a reasonable accurate estimate of the within-group variance.

The majority of studies conducted under such strict conditions consistently shows that serum total and LDL cholesterol are reduced to a similar extent when MUFA or PUFA were substituted for SFA. Most of these studies used olive oil as the MUFA-rich oil, whereas sunflower or safflower oil was used as PUFA-rich oil. Some experimental diets with a very high PUFA content (> 12% of energy) also showed a decrease in HDL cholesterol. This reduction, however, was not observed after diets with a lower PUFA content (< 12% of energy). The substitution of MUFA for SFA did not lead to any significant changes in HDL cholesterol, even if they were consumed in larger amounts (> 15% of energy).

Two recent meta-analyses confirmed that there is no significant difference in total, LDL and HDL cholesterol between diets relatively high in MUFA versus PUFA when fat intake is primarily derived from common vegetable oils, especially when the fatty acid contents are in a range practicable for a long-term diet. The reductions in total and LDL cholesterol are highly significant, and the LDL/HDL cholesterol ratio is also lowered significantly.

On the basis of these results it no longer seems justifiable to recommend the preferential use of PUFA over MUFA. In addition, there is increasing concern about the long-term safety of high intakes of PUFA: First, there is no country world-wide with a long-term intake of PUFA of 10% of energy or even more that could provide the epidemiological evidence that such high intakes indeed will not cause any harmful health effects. Secondly, PUFA easily undergo peroxidation, yielding free oxygen radicals which could cause serious cellular damage. In some animal experiments very large intakes of PUFA were associated with carcinogenesis. High PUFA intake could induce disbalance among different prostaglandins, leading to coagulation disturbances. It has been further discussed if PUFA are associated with a higher risk of cholelithiasis. Taken together, all these aspects have led to a certain caution regarding the recommendations of PUFA intake.

On the other hand, a high MUFA intake has been proved by the „mass-experiment“ of the Mediterranean countries, where olive oil has been used for centuries. In these countries with low CHD mortality the incidence of cancer, gallstones, and other fat-related diseases is not higher than in other countries. Thus, MUFA can be generally regarded as safe.

Carbohydrates, which have been recommended as a substitute for SFA in cholesterol-lowering diets just as PUFA already for many years, can also be regarded as safe. Low-fat, high-carbohydrate diets significantly lower total and LDL cholesterol concentrations, but they also clearly reduce HDL cholesterol levels. In addition, those diets may have untoward effects on plasma triglycerides, glucose and insulin. These adverse metabolic effects may be more pronounced in individuals with pre-existing underlying disorders, such as diabetes mellitus. They also depend on the fibre content of the diets. If a carbohydrate-rich diet does not contain only starch and sugar but has a high fibre content, i.e. if the diet contains a lot of whole-grain cereals, vegetables, legumes and fruit, the untoward metabolic effects can be prevented to a large degree.

In summary, the various dietary approaches which can replace SFA in the diet of individuals and populations at high risk for CHD do not differ substantially in terms of their ability to reduce plasma cholesterol and LDL cholesterol which is the major target of a diet for prevention of atherosclerosis. However, their influence on other lipid parameters, non-lipid cardiovascular risk factors and other diseases are not identical. Taking this into account, MUFA seem to have some advantages over both PUFA and carbohydrates.

From the clinical point of view the best diet in terms of both effectiveness and compliance combines the two dietary approaches of both fat reduction and fat modification. A diet, moderately high in carbohydrates and fibre, not too restricted in total fat, but strictly restricted in SFA, with a moderately high MUFA content seems to be the most feasible approach both to preventing and treating dyslipidaemia.

These scientific findings were of great importance in the formulation of the lipid-lowering diet guidelines of both the European Atherosclerosis Society and the American Heart Association: The total fat intake should be reduced to 30% of the total energy intake, SFA should be reduced below 10% of energy. The intake of PUFA should be not more than 10% of energy (7-10%), whereas the remaining fat proportion should be provided by MUFA (10-15% of energy). The dietary cholesterol content should be below 300 mg/day. Furthermore, the intake of complex carbohydrates and dietary fibre should be increased.

The traditional Mediterranean diet provides an excellent example how these guidelines could be converted into a tasty and appetising diet. It is characterised by an abundance of plant foods such as bread, pasta, vegetables, salad, legumes, fruit; olive oil as the principal source of fat; low to moderate amounts of dairy products; and also only low to moderate amounts of meat, poultry, fish, and eggs. This diet is low in SFA, rich in carbohydrate and fibre, and, as already discussed, has a high MUFA content. The MUFA are primarily derived from olive oil.

In the most Northern and Western European countries the MUFA intake is also relatively high (15% of energy or more), but the MUFA are mainly taken up with foods simultaneously rich in cholesterol-raising SFA, especially with high-fat animal products. So, if the intake of these SFA-rich foods would be reduced to reach the desirable reduction of the SFA intake from 16-20% at present below 10% of energy as recommended, the MUFA intake as well would fall to 10% of energy or less. Thus, to reach the recommended fat content and composition of the diet, clear changes in the food intake must be made in the Western diets. The intake of vegetable foods has to be greatly increased while the consumption of animal foods, primarily high-fat products such as animal fat, high-fat cheese, fatty meats and sausages, as well as of SFA-rich vegetable fats and oils such as palm and coconut oil, and hydrogenated fat should be clearly decreased.

As a substitute for solid (animal and vegetable) fat, vegetable oils are recommended. Due to its high MUFA content olive oil stands out as a vegetable oil with excellent benefits for human health. The consumption of olive oil increases the MUFA intake without any significant elevation of SFA, and ensures an appropriate intake of the essential PUFA. Thus, olive oil can make a valuable contribution to a healthy lipid-lowering and anti-atherogenic diet.

Fact Sheet 2

Scientific Evidence for Olive Oil, the Cardiovascular Risk Factors and Coronary Heart Disease

Author:
Prof. Dr. med Gerd Assmann
Dr. troph. Ursel Wahrburg
The Institute of Arteriosclerosis Research,
University of Münster, Germany

Part A: Olive Oil and Cardiovascular Risk Factors

Part B: Olive Oil and Coronary Heart Disease

Part C: Olive Oil and its Role in Secondary Prevention of Coronary Heart Disease

Introduction

Atherosclerosis and coronary heart disease (CHD) as its main clinical manifestation have a multifactorial origin. The susceptibility to CHD is determined both by genetic and environmental influences. Among the latter, diet is undoubtedly the central factor in the development of CHD. Dietary factors exert their influence largely through their effects on blood lipids and lipoproteins, but also through their great influence on the other established modifiable risk factors (Table 1), with the exception of cigarette smoking.

Table 1: Risk factors for coronary heart disease (CHD)

The dietary factors most directly implicated are dietary fats. Numerous comparisons between populations have shown that there is a strong correlation between the intake of saturated fatty acids (SFA) and CHD morbidity and mortality. A customary diet high in SFA is associated with high levels of CHD. This is the case for the most Western and Northern European countries. On the other hand, in the Mediterranean countries, where people consume their traditional diet in which the majority of fat calories is derived from olive oil, there is a low incidence of CHD. The present paper outlines the effects of monounsaturated fatty acids (MUFA), olive oil, and Mediterranean diet on the different cardiovascular risk factors and on CHD.

Part A: Olive Oil and Cardiovascular Risk Factors

Part A - 1 Olive Oil and Dyslipidaemia

This topic is covered in Fact Sheet 1.

Part A - 2 Olive Oil and Hypertension

Cross-cultural comparisons and prospective observational studies identify a strong relationship between diet and blood pressure. The studies show that vegetarians in particular have lower blood pressure than non-vegetarians. There is also evidence that the Mediterranean diet might exert a beneficial influence on blood pressure. For instance, significantly lower blood pressures were observed in Italian population samples as compared to Finnish and Scottish groups (41). Until now it could not be clearly determined which of the many nutrient differences compared to usual Western diets were responsible for the blood pressure lowering effect. The partial substitution of vegetables and grain for meat decreases SFAs which are largely derived from animal sources. The intake of MUFA is increased due to the olive oil consumption. In addition, Mediterranean as well as vegetarian diets are characterised by a higher intake of fibre, carbohydrates and micronutrients (eg. potassium, calcium, magnesium), and a lower salt intake.

As a consequence of these observations a large number of different studies have examined the correlation between dietary intake of different types of fat and blood pressure. The majority of cross-sectional studies measuring blood pressure and self-reported diet at the same time, provide no evidence to support the hypothesis that dietary fats affect blood pressure. However, the results must be qualified by taking some methodological weakness into account. In most studies either the measurement of usual diet (eg. 24-hour recall) or blood pressure (eg. only one measurement) was inaccurate or there was an inadequate control of both dietary and non-dietary confounding factors.

Epidemiological prospective studies show conflicting results. For instance, in the Multiple Risk Factor Intervention Trial (MRFIT) with approximately 1200 participants, systolic blood pressure was independently related to SFA intake and dietary cholesterol . In contrast, there was no relationship between dietary fat and blood pressure in two large cohort studies: ie: in the Nurses Health Study the 4-year incidence of hypertension was investigated among 58,218 disease-free nurses who completed a dietary questionnaire at baseline. In multivariate analysis controlled for dietary and non-dietary factors, the development of hypertension was not associated with usual intake of total fat, saturated fat, or unsaturated fat. A similar analysis of a cohort of 30,681 US male health professionals confirmed these results.

The large sample size of the two latter studies together with their accurate assessment of usual dietary intake and rigorous statistical control for confounding factors provide support for the absence of an association between dietary fat and hypertension or change in blood pressure.

Experimental data of a relationship between dietary fat and blood pressure are also contradictory ). In many studies the dietary intervention simultaneously included several measures (eg. exchange of animal fat with vegetable oils plus increase in vegetable consumption). Thus, it is impossible to attribute changes in blood pressure to a single nutrient. In addition, the majority of studies were conducted with normotensive subjects, and their results may not be applied to hypertensive patients.

A well-controlled, randomised cross-over trial compared the effects of a high-fat, high-MUFA diet with a high-fat, high-PUFA diet on blood pressure in healthy adults. No change in blood pressure was found after 4 weeks of treatment. There was no evidence of a blood-pressure-lowering effect of either dietary MUFA or PUFA in further studies of normotensive individuals. One study compared a low-fat, carbohydrate-rich diet with a high-fat, olive-oil-rich diet, while holding constant PUFA and SFA. Another replaced about 10% of energy with either oleic acid or SFA in otherwise similar diets , and two separate studies compared diets rich in MUFA with PUFA, with dietary intake of total fat and SFA held constant. The effects of MUFA on blood pressure were not tested among hypertensive patients.

In an Italian study the dietary intervention consisted of a change from Mediterranean type diet to a high-fat, high-SFA diet. This dietary modification was obtained by substituting specific items of the habitual diet (olive oil, cereals, vegetables) with foods rich in SFA such as butter, dairy products, cheese, and meat. At the end of the 6-week intervention period systolic and diastolic blood pressure had increased significantly. After return to the customary diet a rapid decrease in blood pressure levels to preintervention values was observed. However, in this study there was again a multifactorial dietary intervention and not only an exchange of fatty acids.

A recent Spanish study with 20 healthy volunteers evaluated the effects of two high-fat, MUFA-rich diets (40% fat, 22% MUFA), one with virgin olive oil, the other with high-oleic sunflower oil, as compared with the National Cholesterol Education Program (NCEP) Step 1 diet (30% fat, 12% MUFA). SFA content, dietary cholesterol, fibre, and minerals (sodium, potassium, calcium, magnesium) were kept constant throughout the trial. The MUFA diets which were followed for a 4-week period each led to a significant reduction in systolic (from 120 to 110 mmHg) and diastolic (from 73 to 66 mmHg) blood pressure. Both MUFA oils produced similar changes. Thus, the blood pressure lowering effect was likely to be due to the MUFA, and not to unsaponifiable materials mainly present in olive oil. This well-designed, strictly controlled study gives support that MUFA enrichment of an otherwise unchanged diet has a blood pressure lowering effect. The results indicate that there may be a direct and active influence of MUFA on blood pressure. However, they need confirmation by further studies as well as by investigation of the possible underlying mechanisms.

In summary, the question of the relation between dietary fats and blood pressure has not yet been definitively answered. Evidence suggests that Mediterranean diets with a high consumption of olive oil, cereals, vegetables and fruits have favourable effects on blood pressure. However, it still remains a matter of debate if the protective influence is primarily caused by single nutrients, eg. dietary fatty acids, potassium or dietary fibre, or if it can be attributed to the Mediterranean diet as a whole. There is support for the latter hypothesis suggesting that the combination of various favourable factors - low SFA content, high MUFA content, high carbohydrate, fibre and micronutrient content, low salt intake - leads to lower blood pressure values as compared to typical Western diets. Although the effects of a single nutrient may be small, dietary MUFA content may play a more important role in this protective effect than has been assumed in the past.

Part A - 3: Olive Oil and Diabetes

The prevalence of non-insulin-dependent diabetes mellitus (NIDDM) is very different throughout the world: it is particularly high in Western industrialised countries whereas it is low. Furthermore, the prevalence of NIDDM, and presumably the insulin resistance that invariably accompanies this disorder, has increased dramatically since World War II in all developed countries of the world. The association of increased incidence with economic affluence observed in international comparisons and studies of migrants have illustrated the significance of environmental factors in addition to a genetic disposition. Nutritional factors and the degree of habitual physical activity appear to be important determinants of this form of diabetes. Evidence suggests that the progression from glucose intolerance to diabetes can be prevented by dietary treatment and increased physical activity.

