Q J Med 2000; 93: 783-785
© 2000 Association of Physicians
Editorial |
The Mediterranean diet revisited—towards resolving the (French) paradox
Department of Epidemiology and Public Health, The Queen's University of Belfast
The Mediterranean diet traditionally refers to diets typical of the olive-growing regions of the Mediterranean, but has a number of important constituents in addition to olive oil. These include: a large consumption of salads and legumes, and also wheat, olives, grapes, other fruits and their derivative products, including alcohol.1 As noted by Trichopoulou and Lagiou,1 total fat consumption may be high, at around 40% of total energy, as in Greece, or moderate, at around 30% of total energy, as in Italy. In Italy, the diet is characterized by pasta consumption, whereas in Spain fish consumption is particularly high. Ferro-Luzzi and Sette2 and de Lorgeril3 also comment on the difficulty of adequately defining a Mediterranean diet. According to Trichopoulou,1 moderate alcohol consumption is an essential component, but moderate is not defined quantitatively. Ancel Keys, pioneer of the Seven Countries Study, had a hand in promoting the idea of a Mediterranean diet in his cookery book, written with his wife, ‘How to eat and stay well, the Mediterranean way’.
The French paradox is a reference to the observation that a high consumption of animal fats is paradoxically coexistent with one of the lowest incidence of ischaemic heart disease in Europe.4 But others5 have noted that France is not alone and that other southern European countries show this effect. Data from Ancel Keys' epic Seven Countries Study, the planning of which started in 1947, showed that, although edible fat intake in grams per day in Italy and parts of Yugoslavia were quite low, in Crete, where heart disease mortality was very low, intake was second only to Finland's, but was characterized by a high olive oil and a low saturated fat intake.6
The Greeks have the highest olive oil consumption: a staggering 20 kg/person/year,7 whereas in France, even in its southern parts, consumption is only 12% of that in Greece. Olive oil has been said to have a taste as ancient as water: at the north entrance to Knossos on Crete there is a relief of a bull charging an olive tree which dates to the middle Minoan period and so is almost 4000 years old. The reason why the olive tree was associated with peace in Greek culture is that because it takes olive trees 30 years to mature and bear olives, and they were only planted when society was stable. The major fatty acid in olives is oleic acid, a monounsaturated fatty acid comprising 56–83% of olive oil.
The polyunsaturated fatty acids in olive oil are represented by the omega-6 fatty acid, linoleic (3.5–20%) and the omega-3 fatty acid, alpha-linolenic (0–1.5%).8 Perhaps it is not coincidental that these two fatty acids are regarded by most nutritionists as essential to the human diet, although linolenic acid need only be present in very small amounts.9 Two recent meta-analyses have confirmed10,11 that diets which are relatively high in monounsaturated or polyunsaturated fat (in place of saturated fat) both lower total and LDL cholesterol. But substitution with monounsaturates tends to preserve HDL cholesterol levels better, and, as significant reductions in both total and LDL cholesterol occur, the LDL/HDL cholesterol ratio is also significantly lowered. Because of some concern over an excessive intake of polyunsaturated fatty acid, a substitution of saturated by monounsaturated is now the goal, and this may also reduce the susceptibility of LDL cholesterol to oxidative stress.
Bronte-Stewart12 in 1958, in discussing fats and oils, suggested that, in addition to olive oil, the accompanying tocopherols and other antioxidants should also be considered. Certainly the antioxidant hypothesis has gained popularity lately.13 The vitamin E content of olive oil is variable, but is known to be high in extra virgin oil produced in southern France. Olive oil also contains polyphenols, e.g. oleuropein, which again may vary with the quality of the oil.14 Claims of antioxidant activity have also been made for purslane,15 a Greek herb rich in alpha-linolenic acid. Also involved are vitamin C and beta-carotene, and levels in the diet will be affected by consumption of fruits, salads and vegetables. Folate deficiency associated with high blood levels of homocysteine may also be related to per capita consumption of fruits and vegetables.16,17
The long-chain polyunsaturated omega-3 fatty acids, derived particularly from oily fish sources, i.e. eicosapentaenoic acid and docosahexaenoic acid, constitute another component of the Mediterranean diet. Although the omega-3 fatty acids primarily lower serum triglyceride and have only minor effects on total LDL and HDL cholesterol, they may confer an advantage in terms of various haemostatic factors, and may protect the heart from arrhythmias.18 Nuts, garlic and snails also contribute to the Mediterranean diet. In common with the hunter gatherer diet of our forefathers, the Mediterranean diet is highly varied.