It is known people living in Mediterranean regions are at a particularly low risk developing the most common degenerative diseases of the industrialised populations. Therefore, the question arises whether or not the Mediterranean diet protects from diabetes. Unfortunately, information on the prevalence of diabetes in Mediterranean countries is rare, because there is a lack of properly designed studies. However, although no direct evidence exists which suggests that Mediterranean diet protects against the development of diabetes, there are clear indications from cross-cultural comparisons and studies on vegetarians that some of its most important characteristics - namely, the high intake of complex carbohydrate and dietary fibre and the low intake of SFA - may be beneficial in reducing the risk of diabetes.

The key importance of quantity and quality of dietary fat on the development of diabetes has been underlined in several recent studies. In the San Louis Valley Diabetes Study, fat consumption, adjusted for total energy intake, predicted the risk for NIDDM in individuals with impaired glucose tolerance. A high intake of animal fat and cholesterol was found in Japanese-American men with impaired glucose tolerance progressing to NIDDM. In the Nurses Health Study, on the other hand, a high, energy-adjusted intake of vegetable fat was associated with a low relative risk of developing diabetes. The metabolic mechanisms responsible for this associations have not been identified yet. Assumptions that a high-fat diet might adversely influence insulin-related glucose disposal, could not be confirmed in controlled studies. However, a high-fat diet clearly promotes weight gain and obesity which has been shown to be the dominating risk factor for the development of diabetes in genetically disposed individuals. The prevention of obesity is probably the most important measure for reducing the incidence of NIDDM.

Dietary measures are not only important in the prevention of diabetes, but are the cornerstone in the treatment of diabetes. There is no doubt that the basic measure must be a reduction in the intake of SFA. This recommendation is considered of paramount importance by all experts since diabetic patients are exceedingly prone to atherosclerosis. The current guidelines for the treatment of diabetes mellitus favour a diet with a relatively high proportion of carbohydrate-rich foods for most patients.

Although prospective studies seem to support this approach, controlled dietary treatment studies with diabetic individuals have shown controversial results. Some investigators found that a high-fat, MUFA-enriched diet with a low proportion of energy from SFA, was associated with better glycaemic control and reduced insulin requirements compared with a high carbohydrate diet. Furthermore, high-MUFA diets were advantageous because of their effect on lowering of plasma triglyceride and very low density lipoprotein (VLDL) concentrations whilst increasing HDL cholesterol and apo A-I levels. Although weight-reduction is usually facilitated with high-carbohydrate diets containing fibre-rich foods, high-MUFA diets may have some advantages over diets with a higher energy-density for overweight patients.

The results of these studies indicate that a normoenergetic high-MUFA diet can be consumed by NIDDM patients without negative effects on glucose and lipid metabolism. In addition it may even have some advantages compared to a very-low fat, high-carbohydrate diet, albeit not for all patients. With respect to carbohydrates, it should be emphasised that there are large differences in the effects on glucose metabolism depending on the type of carbohydrate. Patients should be generally advised to choose fibre-rich sources of complex carbohydrates in preference of refined sugars.

In summary, the type and amount of dietary fat influences the risk of developing obesity, insulin resistance and NIDDM. The most important measures for preventing diabetes are weight reduction in obese people, and the restriction of dietary fat, especially saturated fat (SFA), together with increased physical activity. For treatment of NIDDM a high-carbohydrate (fibre-rich) or a high-MUFA diet can be recommended. The choice between the two diets should be based on individual requirements and management goals.

The traditional Mediterranean diet meets all the demands for an adequate diabetes diet. It has a low SFA content and is rich in MUFA due to olive oil . With cereals and vegetables, carbohydrates are mainly taken up as fibre-rich, complex carbohydrates. The absolute fat content can easily be varied - depending on the individual needs - by varying the amount of olive oil in the daily diet.

Part A - 4: Olive Oil and Obesity

Overnutrition manifested by obesity has emerged as a major health problem in affluent countries. Although obesity is associated with many risk factors for diseases, the mechanisms whereby it enhances disease risk are not fully understood. However, it is generally agreed that overnutrition and obesity only induce disease states when they are combined with an inherent metabolic weakness or defect. In the absence of obesity such defects may well go unnoticed.

One of the most common consequences of obesity is dyslipidaemia, that is, elevations of very low-density lipoprotein (VLDL) triglycerides, low-density lipoprotein (LDL) cholesterol, and low concentrations of high-density (HDL) cholesterol. The two former effects can be explained by overproduction of VLDL, due to obesity, combined with a genetic defect in clearance of VLDL or LDL. The mechanism whereby obesity causes a lowering of HDL cholesterol is unclear.

Another disease associated with obesity is cholesterol gallstones. The presence of obesity more than doubles the risk for gallstones. Overnutrition promotes the synthesis of whole-body cholesterol, and the only way for excretion of this excess cholesterol is via the biliary tree. Thus, obesity leads to an increased output of cholesterol in the bile. When this reaction is combined with either a deficiency of bile acids or a propensity to crystal formation, the risk for gallstones is greatly increased.

Among patients with NIDDM a high incidence of obesity is well known. It appears that obesity is not the underlying cause of NIDDM, but when combined with a primary defect in insulin metabolism - (a progressive decline in the ability to secrete insulin by beta cells of pancreatic islets or a primary insulin resistance) - NIDDM develops.

Approximately 50% of patients with essential hypertension are obese. The mechanisms whereby obesity raises blood pressure are still uncertain. Since many obese patients do not have hypertension, there must be underlying defects in blood pressure control for hypertension to become manifest in obese individuals.

Finally, limited data suggest that overnutrition increases the risk for certain types of cancer, namely breast, colon, and prostate cancer. If so, it seemingly is a promoter of cancer development rather than a primary initiator. However, the present data are insufficient to conclude that obesity is definitely a risk factor for human cancer.

The role of obesity as an independent risk factor for cardiovascular morbidity and mortality is also a matter of debate up until now. On the other hand, there is no doubt that obesity strongly increases the cardiovascular risk through its detrimental effects on frequency and severity of the other described risk factors. The obesity-mediated risk is not only determined by the degree of obesity, but also by the body-fat distribution. It has been shown that especially abdominal (visceral) obesity is closely related to cardiovascular risk factors and coronary heart disease. Abdominal obesity seems to be associated with a cluster of risk factors, such as dyslipidaemia, NIDDM, and hypertension. This metabolic syndrome is closely linked to visceral fat mass and indicates a very high risk for coronary heart disease.

Obesity is a complex disease with multiple causes. Lifestyle, environment and genetics contribute to its manifestation. Although the pathophysiological mechanisms underlying obesity are not fully understood, it is proven that obesity results from an imbalance in energy intake and energy expenditure. There are many possible causes for this imbalance such as disturbed regulation of food intake, low basal metabolic rate, impaired thermogenesis, and low rates of fat oxidation. Numerous studies have shown that the diet of the majority of obese people is not unusually high in calories, but that the fat content is too high. Since dietary fats - in contrast to carbohydrates and protein - are rather stored than oxidised in the postprandial state, a high-fat diet regularly causes weight gain.

In industrialised countries energy-rich foods are easily available, ubiquitous, and, due to their palatability, are given high preference. The calorific foods are believed to be a major cause of obesity. These foods are low in complex carbohydrates and fibre, and rich in fat. The main sources in the diet of most Western countries are foods from animal origin with a high content of „invisible“ fat. Furthermore, an increasing intake of snacks and sweets leads to a further increase in dietary fat as well as in simple carbohydrates. Consumption of vegetable foods rich in complex carbohydrates and fibre is very low.

Epidemiological data convincingly show that there is a strong inverse relationship between carbohydrate intake and relative body weight. Populations with a high carbohydrate intake in general have lower obesity rates as compared to countries with a high fat intake. Epidemiological studies dating from the beginning of the 1960s noted that the prevalence of overweight and obesity in Mediterranean countries was less than that of other industrialised nations. However, since 1960 both lifestyle and dietary habits have changed in most Mediterranean countries. They have moved from a diet based on cereals, vegetables, and olive oil towards a diet rich in animal products. Consequently, the incidence of obesity in Mediterranean countries is increasing.

Olive oil is a main component in the traditional Mediterranean diet. As well as other pure fats it has a high energy content and provides 9 kcal (38 kJ) per gram. And, theoretically, an excessive consumption of olive oil could therefore also lead to weight gain and obesity. But, in practice, the amount of olive oil in the usual Mediterranean diet is not large enough to cause obesity. Olive oil is the principle fat source in the diet: intake of animal fat is very low, and the diet contains abundant foods of plant origin. As a consequence, the diet has a high complex carbohydrate and fibre content. Such a diet is usually not hypercaloric, but has an energy content according to the individual energy need.

In summary, obesity is one of the principal public health problems of affluent societies. It is a complex disease with multiple causes. One of the major causes is a high fat content in the habitual diet. In Western countries, consumption of fat, particularly fat derived from animal food, is almost twice as high as the recommended amount, while the carbohydrate content is too low. This is the most important nutritional anomaly of the Western world leading to overnutrition and obesity. A diet rich in complex carbohydrates and fibre, on the other hand, means a protection against the development of obesity. The obvious implications are that obesity could be prevented, or treated through use of a diet rich in grain and vegetables such as the traditional Mediterranean diet.

Part A - 5: Olive Oil and Thrombogenic Risk Factors

The importance of factors influencing blood clotting and fibrinolysis in preventing coronary events is now well established. There is also evidence that platelet aggregation, plasma fibrinogen concentration and other haemostaseological factors may be influenced by diet. A high intake of saturated fatty acids is believed to increase the risk of arterial thrombosis. The role of unsaturated fatty acids in thrombogenesis still remains controversial.

Dietary supplementation with type n-3 (omega-3) polyunsaturated fatty acids has been shown to modify platelet function, as evidenced by a prolongation of the bleeding time, diminished platelet aggregation and secretion, and attenuated thromboxane production. These diet-related effects on platelets are considered to be beneficial for the prevention of cardiovascular disorders.

Results concerning the effects of n-6 (omega-6) polyunsaturated fatty acids on thrombosis are contradictory. There are some studies which reported a reduced platelet aggregation, while others found an increase. It seems that not only the absolute content of linoleic acid is of importance with regard to their thrombotic or antithrombotic effects, but also the content of other fatty acids, eg. the amount of saturated fatty acids and the ratio of n-6 to n-3 fatty acids.

There have been hardly any thorough evaluations of the influence of monounsaturated fatty acids on the coagulation system. The studies conducted so far do not give an indication either of a significant pro- or antithrombotic effect of MUFA. Thus, there is no scientific basis to encourage the consumption of olive oil for reducing the risk of thrombosis.

However, the majority of fatty acid and thrombosis studies suggest that both a low-fat or a vegetable-fat diet are preferable to a high-fat diet or a diet high in saturated fatty acids. In this respect, the Mediterranean diet meets the requirements and is a recommendable diet for the prevention of thrombosis.

Part B: Olive Oil and Coronary Heart Disease

The first hypothesis about a possible relationship between the typical diet of a country and a low incidence of coronary heart disease, including the intermediate role of low serum cholesterol levels, was based on exploratory surveys conducted by Ancel Keys and colleagues in the 1950s in Southern Italy, Spain and Greece. They reported a rarity of cases of hospitalised myocardial infarction which was paralleled by low mean levels of serum cholesterol. These observations were particularly impressive when compared with similar data reported from the United States and Finland, where higher levels of serum cholesterol corresponded to many cases of myocardial infarction. Simultaneously, the Mediterranean populations showed a dietary pattern different from that of North American and Northern European populations.

Part B - 1 Epidemiological studies

These non-systematic observations became the basis for methodologically more valid approaches which led to the conduction of the Seven Countries Study. This study probably represents until nowadays the major investigation contributing to knowledge about the relationship between the Mediterranean diet and CHD.

The Seven Countries Study was conducted with almost 13,000 men, aged 40 to 59 years, and healthy at entry examination, enrolled in 15 population samples located in seven different countries (Italy, Greece, former Yugoslavia, the Netherlands, Finland, United States, and Japan) Dietary data were collected at different time points. Serum lipids and other risk parameters were measured at baseline and at 5- and 10-year follow-up. The collection of data on mortality and death was continuous from the beginning.

CHD death rates were related to serum cholesterol, blood pressure, smoking habits, and mean age. The death rates, as well as the average diets differed among the cohorts. The major differences in food consumption between the Mediterranean areas and northern Europe and the United States were in the proportions of saturated fat and not necessarily in overall fat consumption. The polyunsaturated fat consumption had relatively minor relevance since the intercohort differences were limited. The monounsaturated fat consumption showed large differences among the cohorts.

15-year CHD death rates were related positively to average percentage of dietary energy from SFA, negatively to dietary energy percentage from MUFA, and were unrelated to dietary energy percentage from PUFA, proteins, carbohydrates, and alcohol. They were also negatively related to the ratio of monounsaturated to saturated fatty acids. All-cause and CHD death rates were low in cohorts with olive oil as the main fat and as the main source of MUFA (Greece, Italy, former Yugoslavia). The intake of SFA, on the other hand, was low, thus resulting in a high MUFA to SFA ratio. A relatively high MUFA intake was also observed in the US cohort, but there it was accompanied by a high SFA intake, a low MUFA/SFA ratio, and, as a consequence, high CHD mortality rate.

Among all cohorts from southern Europe, Crete showed the lowest mortality from CHD and all causes. More than other Mediterranean diets, the Cretan diet was, at least in the 1960s, rich in legumes, fruit, and edible fats that were mostly olive oil. The Cretan diet contained much less meat, but supplied moderate amounts of fish and alcohol, mostly in form of red wine (Table 2). The extremely low CHD mortality rates were particularly surprising, since the average serum cholesterol concentrations in the Cretan population were similar to those in the other Mediterranean cohorts.

Table 2: Seven Countries Study: Dietary intake and mortality rates in selected cohorts1

1 Adapted from 22,23.
2 n = 9 cohorts.
3 10 years/10,000 men aged 50-69 years.
4 Evaluated in the 1960s.

From the results of the Seven Countries Study, Keys drew the main conclusion that the Mediterranean diet would be ideal for decreasing serum cholesterol concentrations, with most of the cholesterol-lowering effects attributed to both a low intake of saturated fats and to a relatively high intake of monounsaturated fatty acids, specifically oleic acid, which is supplied by olive oil. With regard to the results in Crete, however, it should be emphasised that besides the very important cholesterol-lowering effects due to its favourable fatty acid composition, the Mediterranean diet yields further cardioprotective effects because of the large amounts of cereals, legumes, vegetables, and fruit. These foods contain a variety of nutrients and non-nutrients (antioxidative vitamins and other antioxidants such as polyphenols) that have been recently shown to play an important role in the prevention of CHD and other chronic diseases.

Since the 1960s, ongoing changes in eating habits have occurred in the Mediterranean region. In Italy, eg. food survey data demonstrate a pronounced increase in the consumption of animal foods (meat, milk, and dairy products), and of edible fats other than olive oil during the past 40 years. Consumption of sugar, fruits, and vegetables has also increased, whereas that of cereals has decreased slightly. Comparative changes have been observed in other Mediterranean countries. Increasing evidence suggests that these dietary changes have been accompanied by increases in several cardiovascular risk factors: higher concentrations of serum cholesterol, hypertension, and obesity. In turn, investigators have observed raising rates of CHD and diabetes in various Mediterranean countries. These trends confirm the well-established relations between diet and CHD risk and suggest the need to reverse current practices through widespread efforts to preserve and promote traditional diets within the Mediterranean area.

But despite the unfavourable changes in diet, particularly the observed increase in consumption of animal foods, the dietary profile of the Mediterranean countries has still maintained many of its basic features, and recent vital statistics still demonstrate an advantage of eating behaviour, associated with lower CHD mortality rates (Table 3) as compared to Western Europe and the United States.

Table 3. CHD mortality rates in Europe and the United States in the 1990s1.

1 non age-adjusted mortality rates per year per 100.000 persons; data from 1990, 1991 or 1992.

Source: Statistisches Jahrbuch 1995.

Part B - 2: Interventional studies

Although there is a large body of epidemiological evidence supporting the „diet-heart-hypothesis“, a causal relationship between CHD and the intake of saturated fatty acids or the level of serum cholesterol can only be established by randomised intervention trials. From the 1950s to the 1990s, many intervention trials have been targeted at reducing CHD incidence and mortality. Sometimes, changing a current diet has been used as a single tool for the preventive experiment (such as the Veterans Administration Study, the Finnish Mental Hospital Study); sometimes it has been used in combination with other measures (such as the Multiple Risk Factor Intervention Trial , the North Karelia Project , the WHO European Multifactor Preventive Trial of CHD, the Oslo Preventive Trial). In the latter case, it has not been easy to distinguish the role of diet (and of the subsequent cholesterol changes) from that of the other measures.

In most cases, the suggested or implemented changes in dietary habits were oriented towards a reduction in saturated fat, accompanied by an increase in polyunsaturated fat. None of the original Mediterranean diets were particularly rich in polyunsaturated fat. Also, none of the experimental diets were particularly rich in monounsaturated fat, typical of Mediterranean diets due to high consumption of olive oil. Thus, not a single trial including „hard end-points“ has been conducted with the purpose of testing typical Mediterranean diets for the primary prevention of CHD. However, the positive results of saturated fat-reduction obtained in the majority of the trials mentioned above demonstrate the need to reduce the amount of saturated fat in the diet. Furthermore, other intervention studies with „soft end-points“, such as changes in blood lipids, support, at least indirectly, the benefit of a diet resembling that used in the 1950s and 1960s in the Mediterranean region (for review see). Besides, there is much evidence from numerous controlled dietary studies that MUFA-rich diets efficiently lower serum total and LDL cholesterol without changing HDL cholesterol levels.

In summary, in all dietary intervention studies the cholesterol-lowering effects have been shown to be lower than expected. Metabolic “ward” studies have shown the high efficacy of a cholesterol-lowering diet. However, dietary intake can be maximally controlled and confounding variables, such as changes in body weight and physical activity, are removed. Several requirements must be met before generalisation of the results from metabolic ward studies to free-living populations can be made - the most important is compliance to the dietary regimen. In general, the more intensive the dietary counselling, the greater the compliance. The greater the compliance, the more likely will the results achieved in outpatients approximate the results observed in the metabolic ward setting.

It is well established that a reduction in the cholesterol level will lead to a reduction in morbidity and mortality from CHD. Most of the studies conducted to test this hypothesis have indeed demonstrated a reduction in the incidence of ischaemic cardiac events, and some have also shown a reduction in mortality from cardiovascular disease. In addition, there is also evidence that an intensive lipid-lowering therapy in men with moderate hypercholesterolaemia and no history of myocardial infarction reduces the incidence of myocardial infarction and CHD mortality without adversely affecting the risk of death from noncardiovascular causes.

Part B - 3: Recommendations

All these findings convincingly underline the importance of diet in the prevention of CHD. Statements have been made by different national and international bodies and organisations that recommend preventive diets that are similar to the traditional Mediterranean diet. The intake of total fat should be reduced to 30% of energy, SFA intake below 10%. The intake of PUFA should not be more than 10% of energy (7-10%), whereas the remaining fat proportion should be provided by MUFA (10-15% of energy). Dietary cholesterol content should be below 300 mg/day. Furthermore, the intake of complex carbohydrates and dietary fibre should be increased. The Mediterranean diet provides an excellent example how these guidelines could be converted into a tasty and appetising diet. It contains an abundance of plant foods such as bread and grain products, vegetables, legumes, and fruit. The amounts of animal products are only low to moderate. Olive oil is the principle source of fat. This diet is low in SFA, rich in carbohydrate and fibre, and has a high MUFA content. The MUFA are primarily derived from olive oil.

Part C: Olive Oil and its Role in Secondary Prevention of CHD

In patients with established CHD the European Atherosclerosis Society as well as the National Cholesterol Education Program have recommended more aggressive cholesterol-lowering measures because these individuals are at the highest risk of a CHD event. Several large trials have evaluated the efficacy of cholesterol reduction in secondary prevention of cardiovascular events. Secondary prevention trials are more likely to include subjects who are highly motivated to comply with dietary modifications, thus reducing the problems of nonadherence to diet that is observed in unselected populations or even in high-risk individuals without CHD. The majority of dietary intervention studies achieved at least a 10% reduction in cholesterol levels.

With one exception, the dietary trials support the role for aggressive dietary therapy in the secondary prevention of CHD. However, despite intensive dietary measures, the reduction in cholesterol concentrations frequently is insufficient. In these cases, an additional drug therapy is necessary to achieve the desired very low cholesterol levels. Impressive examples for the efficacy of intensive lipid-lowering measures are the Scandinavian Simvastatin Survival Study (4S) and the CARE Study. They both documented that the significant reduction in total and LDL cholesterol due to a therapy with diet and drugs (HMG-CoA-reductase inhibitor) was associated with a highly significant reduction in non fatal myocardial infarction and CHD mortality in patients after myocardial infarction or CHD.

Until now, only one study has been conducted to investigate the particular effects of a Mediterranean type diet for secondary prevention of CHD. In the Lyon Diet Heart Study a Mediterranean diet resembling that of Crete in the 1960s was compared to the prudent diet usually recommended in France in 605 patients recovering from myocardial infarction. The experimental group received more bread, more root vegetables and green vegetables, more fish, less meat (beef, lamb and pork to be replaced with poultry), no day without fruit, and butter and cream were replaced with a special, rapeseed oil-based margarine. The oils recommended for salads and food preparation were rapeseed and olive oils exclusively whereas sunflower oil was consumed in the control group. Moderate alcohol consumption in the form of wine was allowed at meals. In terms of nutrients the experimental groups showed a lower intake of SFA, cholesterol, and linoleic acid, but a higher intake of oleic acid, -linolenic acid, and vitamin C as compared to the control group.

Throughout the follow-up period, serum total, LDL, and HDL cholesterol, serum triglycerides, body weight, and blood pressure were similar in both groups. After a mean follow-up period of 27 months, there were 16 cardiac deaths in the control and 3 in the experimental group; 17 non-fatal myocardial infarction in the control and 5 in the experimental group. Overall mortality was 20 in the control and 8 in the experimental group.

The protective effects of the Mediterranean diet which were caused by several factors were largely independent of serum lipids and blood pressure since these risk factors did not differ between the groups. The Mediterranean diet contained a larger proportion of -linolenic acid (18:3n-3) which was provided to a large extent by the rapeseed oil and the rapeseed oil-based margarine. As described above (see „olive oil and thrombogenic risk factors“) n-3 fatty acids have been shown to have favourable effects on bleeding time and platelet aggregation, thus reducing the risk of thrombosis. Furthermore, the intake of oleic acid was higher in the experimental than in the control group due to the intake of olive and rapeseed oils. The control group consumed sunflower oil, and therefore had a higher intake of linoleic acid. Because the cholesterol-lowering effects of both fatty acids were similar in this study, one can suggest that the high oleic acid content has yielded beneficial effects by reducing the oxidation of LDL-cholesterol which is assumed to play an important role in the pathogenesis of atherosclerosis . In addition, the increase in natural antioxidants also probably played a protective role by further reducing the susceptibility to lipid peroxidation.

In conclusion, the Lyon Diet Heart Study constitutes the first demonstration that the Cretan Mediterranean diet, even when adapted to a Western population, protects against CHD much more efficiently than a prudent diet, rich in linoleic acid. The specific factors that contribute to the protective effects of this diet need further examination, but the low intake of SFA, the high concentrations of oleic acid, a ratio of 18:3n-3 to 18:2n-6 fatty acids of 1:5, and the high content of natural antioxidants seem to be the most reasonable candidates.

Summary

The Seven Countries Study gave the best scientific proof for the association between a diet low in animal products and saturated fat and low mean population levels of serum cholesterol with low incidence and mortality from CHD. It also documented a negative correlation between the intake of monounsaturated fat and the MUFA to SFA ratio on the one hand and CHD on the other hand. All-cause and CHD death rates were low in cohorts with the MUFA-rich olive oil as the main fat, underlining the favourable role of olive oil. There is a body of indirect evidence from interventional studies that the traditional Mediterranean diet with its abundance in plant foods, preferential and regular intake of olive oil, and low to moderate consumption of animal foods efficiently protects against CHD.

Recent findings indicate that olive oil and the Mediterranean diet yield their benefits not only through their effects on established CHD risk factors such as hyperlipidaemia, hypertension, diabetes, and obesity, but also through directly protective effects, particularly their antioxidative properties. In addition, it has been documented that a Mediterranean diet adapted from the Cretan diet is efficient in the secondary prevention of coronary events and death.

Fact Sheet 3

Scientific Basis for Olive Oil, Mediterranean Diet and Cancer prevention

Author:
Prof. Dr. med Gerd Assmann
Dr. troph. Ursel Wahrburg
The Institute of Arteriosclerosis Research,
University of Münster, Germany

1. Introduction

Cancer accounts for about 20% of all deaths in European countries, but within Europe, there are significant differences in cancer mortality with generally higher rates in the northern and western countries and lower rates in the Mediterranean countries. There is strong evidence that these differences can be attributed to a large extent to dietary factors.

Diet plays an important role in the pathogenesis of cancer, although the exact proportion of dietary involvement is still largely uncertain. It is estimated that about 35% - with a range from 10 to 70% - of all cancer deaths may be attributed to dietary factors. The nutritional factors may relate to the development of cancer in several ways: (1) a particular dietary component, food additives, or contaminants may act as carcinogens, cocarcinogens and/or promoters, (2) dietary constituents act as anticarcinogens, (3) nutrient deficiencies and/or excesses may lead to biochemical alterations that promote neoplastic processes, and (4) changes in the intake of selected macronutrients may induce metabolic and biochemical abnormalities that increase the risk for cancer.

Current knowledge from different kinds of studies consistently shows that foods or food groups are more strongly associated with cancer risk than nutrients. There are some possible explanations for this phenomenon. The food component responsible for the causation of, or protection against, cancer, may be a minor component that has not been considered. For example, the protective effect of vegetables may be due not (or at least not only) to an antioxidant vitamin, but to other potential anticarcinogens that food composition tables do not contain. On the other hand, an effect may indeed be due to the anti-oxidant vitamin being investigated, but the weakness of the correlation may simply reflect the fact that the food tables can only give approximate and average values of the nutrient content, especially with respect to the micro-components.