Recent publications19–21 from the World Health Organization's MONICA Project help resolve some of the issues. Meticulously-validated coronary event rates from many European centres and countries reveal, for example, that coronary event rates in France are similar to those in southern European countries, but also that there exists a considerable gradient in event rates between centres in the north and south of France (Lille and Toulouse). MONICA results21 indicate that trends in classical risk factors—serum cholesterol, hypertension and smoking habit—explain a modest part of the trends in event rates (usually steadily down in the richer northern, western and southern European countries together with Australasia and North America) but that a large component remains unexplained and could be explained by as yet undefined risk factors in diet or lifestyle.
One factor which has proven especially seductive is alcohol, in particular in the form of red wine, which may be protective against heart disease in countries which consume adequate quantities. Indeed the French have a proverb that runs ‘Il y a plus ivrognes anciens que médecins anciens’. Not only may the type of alcohol consumption be important but also the pattern of drinking which differs between northern and southern Europe.22 Alcohol consumption in moderation has been shown in several prospective studies23 to be protective for coronary heart disease, having beneficial effects on HDL cholesterol and possibly platelets.24 In their review of ischaemic heart disease deaths in 18 developed countries in 1979,25 St Leger and his colleagues looked for correlations between many factors. The principal finding was a strong and specific negative association between IHD deaths and alcohol consumption. In particular, they concluded that ‘If wine is ever found to contain a constituent protective against IHD then we consider it almost a sacrilege that this constituent should be isolated. The medicine is already in a highly palatable form (as every connoisseur will confirm).’ Nevertheless several attempts have been made to do just this. Anthocyanin, which gives red wine and black olives their colour,8 may have antioxidant effects, as may a host of polyphenols, including bioflavonoids.26,27 Despite this Cricqui and Ringel28 and others29 have soberingly poured cold water on any promotion of alcohol for cardioprotective purposes. The dangers of excessive alcohol consumption are well recognized and in the 30 years between 1961 and 1991 the annual consumption of wines in France and Italy has almost halved to 67 and 57 l respectively per capita (compared to a stable 12 l per capita in the UK).22 Within France, alcohol consumption is largely in the form of wine, particularly in the south where the incidence of IHD is at its lowest, and less wine and more beer and spirits are consumed in the north where IHD is at its highest.30 Interest has focused on quercetin, a bioflavin present in red wine but also available from other sources including tea, onions and apples. In the Zutphen Elderly Study,31 61% of flavonoids came from tea, 13% from onions and 10% from apples. It is possible that alcohol promotes the bioavailability of these substances. A galaxy of claims have been made for resveratrol, another component of red wine whose only other source is peanuts.32 It is reputed to have greater antioxidant activity than vitamin E. Lately, researchers at the University of Omaha claim to have identified a ‘super antioxidant’ in red grape pips: they have called it ‘activin’.32 Alcohol provokes strong emotions and Thomas Jefferson, wrote ‘I think it is a great error to consider a heavy tax on wines as a tax on luxury; on the contrary, it is a tax on the health of all our citizens’.33 Medical opinion, if not practice, would tend to be more temperate.34
One of the weaknesses of the bulk of the data quoted in this short review is that they deal with cross-cultural comparisons and only the variables which have been measured can be compared, i.e. a host of other factors could be operating, e.g. nutrient, lifestyle, genetic, etc. Unfortunately, the studies which might build up hard evidence for or against the Mediterranean diet as protective for cardiovascular disease are difficult to accomplish. However, good evidence comes from small, well-planned, intervention studies using discrete nutrients. These will pave the way for larger, more clinically relevant studies which have clinical disease or death as end points.
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