The nutrition-cancer hypothesis has been primarily based on ecological observations. Strong correlations between cancer and dietary habits have been found throughout the world, however, it is well known that correlation does not necessarily mean causation. Besides population studies information on the diet-cancer relationship in humans has derived from case-control and cohort studies. Unfortunately, the evidence from various types of data is partly inconsistent. Furthermore, due to the lack of interventional studies very little is known at present about the impact of dietary modifications or supplements on subsequent cancer rates. Although controlled intervention studies would strongly support the hypothesis of a causal link between diet and cancer, they have several disadvantages. First, a dietary intervention study of cancer risk needs to be very large, in order to receive the number of cases needed for statistical power. This makes such studies extremely expensive. Second, an intervention study can only run for a limited time, whereas cancer is the end-result of a process lasting years or even decades. In addition, there is a great problem of compliance, since it is nearly impossible to change dietary habits of the intervention group for at least a decade, and, simultaneously, to maintain the dietary habits of the control group unchanged. Due to these limitations, there are hardly any results from well-controlled, powerful interventional studies to provide the necessary strong evidence for an effect of single nutrients or specific non-nutrient components on cancer.

The traditional diet in Mediterranean countries is characterised by a high availability of potentially „protective“ foods, such as fruit and vegetables. In addition, there are findings indicating that olive oil as a main component of the Mediterranean diet may also have a cancer-protective effect. The present paper reviews the scientific evidence for the role of olive oil and the Mediterranean diet in cancer prevention. Relevant findings of a relationship between foods, nutrients and cancer risk will be discussed, and international recommendations on the basis of current knowledge will be presented.

2. Cancer and Mediterranean diet

2.1 Role of body weight

The relationship between body weight and cancer has been extensively studied. Obesity is a well-established risk factor for post-menopausal breast cancer and cancer of the prostate, endometrium, and gall-bladder. There is also evidence that obesity is a risk factor for renal-cell carcinoma and cervical cancer. The relationship between overweight and digestive tract cancers is difficult to study, because weight loss is an early symptom of such cancers.

The mechanisms whereby over-nutrition and obesity may promote the development of cancer in humans are unknown. One hypothesis is that over-nutrition in general, regardless of the composition of the diet, is a risk factor for cancer. However, it is also conceivable that a higher intake of special dietary components, eg. dietary fat or potentially carcinogenic substances, may play a crucial role in obesity-related cancer.

At the present stage it is justified to conclude that obesity is an important risk factor for cancers at a large number of sites, and, simultaneously, there is no evidence that it is protective at any site. Therefore, the public health importance of avoiding overweight and obesity is very clear. This is further supported by the fact that obesity is also associated with a number of non-malignant diseases such as gallstones, heart disease, diabetes etc. Due to the high consumption of fruit, vegetables, and cereals, thus leading to a high content of complex carbohydrates and dietary fibre, the Mediterranean diet is suitable for the prevention of obesity and, as a consequence, for the prevention of obesity-related cancer.

2.2 Role of dietary fat

There is a body of evidence from population studies associating total fat intake with cancer at a number of sites, particularly colon, breast, endometrium, ovary, prostate. All of these cancers are related to a Western-type diet and to excess energy intake. However, the evidence is far from clear. For example, at all sites there is simultaneously a strong association with total energy intake, and it is not clear which of these two is the primary factor. Prospective studies have failed to detect any relation between total fat intake and breast cancer (see below). Besides, another study suggests an even protective effect of fat intake due to the consumption of meat and dairy products against gastric carcinogenesis.

International correlation studies have differentiated between per capita consumption of animal and vegetable fat. There is a positive correlation between colon, prostate, breast, and ovary cancer mortality rates and animal fat consumption per capita. Population studies as well as several case-control studies have particularly supported a strong correlation between animal-fat intake and colorectal cancer risk. In contrast, mortality rates of colon cancer are relatively low in Greece, Spain, and Southern Italy, where olive oil is by far the most common type of fat consumed. In a large prospective study animal fat was confirmed as a major risk factor for colorectal cancer, while vegetable fat was neutral and fish oil was protective. These studies suggest that not only the amount but the type of dietary fat is of importance in the aetiology of fat-related cancers.

Vegetable fats/oils are considered to be neutral with regard to cancer risk, but there is little evidence for a special protective effect of vegetable oils per se. However, recent analyses suggest that actually olive oil may have a protective effect against cancer at different sites, particularly breast cancer, which will be discussed in detail below.

Among the PUFA there is evidence from human studies that fatty acids of the n-3 series are protective whereas the n-6 series appears to be neutral with respect to cancer risk. On the other hand, it should be mentioned that the role of PUFA becomes less clear when also evaluating animal studies. Investigations in laboratory animals have indicated that n-6 PUFA are more likely to increase cancer risk than other types of fatty acids. Several mechanisms whereby PUFA might promote the tumour development in animals are discussed: (1) Peroxides produced from PUFA might activate cell proliferation, (2) PUFA could increase membrane fluidity which could promote cell division, or (3) PUFA might suppress the immune system which could allow for the development of cancer by decreased „immune surveillance“. However, these hypotheses should be assessed with great caution, and the findings have never been confirmed in humans.

2.3 Role of protein

In epidemiological studies it is very difficult to separate the effects of animal fat and animal protein. In a recent review of diet in the aetiology of cancer it is concluded that all of the correlations observed between cancer risk and animal protein were secondary to those caused by the animal fat component. This conclusion has been confirmed by other groups. All in all, there is no evidence showing that there is an independent relationship between protein consumption and cancer risk.

2.4 Role of complex carbohydrates and dietary fibre

As it is the case for dietary fat, the relation between complex carbohydrates and cancer risk is not clear. Although there is some evidence for a protective effect, the question has been complicated by methodological problems. Complex carbohydrates are a group of heterogenous substances, including starch, non-starch polysaccharides, and dietary fibre, and it is not clear to which component of the total complex carbohydrates protective effects can be attributed.

These problems do not affect studies of the relation between cancer and food intake. So, when relating high-fibre food groups and cancers at various sites, it has been found that cereals are highly protective against cancers of the colon, breast, endometrium, and prostate. Case-control studies did not show such consistent results, but a protective effect of high-fibre foods against colorectal cancer could be confirmed in a prospective study by Willett et al.

2.5 Role of fruit and vegetables

There is a body of epidemiological evidence based on population, cohort, and case-control studies that a high intake of fruit and vegetables, particularly raw vegetables, protects against cancers at different sites, especially those of the digestive and respiratory tracts, and the hormone related cancers. One important feature of fruit and vegetables is that they have an anti-carcinogenic action at a wide rage of sites, and, simultaneously, that there is not any positive correlation between fruit and vegetable intake and cancer.

Fruits and vegetables contain a variety of anti-carcinogenic agents. These include carotenoids, vitamin C and E, dietary fibre, selenium, glucosinolate, indoles, flavonoids, protease inhibitors, and plant sterols. These and other agents show both complementary and overlapping mechanisms of action, including induction of detoxification enzymes, antioxidant effects, inhibition of the formation of nitrosamine, binding and dilution of carcinogens in the digestive tract, alteration of hormone metabolism and immune system, and others. The exact mechanisms of single agents, however, are largely unclear. Until now, only the actions of anti-oxidant vitamins and pro-vitamins have been supported by human epidemiology. It is likely that not a single agent functions as key agent, but that all of them play some part in the protective action under some circumstances.

3. Role of olive oil in cancer

Various epidemiological studies indicate that the regular consumption of olive oil is inversely associated with cancer at different sites. Most of the studies address the relationship between olive oil and breast cancer or gastric cancer. They will be detailed below. Although there are also findings which suggest protective effects of olive oil for cancer at other sites, eg. colon, endometrium and ovary, the evidence is limited, because the number of studies is rather small, and their results can be not more than an indication for a possible effect.

3.1 Risk of breast cancer

Breast cancer incidence varies more than fivefold around the world, and the offspring of migrants moving from countries with low breast cancer incidence to countries with high incidence acquire rates close to those of the new country, suggesting that environmental and lifestyle influences are important in the aetiology of breast cancer. Diet may be a major factor in the international variation in the incidence of breast cancer. This is supported by the fact that the incidence of, and mortality from breast cancer is lower in Mediterranean countries than in countries of western and northern Europe or the United States. As already pointed out, breast cancer, as well as other types of cancer, is related to a Western-type diet with excess energy intake, high intake of animal products and animal fat, and low intake of vegetable foods. The Mediterranean diet, on the other hand, contains a variety of potentially protective food items such as fruit and vegetables. The content of animal fat is low, and olive oil is the major source of dietary fat.

Based on the population studies it seems likely that dietary fat is a key factor in the relationship between diet and breast cancer. Evidence from animal experiments and case-control studies support the association between dietary fat intake and the incidence of breast cancer. However, a recent meta-analysis of seven large prospective cohort studies could not corroborate the association. In the included cohort studies, the baseline diet of a large number of women has been assessed and the diets of women who subsequently develop breast cancer have been compared with the diets of women who did not. In the analysis, information about 4,980 cases from studies including 337,800 women was available. The authors found no evidence for a positive association between total dietary fat intake and the risk of breast cancer. Relative risks for saturated, mono-unsaturated, and polyunsaturated fat and for cholesterol, considered individually, were also close to unity. There was no risk reduction even among women whose energy intake from fat was less than 20% of total energy intake. Correction for error in the measurement of nutrient intake did not substantially alter these findings.

Nevertheless, despite these findings which focus on the effects of the nutrients fat and fatty acids, it cannot be overlooked that there is some evidence for an inverse correlation between breast cancer risk and olive oil consumption. Recently, four noteworthy case-control studies have especially examined the role of olive oil in the aetiology of breast cancer. All four studies point to a modest beneficial effect of olive oil consumption on breast cancer risk. One of two Spanish studies included 100 women with breast cancer and 100 hospital controls, who were interviewed using a food-frequency questionnaire. Cases reported significantly lower consumption of fruits, vegetables, and fish. They showed a lower intake of vitamin C and mono-unsaturated fatty acids (MUFA) as compared to the controls. After controlling for total energy intake the relative risk (RR) for women with the highest intake of MUFA was 0.3 as compared to those with the lowest intake.

In the second Spanish study a validated semi-quantitative food-frequency questionnaire was completed by 760 women with histologically confirmed breast cancer and 990 randomly selected female controls. The study subjects were divided into quartiles according to their intake of food items and nutrients. Data were adjusted for total energy intake and other potential confounding factors by multiple logistic regression analysis. Neither the total fat intake nor the different types of fatty acids were significantly associated with breast cancer. There were negative associations observed for MUFA and oleic acid, but these trends did not reach statistical significance. However, a higher consumption of olive oil was significantly related to a lower risk of breast cancer (for highest vs. lowest quartile of consumption odds ratio (OD) = 0.66), with a significant dose-response trend).

In Italy, data from a multi-centre case-control study, including 2,560 cases with confirmed breast cancer and 2,590 controls and using a validated food-frequency questionnaire, were analysed . The data were controlled for confounding factors by multiple logistic regression analysis. The results showed an inverse relationship of breast cancer risk with the intake of olive oil and also with other vegetable oils, but not with butter or margarine.

One further study in Greece also tried to quantify the effect of consumption of olive oil with a comparable design as the studies mentioned above (820 cases, 1550 controls; food-frequency questionnaire; adjustment for confounding factors). Increased olive oil consumption was associated with significantly reduced breast cancer risk. The relative risk was 0.75 in the quintile with the highest intake. Again, there was no relationship between total fat intake and breast cancer risk.

Although these studies give evidence for a protective role of olive oil against breast cancer, several questions remain: Is the inverse association between olive oil consumption and breast cancer indeed genuine and unconfounded ? And if the association is real and causal, is it specific for olive oil or does it apply to MUFA in general? Is the effect actually attributable to the fatty acid composition of olive oil or to its content of specific micronutrients (eg. vitamin E and other antioxidative substances)? In addition, the reported associations are modest, the possibility of selection bias can never be completely excluded, and residual confounding by vegetable intake is theoretically possible since vegetables frequently are consumed with olive oil and are also likely to protect against breast cancer.

In summary, there is a great need for future research work on the relationship between olive oil and breast cancer. Nevertheless, the necessary caution should not overshadow that the existing evidence consistently supports a protective role of olive oil in breast cancer prevention.

3.2 Risk of gastric cancer

Gastric cancer is the third most common cause of cancer mortality in the European Union. Among the European countries, Italy is one with the highest mortality rates for gastric cancer in both sexes. Within Italy a wide geographical variability for gastric cancer has been found, with high risk areas located in central-northern regions and low rates in southern Italy and on the islands (5). These findings are rather surprising, because usually gastric cancer rates are higher in low social classes and in less developed areas. Therefore, the decreasing geographical gradient from north to south is in contrast with most reports from other western countries and represents a specific Italian pattern. Since the limited information available suggests that dietary habits vary widely in different regions within Italy, it can be speculated that these differences in the diet play a role in the geographical variability of gastric cancer(5).

One recent multi-centre case-control study evaluated possible reasons for the differences in cancer mortality, focusing on dietary factors. The study included 1015 cases with histologically confirmed gastric cancer and 1160 controls. Dietary patterns were assessed using a quantitative food-frequency questionnaire. The study subjects frequently consuming fresh fruits, citrus fruits, and raw vegetables had a gastric cancer risk reduced to only 30 % compared to those who only rarely consumed such foods. These findings were consistent with other studies. Furthermore, the consumption of olive oil, as well as of garlic and spices was inversely related to gastric cancer risk. On the other hand, the consumption of meat, salted and dried fish, and seasoned cheeses was associated with an increased risk of gastric cancer.

Since there are no further well-designed studies which evaluated the role of olive oil in gastric cancer prevention and thus could support it, it must be stated that a protective effect of olive oil is not proven until now. The only conclusion that can be drawn at the present stage for gastric cancer prevention is that increased fruit and vegetable intake seems to be helpful.

3.3 Olive oil and cancer - open questions

Although all the findings concerning the role of olive oil in cancer prevention are encouraging, they need confirmation by further studies, particularly prospective cohort studies and well-designed, strictly controlled intervention studies. As already pointed out, intervention studies in the field of cancer are hard to conduct. They would be greatly facilitated if there were biological markers which (1) were proven as a risk factor for cancer, (2) could be easily measured in the laboratory and, especially, (3) were diet-dependent. Eg., one such biomarker is the excretion of secondary bile acids. It has been demonstrated that secondary bile acids induce cell proliferation and act as promoters for colon cancer. In addition, dietary intervention studies in humans have demonstrated that different diet regimens led to different faecal bile acid levels. To identify more valid parameters as risk factors for cancer at different sites is an important objective of current research work.

Another question that arises is which component (or components) of olive oil is responsible for the eventual protective effect. The identity of olive oil is dominated by mono-unsaturated fatty acids (MUFA), but undoubtedly, olive oil is more than a mono-unsaturated fat. It is likely that the MUFA play a central role in the cancer-protective effect, but it is difficult to demonstrate this effect in epidemiological studies. In northern Europe and the United Stated most MUFA are supplied with animal products, ie., their intake is accompanied by a high intake of saturated fat, and the effects of SAFA and MUFA can be hardly evaluated independently. On the other hand, if olive oil is the primary source of MUFA, the diet simultaneously has a high content of antioxidative substances, which are also present in olive oil. Until now, there is not a single study which could precisely discriminate between the different types of fatty acids and which had adjusted the results for the other dietary confounding factors. An active effect, either protective or promoting, in the pathogenesis of cancer has not been proven either for oleic acid or for MUFA as a whole.

Another explanation for a cancer-protective effect of olive oil could be that the responsible component would not be the MUFA but other components of the oil. It is possible that tocopherols or other micronutrients in olive oil are important mediators of its effects. Again, the evidence is limited, and no conclusive statements can be made at present.

In summary, at this stage the question on the role of olive oil in the pathogenesis of cancer cannot be definitively answered. A beneficial effect of olive oil consumption on breast cancer risk is highly likely although not conclusively established. The knowledge with regard to cancer at other sites is less clear. However, even if an active cancer-protective effect of olive oil could not be definitively proven until now, there are on the other hand no studies at all which would support a tumour-promoting effect of olive oil. Thus, as a conclusion, olive oil is at least neutral with respect to tumorgenesis, and there is some evidence that it is not simply non-toxic, but has its own protective effects.

4. International recommendations for cancer prevention

Several health authorities have made dietary recommendations for cancer prevention. The recent guidelines of the American Cancer Society consist of six points (1):

1. Avoid obesity.
2. Cut down on total fat intake.
3. Include a variety of vegetables and fruits in the daily diet.
4. Eat more high-fibre foods, such as whole grain cereals, vegetables, and fruits.
5. Limit consumption of alcoholic beverages, if you drink at all.
6. Limit consumption of smoked, salt-cured, and nitrate-cured foods.

Similar recommendations are given by the National Cancer Institute, but their guidelines differ from those of the American Cancer Society by specifying levels of nutrient intake for the general population (no more than 30% of total calories from fat and 20-30 grams of dietary fibre daily).

The United States have started to implement these guidelines with a nationwide programme, called the „5-a-day for better health programme“. This programme is a joint campaign of health authorities, state, and food the industry. The recommendations are to consume mainly vegetable foods, to eat five or more portions of fruits and vegetables per day, and in addition, to eat six or more portions of breads, cereals or grain per day. With this strategy a simple and positive message is given in the context of healthy nutrition to the population. It is disseminated via super markets, restaurants, media, the public, and research. In the following years it will be evaluated if and to what extent the campaign has led to changes in the dietary habits in the United States.

In Europe, the programme „Europe against cancer“ by the European Commission has worked out a codex for cancer control which has been updated in 1994. In the framework of this programme, there are also some recommendations concerning nutrition and diet:

1. Increase the daily intake of fresh fruits and vegetables, as well as of high-fibre grain products.
2. Avoid obesity, increase regular physical activity, and limit the intake of high-fat foods.
3. Reduce the alcohol consumption.

In November 1996 the WHO conference „nutrition in prevention and therapy of cancer“ took place in Stuttgart, Germany. The purpose of this conference with internationally established scientists was to summarise current knowledge in the field of colon, gastric, breast, and lung cancer with regard to the role of nutrition in their prevention and therapy. The resulting consensus statements will be published in detail in the course of the year 1997. As a general policy statement for reducing the risk of cancer it was formulated that fruits, vegetables, and whole-meal cereals should be the main components of the daily diet. Avoidance of obesity and of high alcohol intake as well as regular physical activity can contribute to a reduction of cancer risk. In addition, it was stated that there is no kind of diet with which colon, gastric, breast, or lung cancer could be treated.

5. Summary and conclusions

There is general consensus that diet is an important component in the aetiology of cancer. Scientific evidence is primarily derived from epidemiological studies as well as from animal and in vitro experiments. In the former, foods or food groups are more strongly associated with cancer risk than nutrients, and for many foods the results are not persuasive or consistent. Well-designed, strictly controlled intervention studies in humans which could support the role of single foods or nutrients in cancer prevention with sufficient strength are missing. Thus, the scientific evidence for detailed recommendations with respect to cancer prevention is limited.

In Europe, but also in the United States, experts in the field of medicine and nutrition have lost much of their credibility in the eyes of the public, because sometimes dietary advice has been given on the basis of passionate belief rather than solid evidence, and has subsequently had to be withdrawn. In addition, risk-benefit analyses for recommendations must be carried out in the context of general health and not regarding one disease in isolation (13). For instance, it is known that there is an inverse correlation between the mortality from colon cancer which is associated with over-nutrition, and gastric cancer which is related to poor nutrition. So, any advice given for the prevention of cancer at one site must imply that it will not lead to an increase in cancer at another site.

As a consequence of these deficiencies in knowledge and in the light of current evidence, only few recommendations are justifiable as formulated by international health authorities: Obesity should be avoided; vegetable foods should be the main components of the daily diet, ie. the intake of fruits, vegetables, and whole-grain cereals should be greatly increased; the amount of total fat should be reduced; the alcohol consumption should be limited. Furthermore, it is likely that it will be reasonable to advice the general public to eat decreased amounts of animal fat. These recommendations are largely in agreement with those internationally given for the prevention of cardiovascular risk factors and coronary heart disease

The traditional Mediterranean diet is characterised by a high consumption of fruits, vegetables, and cereals which are the major sources of dietary fibre. Due to the high content of complex carbohydrates it has, on average, a lower energy content than a high-fat diet which makes it suitable for the prevention of obesity. The intake of animal products and animal fat is low, most dietary fat is provided by olive oil. Thus, the Mediterranean diet can be considered as an excellent example for a diet complying with the requirements for a cancer-protective diet. This has been confirmed by lower cancer mortality rates in the Mediterranean region as compared to western and northern Europe and the United States. However, until now, it is not known if the protective effect of the Mediterranean way of eating is due to specific food items or nutrients, or to the presence and interaction of a variety of protective components among nutrients and non-nutrients. It is likely that olive oil is one of the relevant foods, but its definitive role in cancer prevention remains to be proven.

Fact Sheet 4

Scientific Basis for Olive Oil, monounsaturated fatty acids, antioxidants and LDL oxidation

Author
Prof. Dr. med Gerd Assmann
Dr. troph. Ursel Wahrburg
The Institute of Arteriosclerosis Research,
University of Münster, Germany

1. Introduction

A high concentration of plasma low-density lipoprotein (LDL) cholesterol is a dominant risk factor for the development of atherosclerosis. However, the precise mechanisms by which LDL causes atherosclerosis, i.e. the steps between infiltration of LDL into the arterial wall and the formation of an atherosclerotic lesion, are not well understood. There is increasing evidence that LDL must be modified in some way before it can become pathogenic, and recent data from biochemical, animal and epidemiological studies strongly support the hypothesis that oxidative modification of LDL plays a crucial and causative role in the pathogenesis of atherosclerosis. The susceptibility of LDL to become oxidised is determined by a variety of endogenous and exogenous factors. Among the latter, nutritional factors are of outstanding importance, in particular the types of dietary fatty acids and antioxidant vitamins. The present paper outlines the oxidation hypothesis of atherosclerosis and the role nutritional factors could play in modulating this process.

2. The role of LDL oxidation in atherogenesis

2.1 LDL oxidation

LDL is a spherical particle consisting of a large protein, termed apolipoprotein B (apo B), which is embedded in an outer mono-layer of phospholipids and free cholesterol molecules. This monolayer surrounds a central core of cholesterylesters and triglycerides. One such LDL-particle contains about 3600 fatty acids, roughly half of them being polyunsaturated fatty acids (PUFA). Additionally, LDL contains several antioxidants, the most important being -tocopherol.

Oxidation of LDL is a lipid peroxidation chain reaction, initiated by so-called free radicals. Radicals are molecules with an unpaired free electron, making them highly reactive. In particular, oxygen radicals (eg. hydroxyl-, hydroperoxyl-, superoxidradical), which are produced in the cells as by-products of oxidative metabolic processes, are of importance. The chain reaction begins when a reactive free radical removes a hydrogen atom from a polyunsaturated fatty acid molecule in the LDL particle. PUFAs are highly susceptible to lipid peroxidation, because the susceptibility of a fatty acid to oxidation increases with its number of double bonds. Lipid peroxyl radicals are formed which in turn can initiate oxidation in neighbouring fatty acids. This process leads to a breakdown of PUFA, yielding a variety of reactive aldehydes, ketones, and other products, some of which form covalent bonds with LDL apo B).

It is well known that oxidation of LDL can be initiated in vitro by incubating isolated LDL particles with cells (macrophages, lymphocytes, smooth muscle cells, and endothelial cells), metal ions (copper or iron), enzymes, oxygen radicals, or UV-light (10,19,40). However, little is known about the mechanisms by which LDL becomes oxidised in vivo. There is evidence that LDL in the plasma is protected against oxidation, because the plasma contains a lot of water-soluble antioxidative substances, such as ascorbic acid, ureic acid, or bilirubin. Thus, it is likely that the majority of oxidative modification of LDL occurs in the artery wall and not intravascularly. In the arterial wall LDL is largely isolated from the many antioxidants present in the plasma. Furthermore, the LDL particles are exposed to a variety of free radical species and oxidative enzymes produced by artery wall cells.

If LDL is exposed to pro-oxidative conditions, it becomes depleted of its antioxidants, with -tocopherol being the first to be lost. The oxidation of the LDL PUFA to lipid hydroperoxides starts when most of the antioxidant defence has been lost. The rapid decomposition of PUFA leads to a lot of further modifications in the LDL-particle, eg. oxidation of cholesterol, modifications of apo B and release of several bioactive substances.

Nilsson et al., Herz 1992;263

2.2 LDL oxidation and atherosclerosis

Early atherosclerotic lesions are characterised by the presence of fatty streaks, which are composed of so-called foam cells, derived from smooth muscle cells and monocyte-macrophages. In a first step monocytes invade from the bloodstream into the subendothelial space and become resident macrophages. They then take up lipids and lipoproteins, predominantly cholesterylesters, infiltrated and deposited in those regions. However, the precise mechanisms of this lipid uptake and accumulation is not known. It is known that the LDL-receptor is down-regulated when the intracellular cholesterol content increases, so the uptake of cholesterol via the classic LDL-receptor pathway cannot result in a pathological cholesterol accumulation. On the other hand, oxidatively modified LDL (ox-LDL) is no longer recognised by the LDL-receptor, but can be taken up by the so-called scavenger receptor on macrophages which is not regulated by intracellular cholesterol. Thus, the scavenger receptor mediated uptake of ox-LDL could lead to a substantial cholesterylester accumulation in macrophages.

The study of ox-LDL has shown that oxidation of LDL changes it in many ways that make it more atherogenic than native LDL. These effects are summarised in Table 1.

Table 1: Atherogenic properties of ox-LDL.

3. Olive oil and LDL oxidation

There is no doubt that nutrition is of great importance in LDL oxidation. In particular, the amount and type of fat in the diet as well as the content of antioxidative components affect the susceptibility of LDL and cells to oxidative damage. There are several potential ways by which dietary fatty acids may influence the oxidation of LDL. First of all, the amount and composition of dietary fat affects the amount of LDL particles present in the artery wall. Replacement of dietary saturated fatty acids with MUFA or PUFA lowers total and LDL cholesterol levels . This reduction in LDL levels would likely decrease the amount of LDL entering the artery wall and theoretically would directly reduce the amount of LDL available for oxidation. Dietary fatty acids may also directly influence LDL susceptibility to oxidation by changing its fatty acid composition. Furthermore, dietary fatty acids may change the fatty acid composition of the artery wall cells, thus altering their pro-oxidant activity and their response to oxidative stress.

Due to its high MUFA content olive oil seems to have protective properties with regard to LDL oxidation. Additionally, olive oil may further provide some protection by supplying LDL with potent antioxidants, such as Vitamin E and polyphenolic compounds. These protective effects of olive oil are detailed below.

3.1 Effects of dietary fatty acids on LDL oxidation

Several investigators have compared the influence of dietary MUFA and PUFA on LDL oxidation. First, it could be shown in rabbits, that oleate-rich LDL particles were remarkably resistant to oxidative modification (32). In subsequent studies small groups of human subjects were fed diets differing in their MUFA and PUFA content.

Reaven et al. provided their study participants with high-fat liquid formula diets which had a MUFA content of about 80% of total fatty acids or a PUFA content of about 60%. After these diets extremely enriched with MUFA (derived from high-oleic sunflower oil) or PUFA (derived from sunflower oil) the fatty acid composition of isolated LDL particles reflected the fatty acid composition of the diet, and the fatty acid distribution was similar in the different lipid fractions of the LDL-particle. The linoleic acid (C18:2) content of LDL was strongly related to the rate and the extent of oxidation, whereas the amount of oleic acid (C18:1) in the LDL-particles was inversely correlated to the extent of oxidation.

Some other studies were conducted with solid food diets. For instance, Bonanome et al. compared a grapeseed oil-enriched diet (45 % fat; 5 % MUFA, 30 % PUFA) to a diet enriched in olive oil (45% fat; 30% MUFA, 5% PUFA) in 12 healthy subjects. Again, the rate of LDL oxidation during the PUFA diet was increased compared to the MUFA diet. In the other studies it could be confirmed that the linoleic acid content of LDL strongly correlated with either the rate or the extent of oxidation.

Although these results were unambiguous, several questions remain open. On the basis of the studies conducted so far, it is not clear whether only the decrease in easily oxidisable linoleic acid in LDL is responsible for the decrease in lipid peroxidation after a MUFA-rich diet, or whether this decrease is due to direct antioxidant properties of oleic acid? Or, are both mechanisms together involved in reducing the susceptibility of LDL to oxidation? Do PUFA enhance or do MUFA decrease LDL oxidation?

Until now, there are only two studies dealing with these questions: Aviram and Eias compared the effects of an olive oil supplement (50g/day) to the baseline diet (30% fat, 50% carbohydrates). It could be demonstrated, that the LDL obtained after one and two weeks of the olive oil-rich diet showed a reduced susceptibility to oxidation as well as reduced cellular uptake by macrophages. Thus, it can be suggested that MUFA supplementation can cause an absolute decrease in LDL susceptibility to oxidation. Berry et al confirmed these results in their study in which they compared a MUFA-rich diet (17% of energy, total fat: 33% of energy) with a carbohydrate-rich diet (65% of energy; MUFA: 7%), keeping the PUFA content constant in both diets. The MUFA-diet led to a significant reduction in the susceptibility of LDL to oxidative stress. These data support the concept, that oleic acid enriched diets may reduce LDL oxidation both through intrinsic antioxidant properties of the MUFA, as well as by reducing the content of linoleic acid in LDL.

3.2 Effects of dietary fatty acids on cellular pro-oxidant activity and cellular susceptibility to oxidative stress

Dietary fatty acids can also have effects on cellular pro-oxidant activity. Several investigators have demonstrated that dietary supplementation with different types of fatty acids leads to changes in the fatty acid composition of the monocyte membrane composition and that this influences the production of oxygen radicals, particularly superoxide anions, in monocytes and macrophages. The production of superoxide anions by these cells undoubtedly contributes to LDL oxidation. In a comparison of the effects of dietary supplementation with MUFA, n-3-, or n-6-PUFA on superoxide anion generation a decrease in superoxide production was only observed after n-3 fatty acid supplementation, while the monocytes from the MUFA or n-6-PUFA supplemented groups showed no change or had increased superoxide anion levels. The mechanism by which n-3 fatty acids may reduce the oxygen radicals is not known, and other studies could not confirm these effects. Further investigation is needed to exactly evaluate the role of the different fatty acids on cellular pro-oxidant activity.

Furthermore, dietary fatty acids can influence the susceptibility of cells to oxidative stress, probably also by changing cell membrane fatty acid composition. Cells enriched with MUFA have been shown to be less susceptible to oxidative damage, whereas n-6 PUFA increased the susceptibility to oxidative damage. Oxidative damage of cells of the artery wall can contribute to the progression of atherosclerotic lesions. For instance, oxidation-induced injury to endothelial cells may increase the likelihood of plaque rupture and clot formation.

3.3 Antioxidative constituents of olive oil and LDL oxidation

3.3.1 Vitamin E (-Tocopherol)

Oxidative injury is assumed to play a crucial role in the development of several chronic diseases, eg. coronary heart disease (CHD) and cancer, and the possibility that dietary antioxidants may protect against LDL oxidation and oxidative injury has received growing attention in the past few years.

Since the 1980s several epidemiologic studies have been carried out to evaluate the relationship between the intake of antioxidants, with main emphasis on vitamin E, and cardiovascular disease. It could be observed, that high-dose vitamin E supplements (>100 IU/d = 67 mg -tocopherol/d) over at least two years significantly lowered the CHD risk (risk reduction 31-65%) (reviewed in Jha et al 95). On the other hand, short-term supplementations as well as low-dose supplementations (< 100 IU/d) had no significant effects on CHD.

By contrast, the majority of randomised intervention trials completed so far showed no significant reduction in cardiovascular disease with vitamin E supplementation (for review see. However, it should be mentioned that these trials had several limitations: They were not specifically designed to assess cardiovascular disease, did not provide data on nonfatal cardiovascular events, the treatment duration was insufficient and they used suboptimal vitamin E doses. It can be expected that the contrast between epidemiological and interventional findings could be resolved after completion of the ongoing large-scale and long-term randomised intervention trials designed specifically to evaluate the effects of antioxidants on cardiovascular disease.

Until now, only the CHAOS (Cambridge Heart Antioxidant Study) has been completed. In this double-blind, placebo-controlled study 2000 patients with established coronary atherosclerosis received either a vitamin E supplementation ( 400 or 800 IU daily) or a placebo for about one year. The -tocopherol treatment led to a substantial reduction of non-fatal myocardial infarction.

However, intervention trials cannot provide sufficient evidence for a causal relationship between the intake of antioxidants and LDL oxidation and atherogenesis. In addition, there are several unresolved questions with respect to antioxidant intervention studies. It can be speculated that a study duration of only a few years may be inadequate, because - based on the hypothesis that the anti-atherogenic effects of antioxidants are mediated by the inhibition of LDL oxidation and that LDL oxidation is one of the earliest steps in atherogenesis - there might be no demonstrable effect on clinical events for some time. It is not known how long it takes for a new fatty streak to become a clinically relevant lesion, and thus it may be sensible to investigate the effects of antioxidant supplementation over a longer period, possibly over twenty or more years.

In addition to epidemiological observations and intervention studies the effects of antioxidants on oxidative modifications of LDL have been investigated directly with controlled experimental studies. As already pointed out (see pt. 2.1), -tocopherol is the most important antioxidant in LDL particles, and the main research interest has focused on the relationships between vitamin E and LDL oxidation.

Several studies with healthy subjects whose diet was supplemented with vitamin E were conducted in order to investigate how increased vitamin E intake would affect the oxidation resistance of LDL. In a study by Esterbauer et al. healthy volunteers were given -tocopherol doses from 150 to 1200 IU for three weeks. The plasma and LDL antioxidant status and the oxidation resistance of LDL (lag time and oxidation rate to copper-induced LDL oxidation) were measured before, during and after supplementation. It could be demonstrated that the -tocopherol supplementation led to an increase in the amount of -tocopherol in the plasma and in the LDL particles. During the supplementation period the isolated LDL showed in vitro a higher oxidation resistance compared to the initial value determined before the study. The degree of oxidation resistance correlated closely with the dosage of vitamin E. One week after the supplements were stopped the oxidation resistance had returned to the initial basal values. Similar results were obtained in comparable studies.

Furthermore, it could be observed that the oxidation resistance of LDL was also increased in non-vitamin E supplemented men at high vitamin E plasma levels as compared to subjects with a lower plasma content, suggesting that even an amount of -tocopherol that can be taken up only with vitamin E rich foods would result in a plasma vitamin E level which decreases the susceptibility of LDL to oxidation.

3.3.2 Phenolic compounds

In addition to its vitamin E content olive oil contains a variety of further minor components that are responsible for its unique flavour and taste. This is related to the fact that olive oil is the only vegetable oil obtained from whole fruits rather than from seeds, which allows it to retain all the organoleptic properties of olives.

Among these minor constituents (adding up to 2-3% of unrefined oil), phenolic compounds are of outstanding importance. The amount of phenolic compounds – in a range from 50 to 800 mg/kg oil - depends on factors such as climate, cultivation, stage of maturation. Furthermore, unrefined olive oil has a much higher content of phenolic compounds than refined oils.

Phenolic compounds in foods include simple phenols and phenolic acids, eg. hydroxycinnamic acid derivatives, and flavonoids. These phenolic classes contain numerous compounds that are widespread in plant foods. Phenolic compounds influence the quality, palatability, and stability of foods by acting as flavourants, colourants, and antioxidants. The presence of conjugated ring structures and hydroxyl groups allows phenolics to actively scavenge and detoxicate free radicals, and they can sequester metal ions through liganding. They have been shown to inhibit lipid oxidation in biological systems. Additionally, phenolic compounds are able to inhibit the activities of the pro-oxidant enzymes lipoxygenase and cyclooxygenase. In numerous animal models phenolic compounds exhibit further pharmacological effects, such as anticarcinogenic, antiinflammatory, and antihaemorrhagic effects (inhibit lipid oxidation in biological systems.

Until now, not all of the numerous phenolic compounds in olive oil have been chemically identified. Some of the major compounds with potent antioxidative properties are dihydroxyphenylethanol (DHPE), hydroxytyrosol, caffeic acid and oleuropein. The concentration of DHPE in olive oil is considered to be a marker of its quality and has been shown to be a good index of its stability. Various phenolic components have been tested for their ability to prevent the accumulation of peroxides in the oil, but only little is known about their potential biological activities.

In a recent study isolated LDL particles obtained from healthy subjects were incubated with different olive oil polyphenols, leading to an inhibition of LDL oxidation, measured as effects on various parameters of lipid oxidation, such as formation of lipid peroxides and conjugated dienes. An animal experiment in rats which were fed with different MUFA-rich oils with standardised vitamin E contents demonstrated that the LDL particles from the olive oil-fed rats were more resistant to oxidative modification than those of the triolein fed-rats, suggesting that the phenolic compounds present in the olive oil may be responsible for the increased resistance of LDL to oxidation. These results could be confirmed with comparable experiments in rabbits.

The findings available so far are encouraging and indicate that phenolic compounds present in olive oil, particularly in unrefined oils, may contribute to the prevention of processes, such as lipoprotein oxidation, that are considered to be relevant in promoting atherogenesis. However, this evidence is only based on in vitro and animal studies, and much more research work, especially controlled human dietary studies, has to be done to evaluate the possible beneficial effects of phenolic compounds in olive oil in vivo.

As described above, phenolic compounds are not only found in olive oil, but are widespread in vegetable foods. In particular, flavonoids as a large group of potent polyphenolic antioxidants are naturally present in vegetables, fruits, and in beverages such as tea and wine. Some of the major food flavonoids are quercetin, kaempferol, myricetin, and luteolin. In the Seven Countries Study as well as in the Zutphen Elderly Study the average intake of flavonoids was inversely and independently correlated with mortality from CHD. It can be assumed that the flavonoids may be partially responsible for the positive effects of fruit and vegetable consumption on CHD. Nevertheless, more supportive epidemiological data and more experimental studies on the mechanisms involved are needed before firm conclusions on the protective effects of flavonoids on CHD risk can be drawn.

4. Summary and conclusions

There is extensive evidence that oxidative modifications of LDL play a crucial and causative role in the pathogenesis of atherosclerosis. Oxidation of LDL begins with peroxidation of the PUFA in the LDL particle. Thus, LDL fatty acid composition undoubtedly contributes to the process of LDL oxidation. The fatty acid composition of LDL is influenced by dietary fatty acids, and, as a consequence, the amount and type of fat in the diet also affects the susceptibility of LDL to oxidative damage. Diets rich in MUFA render LDL more resistant to oxidative modifications compared with diets rich in linoleic acid, due to an enrichment of LDL particles with oleic acid instead of linoleic acid. In addition, the fatty acid composition of cell membranes is diet-dependent, and MUFA-rich diets also lead to a higher MUFA content of cell membranes, and therefore to a higher cellular resistance to oxidative damage.

A second important dietary factor which provides protection against oxidative stress are antioxidants mainly derived from plant foods, such as vitamin E, -carotene, vitamin C, flavonoids and other phenolic compounds. Recent studies indicate that not only -tocopherol, but also several phenolic compounds in olive oil may inhibit also LDL oxidation, leading to a reduced risk of atherosclerosis. Although these results are promising, many questions remain unanswered and more research work is needed to investigate the exact mechanisms of actions of the phenolic compounds and to confirm the protective effects in vivo.

Up to now, attention to the benefits of the Mediterranean diet has been focused on its favourable effects on hyperlipidaemia and other established cardiovascular risk factors, due to its low content of SAFA and its high content of MUFA as well as of complex carbohydrates and dietary fibre. Currently evidence is provided that additional components typically present in abundance in the Mediterranean diet, namely antioxidants derived from vegetables, fruit and beverages, but also from olive oil, might contribute to the protection against CHD and, probably, cancer and other diseases. Furthermore, the high intake of MUFA in the Mediterranean diet, due to the olive oil consumption may combine the advantages of lowering cholesterol levels and decreasing LDL and cell susceptibility to oxidation.

Fact Sheet 5

Olive oil: production, trade quality, and composition

Author: Eurosciences Communication in co-operation with the Institute for Arteriosclerosis Research, University of Münster, Germany

The olive tree belongs to the botanical family of Oleaceae, the most important species being Olea europea sativa. It is characterized by its extended life span. The olive tree has adapted to heat and dryness and is therefore well suited to the environment in which it grows. The ideal conditions for its growth are at a mean temperature of 15 to 20 °C, i.e. especially in Mediterranean countries. During maturation, the oil content of the olive increases and reaches 15 to 30% weight of the total fruit. Harvesting takes place from November to February, using the traditional method of hand picking. Beating the olives off the tree with poles considerably increases the quantity of olives collected per day. The olives are collected in large light synthetic fibre nets positioned under the trees. After harvesting, the fruit has to be sorted, especially when it has been collected from the ground.

• Native olive oils

Native olive oil is oil obtained from the olive by mechanical or other physical means. It is the oily juice of the fruit and not (in contrast to other vegetable oils) a seed oil. Native olive oil is virtually the only oil that can be consumed as it is actually obtained from the fruit, and when properly processed, maintains the taste and odour of the fruit unchanged.

Firstly, the olives are washed to eliminate any remaining impurities (e.g. dust or soil). Then they are crushed whole, without prior stoning, in roller mills or by modern hammer crushers. For separating the solids and liquids, the olive paste is spread onto a pulp mat, which is then stacked onto other mats to form a cylindrical load held fast by a central guide. The pressure exerted on the stack causes the liquids to run while the solids (pomace) are retained on the pulp mats. The vegetable water and oil gradually seep out, running down into a set of decanters. The mixture of water and oil produced by this traditional pressing method can be separated by gravity in decanting vats. A more rapid separation can be achieved in centrifuges.

By using modern technology the process is simplified. The pomace, oil and vegetable water are separated by continuously centrifugating the paste. After being suitably thinned with lukewarm water, the paste is injected into centrifuges. Because of the different densities of the three substances, an immediate separation can be achieved.

• Refined olive oil

Native olive oils that have defects in the form of a low sensory rating (see appendix) and/or a high free acidity have to be made fit for consumption by refining. The process of refining includes neutralisation, decolouration and deodorising. Neutralisation is for eliminating the excess of free fatty acids in the oil. Alkaline bleaches (e.g. sodium hydroxide) are normally used for this process. Minor quality oils often have an intense or abnormal colour which has to be corrected by decolouration. This is a physical process carried out by "surface absorption" with natural colourings being absorbed onto substances such as bleaching clay and active carbon. The purpose of deodorising (treating the oil with steam in a vacuum at high temperature) is to eliminate defective odours and flavours in the oil.

Olive oils that have been refined are pale in colour and not very viscous. They have little or no taste or odour and a very low acidity.

• Trade quality

Olive oil is traded on the international market at a higher price than other vegetable oils. Consequently, adulteration of olive oil with cheaper oils is a temptation. To guarantee a fair trade and to protect consumers, the European Commission (EC) (1) has introduced definitions and fixed criteria for olive oil and olive-pomace oil. These criteria are set in place to distinguish the different types of olive oils and to protect their quality and purity. They include limits for the fatty acid composition, free fatty acids, aliphatic alcohols, the content and composition of sterols, erythrodiol and uvaol, the peroxide level and the presence of saturated fatty acids at the 2-position within the triglycerides and trilinolein. The criteria also define the sensory characteristics for virgin olive oils.

Since 1991 the regulation No. 2568/91 is obligatory for the member states of the European Union. According to these standards there are different quality grades for olive oil and olive-pomace oil. However, only four of them (marked in grey) are directly available for consumers.

Used generically, the term OLIVE OIL means the oil obtained solely from the fruit of the olive tree. It also excludes mixtures with oils of other kinds. Olive-pomace oil may not use this term either. Olive oil may be called by one of the following designations provided it complies with the relevant criteria fixed in the standard.

VIRGIN (OR NATIVE) OLIVE OILS are oils obtained from the fruit of the olive tree by mechanical or other physical means under conditions, particularly thermal conditions, that do not lead to the deterioration of the oil. Virgin oils have not undergone any treatment other than washing, crushing, pressing, centrifugation, and filtration. When virgin olive oil is intended for consumption in its natural state, it is called by one of the following names:

• Extra virgin olive oil - a virgin olive oil that has a sensory rating of 6.5 or more (see appendix) and a content of free fatty acids, expressed as oleic acid, of not more than 1 gram per 100 grams.

• Virgin olive oil - a virgin olive oil with content of free fatty acids of not more than 2 g per 100 g. (Sensory rating of at least 5.5.)

• Ordinary virgin olive oil is a virgin olive oil that has a content of free fatty acids of not more than 3.3 g per 100 grams. (Sensory rating not more than 3.5.)

REFINED OLIVE OIL is the oil obtained when virgin olive oil is refined (see pt. 1.2).

OLIVE OIL is the oil consisting of a blend of refined olive oil and virgin olive oil that is deemed independently fit for consumption.

OLIVE-POMACE OIL is the oil obtained by treating olive pomace with solvents. It can be classified as follows:

• Refined olive-pomace oil is obtained from crude olive-pomace oil by refining.

• Olive-pomace oil is a blend of refined olive-pomace oil and virgin olive oil that is deemed independently fit for consumption. In no case whatsoever may it be called "olive oil".

The different grades of olive oil and olive-pomace oil are identified by the purity and quality criteria laid down in the trade standards of the EC. For each grade, minimum or maximum limits for the content of the different parameters are prescribed.

• Composition of olive oil

Olive oil is a mixture of glycerides, which are esters of glycerol with fatty acids. In addition, olive oil contains small amounts of free fatty acids, glycerol, phosphatides, flavour compounds, sterols and other minor components.

The major fatty acid of olive oil is the monounsaturated oleic acid (C18:1). The mean fatty acid composition of olive oil is as follows:

Palmitic acid (C16:0) 7.5-20%

Palmitoleic acid (C16:1) 0.3-3.5%

Stearic acid (C18:0) 0.5-5.0%

Oleic acid (C18:1) 55.0-83.0%

Linoleic acid (C18:2) 3.5-21.0%

Others 1.5-3.2%

The fatty acid composition of olive oil is influenced by different factors, such as the variety of the olive tree, agricultural and climate factors.

The nonglyceride fraction of olive oil comprises several groups of compounds: nonglyceride fatty acid esters; hydrocarbons; sterols; triterpene alcohols; tocopherols; phenols; phospholipids; chlorophylls; and flavour compounds (see below).

Although the largest proportion of fatty acids is esterified with glycerol, small quantities of fatty acids form esters with a variety of other alcoholic compounds, including methanol, ethanol and triterpene alcohols. The main triterpene of olive oil is squalene, a biochemical precursor of sterols. Olive oil is richer in squalene than most vegetable oils. Furthermore, there are different polycyclic aromatic hydrocarbons present in olive oil, as well as small quantities of b -carotene. The main sterol present in olive oil is b -sitosterol which accounts for about 95% of the total sterols. Campesterol represents 3%, the remaining 2% are a mixture of other sterolic constituents.

Most of the tocopherol in olive oil is a -tocopherol which has the highest vitamin E activity. Its content is on average 15-25 mg/100g. In general, native olive oils have a higher vitamin E content than refined olive oils.

The olive mesocarp contains phenolic compounds which are mainly water soluble. Some quantities of phenolics, however, are carried into the olive oil. The main polyphenols of olive oil are tyrosol and hydroxytyrosol, derived from the hydrolysis of oleuropein, the bitter component of the olives. Benzoic acid and cinnamic acid, probably originating from the degradation of flavonoids, are also present in olive oil. The phenolics of olive oil decrease its oxidation rate, because they are potent antioxidants. Virgin olive oil has a characteristically pleasant flavour. The main groups of flavour substances are aliphatic and aromatic hydrocarbons, aliphatic and terpenic alcohols, aldehydes, ketones, ethers, esters, furan and thiophene derivatives. The flavour complex changes as the oil deteriorates with storage time.

Appendix for additional information

Analytical quality controls for olive oil

Acidity

Lipolytic processes in the olive start to break down the triglycerides in the maturation stage. This becomes more prevalent at harvesting. These lipolytic processes are intensified by hydrolysis and autoxidation, leading to the formation of free fatty acids which decrease the sensorial quality of the oil. The lower the content of free fatty acids the higher the quality of the oil.

The content of free fatty acids is mainly influenced by the time of harvesting, the duration between harvesting and processing, and storage conditions of the olives.

Peroxidation

Lipid peroxidation leading to oxidative rancidity is the main change causing deterioration of olive oil during storage. It is due to oxidation of unsaturated fatty acids initiated by free radicals and the subsequent formation of compounds possessing unpleasant taste and odour.

The formation of peroxides depends on several factors. All influences which promote the formation of free radicals such as light, high temperature and contact with metals increase lipid peroxidation. Thus, conditions and duration of storage of both the olives and the oil are of great importance.

Purity parameters

The sterol, erythrodiol, uvaol and alkanol contents are very important for the investigation of the quality and purity of olive oil. These minor components cannot be converted or broken down but can be separated from it by suitable techniques.

The composition of sterols is unique for each vegetable oil. For olive oil, b -sitosterol is the main sterol, and also campesterol is present in measurable amounts. If other sterols can be detected, it is an indicator for an adulteration with other oils or fat, for example, substitution of sunflower oil for part of the olive oil is reflected by the presence of stigmasterol, which is absent from olive oil.

The total sterol content is much higher in native olive oils than in refined oils. Thus, the amount of sterols can indicate if a native olive oil has been adulterated with a refined olive oil. A minimum content for native olive oils is prescribed according to the EC-regulation.

The content of erythrodiol and uvaol is much higher in olive-pomace oils than in olive oils, so their presence shows if part of the olive oil has been substituted with olive-pomace oil.

Another parameter of purity is the stereospecific distribution of major fatty acids between the 1,3 and 2-position of glycerol in the oil. All vegetable oils are characterised not only by a specific fatty acid composition, but also by a specific distribution of their fatty acids within the triglycerides: saturated fatty acids are concentrated at the 1,3-positions and almost absent at the 2-position which is generally occupied by unsaturated fatty acids. If the proportion of saturated fatty acids at the 2-position is increased in olive oil then it can be assumed that there is an adulteration with a synthetic ester oil.

Sensory quality criteria of olive oil

The sensory analysis of virgin olive oil is based on a panel test, developed by the International Olive Oil Council. In this test, 8-12 selected, trained tasters analyse the flavour (which includes taste and odour) of virgin olive oil as well as the intensity of the different flavour attributes. (Examples for positive attributes are: "apple", "fruity", "green leaves", "grass", "bitter", "harsh", "sweet"; and for negative attributes: "winey-vinegary", "metallic", "earthy", "muddy sediment", "fusty", "rancid".)

The final rating is awarded on the basis of a scale of points running from 0, which indicates that the oil has extreme defects, to 9, which indicates that it has no defects at all. For extra virgin olive oils, the rating must be at least 6.5.

There are more than 50 varieties of olive trees leading to the specific odours, tastes and colours of the olive oils. For example, the olive oil from Tuscany may be fruity, but a little bit pungent, while oils from Malaga are often characterised by a light taste and a golden colour.

Furthermore, the sensory properties of an oil are influenced by agricultural and climatic factors as well as by time and method of harvesting.

References

• Commission Regulation (EEC) No 2568/91 of 11 July 1991 on the characteristics of olive oil and olive-residue oil and on the relevant methods of analysis, Official Journal L 248, 05.09.1991, p. 1 – 82.

• International olive oil council (IOOC): Trade standard applying to olive oil and olive pomace oil. COI/T.15/NC No.2/Rev.9, 10 June 1999.

Additional literature

• Aparicio R, Morales MT, Alonso V: Authentication of European virgin olive oils by their chemical compounds, sensory attributes, and consumers’ attitudes. J Agric Food Chem 45: 1076-1083 (1997).

• Angerosa F, d’Alessandro N, Basti C, Vito R: Biogeneration of volatile compounds in virgin olive oil: Their evolution in relation to malaxation time. J Agric Food Chem 46: 2940-2944 (1998).

• Cinquanta L, Esti M, La Notte E: Evolution of phenolic compounds in virgin olive oil during storage. JAOCS 74: 1259-1264 (1997).

• García JM, Seller S, Pérez-Camino MC: Influence of fruit ripening on olive oil quality. J Agric Food Chem 44: 3516-3520 (1996).

• García JM, Gutiérrez F, Castellano JM, Perdiguero S, Morilla A, Albi MA: Influence of storage temperature on fruit ripening and olive oil quality. J Agric Food Chem 44: 264-267 (1996).

• Gutiérrez F, Jímenez B, Ruíz A, Albi MA: Effect of olive ripeness on the oxidative stability of virgin olive oil extracted from the varieties picual and hojiblanca and on the different components involved. J Agric Food Chem 47: 121-127 (1999).

• Morales MT, Aparicio R: Effect of extraction conditions on sensory quality of virgin olive oil. JAOCS 76: 295-300 (1999).

• Morales MT, Rios JJ, Aparicio R: Changes in the volatile composition of virgin olive oil during oxidation: flavors and off-flavors. J Agric Food Chem 45: 2666-2673 (1997).

• Ranalli A, Ferrante ML, De Mattia G, Costantini N: Analytical evaluation of virgin olive oil of first and second extraction. J Agric Food Chem 47: 417-424 (1999).

• Satue MT, Huang S-W, Frankel EN: Effect of natural antioxidants in virgin olive oil on oxidative stability of refined, bleached, and deodorized olive oil. JAOCS 72: 1131-1137 (1995).

• Tsimidou M: Polyphenols and quality of virgin olive oil in retrospect. Ital J Food Sci 10: 99 – 116 (1998)

Fact Sheet 6

Olive oil and the Gastrointestinal Tract

Author: Eurosciences Communication in co-operation with the Institute for Arteriosclerosis Research, University of Münster, Germany

Introduction

From an increasing number of publications, it is becoming more and more evident, that the quality of dietary fat influences physiology and pathophysiology of the gastrointestinal tract. The main focus of these studies has been the effect of different dietary fatty acids on gastric acid secretion and gallstone formation. Diseases of the stomach and in particular gallstones are very common in western industrialised countries, e.g. the prevalence of gallstone disease is up to 38% in Europe and North America (4).

Gastric acid secretion

In 1886, in what was probably the first study to investigate the influence of dietary fat on gastric function, Ewald and Boas observed that the addition of olive oil to a test meal suppressed gastric acid secretion (6). Since then, numerous studies have confirmed, that the presence of fat in different segments of the intestinal tract inhibits gastric acid secretion. In most of these studies olive oil was used as a source of dietary fat. The intraduodenal presence of olive oil reduces gastric acid secretion in dogs (11), rats (20) and humans (18, 23). However, until recently it was unknown, whether this effect was also verifiable for other dietary fats, or if it was a specific feature of olive oil or monounsaturated fatty acids. In 1997, Serrano et al. compared the effects of diets rich in monounsaturated fatty acids (olive oil) with diets rich in polyunsaturated fatty acids (sunflower oil) on gastric acid secretion. They were able to show, that "a 30-day period of diets containing olive oil [ ...] resulted in attenuated gastric acid secretion in response to a liquid meal when compared with those containing sunflower oil" (22). Rhee et al. (20) also investigated the mechanical aspects of this suppressive action of oleic acid. They were able to show in rats, that the inhibitory effect of oleic acid on gastric acid secretion is mediated by a peptidic hormone released into the blood when the duodenal mucosa comes into contact with this fatty acid.

These findings consistently indicate that the consumption of olive oil reduces gastric acid secretion. This effect might be beneficial with regard to diseases such as gastric or duodenal ulcers, where the attenuation of gastric acid secretion is a key therapeutic goal.

Gallstone formation

There are numerous investigations into the relationship between diet and the formation of gallstones. Unfortunately, in some of these studies (9, 10, 15, 17, 19, 21, 25), the dietary fatty acid composition has not been determined, so these studies do not allow an evaluation of the relationship between gallstones and dietary fatty acids. Furthermore, wide variations in study design, methods of diet assessment or diagnosis of gallstone disease make a comparison of the studies on this topic rather difficult.

One of the first investigations describing a relationship between dietary fat and gallstone disease was a case-control study by Linos and colleagues in 1989. In their study they found that "from all the dietary factors the only one that showed a positive statistically significant (p<0.05) association was consumption of animal fat [ ...] . Interestingly high consumption of olive oil had a negative (i.e. protective) association with the disease" (12). Recently Misciagna and colleagues observed in a population-based case-control study that among other factors, saturated fats were a risk factor for gallstone formation while dietary monounsaturated fatty acids were inversely associated with this disease (14). In a prospective study, Gilat et al. observed a higher intake of energy, carbohydrates, fibre and unsaturated fatty acids in Arabs who had a low incidence of gallstones compared to Jews with a higher incidence of the disease (7). However, they concluded that "it cannot be determined which, if any, of these dietary differences is related to the lower frequency of gallstones". Further evidence of an association between the quality of dietary fat and gallstone formation is provided by a finding from the Nurses Health Study, where the authors observed an inverse association between the intake of vegetable fat and the incidence of gallstones (13). On the other hand, these authors did not observe a significant association between the disease and polyunsaturated or monounsaturated fatty acid intakes. Bravo and colleagues showed, that both dietary monounsaturated and polyunsaturated fatty acids increase the biliary excretion of cholesterol in rats (3). This was accompanied by an increased cholesterol saturation of the bile in the animals fed the polyunsaturated fatty acids, but not in those fed the monounsaturated fatty acids. The authors concluded, that this "may have implications for the risk of the development of cholesterol gallstone disease" (3).

These findings are in agreement with two studies conducted in hamsters, where saturated fatty acids were found to intensify gallstone formation while monounsaturated and polyunsaturated fatty acids caused a reduction (5, 8). Although in two other studies an association between dietary fat and gallstones could not be found (16, 24), and one of these even observed a higher intake of monounsaturated fatty acids in patients with gallstones (16), the general consensus from the studies conducted so far is that a high intake of saturated fatty acids appears to be a risk factor for gallstone formation, while the intake of monounsaturated fatty such as olive oil, and possibly also polyunsaturated fatty acids, might protect against gallstone formation. On the other hand there are still some open questions, e.g. why some investigators did not observe a protective effect of monounsaturated fatty acids while others did, and what the implications of dietary fatty acids in the aetiology of gallstone formation are. Thus, further investigation is needed to clarify these points.

Summary

From the existing studies on the relationship between dietary fat intake and physiology and pathophysiology of the gastrointestinal tract, there is evidence, that a high intake of monounsaturated fatty acids exerts beneficial effects on the gastrointestinal tract by reducing gastric acid secretion and preventing gallstone disease. The impact of the dietary fat composition on other gastrointestinal diseases, e.g. reflux oesophagitis or constipation, has not been evaluated thoroughly. There are, however, some studies, which suggest further favourable effects of diets rich in monounsaturated fatty acids. Barltrop and Oppe observed in infants, that olive oil is more quantitatively absorbed than butterfat (2). Ballesta and colleagues were able to show, that in dogs olive oil improves the digestibility and metabolic utilisation of dietary protein (1). Furthermore, initial studies on the effect of olive oil or oleic acid with regard to gastrointestinal motility and gastric emptying, show that oleic acid-rich meals delay gastric emptying compared with saturated fatty acid-rich meals, therefore supporting the reservoir function of the stomach (27). Spiller et al. (26) described an accelerated colonic transit when oleic acid was added to test meals. However, they did not compare oleic acid with other fatty acids, so it remains to be proven if this effect is true for fat in general or if it is a specific feature of monounsaturated fatty acids.

In conclusion, although many questions are still open, there is a body of evidence that the consumption of olive oil has beneficial effects on different metabolic functions of the gastrointestinal tract.

References

1. Ballesta MC, Martinez-Victoria E, Manas M, Mataix FJ, Seiquer I, Huertas JH: Protein digestibility in dog. Effect of the quantity and quality of dietary fat (virgin olive oil and sunflower oil). Nahrung 35: 161-167 (1991).

2. Barltrop D, Oppe TE: Absorption of fat and calcium by low birthweight infants from milks containing butterfat and olive oil. Arch Dis Child 48: 496-501 (1973).

3. Bravo E, Flora L, Cantafora A, De Luca V, Tripodi M, Avella M, Botham KM: The influence of dietary saturated and unsaturated fat on hepatic cholesterol metabolism and the biliary excretion of chylomicron cholesterol in the rat. Biochim Biophys Acta 1390: 134-148 (1998).

4. Brett M, Barker DJP: The World Distribution of Gallstones. Int J Epidemiol 5: 335-341 (1976).

5. Cohen BI, Mosbach EH, Ayyad N, Miki S, McSherry CK: Dietary fat and fatty acids modulate cholesterol cholelithiasis in the hamster. Lipids 27: 526-532 (1992).

6. Edwald CA, Boas J: Beiträge zur Physiologie und Pathologie der Verdauung. Virchows Arch Path Anat Physiol Klin Med 104: 271-305 (1886).

7. Gilat T, Horwitz C, Halpern Z, Bar Itzhak A, Feldman C: Gallstones and diet in Tel Aviv and Gaza. Am J Clin Nutr 41: 336-342 (1985).

8. Jonnalagadda SS, Trautwein EA, Hayes KC: Dietary fats rich in saturated fatty acids (12:0, 14:0, and 16:0) enhance gallstone formation relative to monounsaturated fat (18:1) in cholesterol-fed hamsters. Lipids 30: 415-424 (1995).

9. Jorgensen T, Jorgensen M: Gallstones and diet in a Danish population. Scand J Gastroenterol 24: 821-826 (1989).

10. Kato I, Nomura A, Stemmermann GN, Chyou PH: Prospective study of clinical gallbladder disease and its association with obesity, physical activity, and other factors. Dig Dis Sci 37: 784-790 (1992).

11. Kihl BO, Boden G, Landor JH: Bile-fat relationships in gastric secretory inhibition and bile flow stimulation. Surgery 81: 386-391 (1976).

12. Linos AD, Daras V, Linos DA, Kekis V, Tsoukas MM, Golematis V: Dietary and other risk factors in the aetiology of cholelithiasis: a case control study. HPB Surgery 1: 221-227 (1989).

13. Maclure KM, Hayes KC, Colditz GA, Stampfer MJ, Willet WC: Dietary predictors of symptom-associated gallstones in middle-aged women. Am J Clin Nutr 52: 16-22 (1990).

14. Misciagna G, Centonze S, Leoci C, Cisternino AM, Ceo R, Trevisan M: Diet, physical activity, and gallstones - a population-based, case-control study in southern italy. Am J Clin Nutr 69: 120-126 (1999).

15. Moermann CJ, Smeets FWM, Kromhout D: Dietary risk factors for clinically diagnosed gallstones in middle-aged men. Ann Epidemiol 4: 248-254 (1994).

16. Ortega RM, Fernandez-Azuela M, Encinas-Sotillos A, Andres P, Lopez-Sobaler AM: Differences in diet and food habits between patients with gallstones and controls. J Am Coll Nutr 16: 88-95 (1997).

References

17. Pastides H, Tzonou A, Trichopoulos D, Katsouyanni K, Trichopoulou A, Kefalogiannis N, Manousos O: A case-control study of the relationship between smoking, diet, and gallbladder disease. Arch Intern Med 150: 1409-1412 (1990).

18. Petersen F, Olsen O, Jepsen LV, Christiansen J: Fat and gastric acid secretion. Digestion 52: 43-46 (1992).

19. Pixley F, Mann J: Dietary factors in the aetiology of gall stones: a case-control study. Gut 29: 1511-1515 (1988).

20. Rhee JC, Chang TM, Lee KY, Jo YH, Chey WY: Mechanism of oleic acid-induced inhibition of gastric acid secretion in rats. Am J Physiol 260: G564-G570 (1991).

21. Sarles H, Gerolami A, Cros RC: Diet and cholesterol gallstones. Digestion 17: 121-127 (1978).

22. Serrano P, Yago MD, Manas M, Calpena R, Mataix J, Martinez-Victoria E: Influence of type of dietary fat (olive and sunflower oil) upon gastric acid secretion and release of gastrin, somatostatin, and peptide yy in man. Dig Dis Sci 42: 626-633 (1997).

23. Shiraori K, Watanabe SI, Takeuchi T: Intestinal fat digestion plays a significant role in fat-induced suppression of gastric acid secretion and gastrin release in the rat. Dig Dis Sci 38: 2267-2272 (1993).

24. Sichieri R, Everhart JE, Roth H: A prospective study of hospitalisation with gallstone disease among women: role of dietary factors, fasting period, and dieting. Am J Public Health 81: 880-884 (1991).

25. Smith DA, Gee MI: A dietary survey to determine the relationship between diet and cholelithiasis. Am J Clin Nutr 32: 1519-1526 (1979).

26. Spiller RC, Brown ML, Phillips SF: Decreased fluid tolerance, accelerated transit, and abnormal motility of the human colon induced by oleic acid. Gastroenterology 91: 100-107 (1986).

27. Thomsen C, Rasmussen O, Lousen T, Holst JH, Fenselau S, Schrezenmeir J, Hermansen K: Differential effects of saturated and monounsaturated fatty acids on postprandial lipemia and incretin responses in healthy subjects. Am J Clin Nutr 69: 1135-1143 (1999).