Diet Digest
Crete, the legendary birthplace of Zeus, has been a part of human history for 8,000 years. Paleolithic man arrived there around 6000 BC, and over millennia, people from a wide variety of cultures — Minoans, Romans, Arabs, Turks, and others — came to conquer and control the often prosperous and fertile island. Roughly 40 years ago, Crete became the birthplace of something new: the Mediterranean diet, a heart-healthy eating pattern that has become, for many, the de facto diet of anyone living in countries bordering the northern Mediterranean Sea.
Unfortunately, many details of the original research about the Cretan diet have been lost in translation, and nutrition recommendations developed from those studies are often condensed into three generalities: consume more olive oil, fish, and wine. A look back at the original research about the Cretan diet reveals an indigenous pattern of eating far more nutritionally complex and, in reality, not easily replicated outside its place of origin.
More than 20 countries border or lie inside the Mediterranean Sea. The diets of people living in Spain, Italy, Albania, Morocco, Egypt, and Serbia are as varied and diverse as their cultures and weather. Thus, the term Mediterranean diet is rather misleading. The diet recommendations with this regional characterization are actually based, in large part, on an epidemiological study of men living in rural Crete in the 1950s. This, the Seven Countries Study, investigated the incidence of cardiovascular disease (CVD) in 18 regions in seven countries: Italy, Yugoslavia, the United States, the Netherlands, Japan, Finland, and Greece. This study was unique and progressive for its time, comparing the lifestyle, diet, and disease risk among contrasting populations with the hypothesis that CVD risk factors are variable and modifiable. The study included only men because heart disease was then rare among women, and researchers also considered many of their field exams inappropriately invasive.
Ancel Keys and the Seven Countries StudyAncel Keys, PhD, a professor of physiology at the University of Minnesota from 1936 to 1975, was the chief investigator in the Seven Countries Study. Keys was interested in the relationship among nutrition, health, and disease. In addition to being well-known (and maligned) for his formulation of “K rations,” he was also one of the first research scientists to conduct a large, prospective epidemiological study of CVD incidence and risk. After observing significant differences in the incidence of CVD among groups of well-fed Minnesota businessmen (high) and malnourished post-World War II Europeans (low), he postulated a correlation between serum levels of cholesterol and disease risk. Wanting to further test his hypothesis, he and like-minded colleagues devised and conducted the Seven Countries Study, the first to systematically examine links between lifestyle, diet, and rates of heart attack and stroke in contrasting populations.
The Seven Countries Study surveyed rural men aged 40 to 59 from 1958 to 1970.1 At the time and in many of the regions investigators chose to collect data, the traditions of subsistence living were continuing much as they had for centuries. Because of this, the scientists had the opportunity to accurately assess traditional diets in context with traditional lifestyles, gaining a more accurate picture of the factors protecting populations from or subjecting them to CVD risks. Blood tests, evaluations of exercise tolerance, electrocardiograms, diet histories, and chemical analyses of local foods were some tools investigators used to assess cardiovascular health and eating patterns. The researchers intended to be thorough and accurate in their data collection and interpretation of results. Thus, they created unique and standardized measures of diet, risk factors, and disease and also blindfold coded the data.
The Seven Countries Study is still regarded as a landmark project that directed attention to the importance of comparative population studies in epidemiology.2 It was also the first to demonstrate the degree to which dietary intake of saturated fatty acids and mean serum cholesterol levels predict present and future rates of coronary heart disease.
It is interesting to note the reasoning researchers used to determine which regions of the world to study.2 Yugoslavia was chosen because of great regional variations in diet—the fats in foods eaten by eastern populations were largely of animal origin, while the fats in the diets of people living in the west were primarily vegetable oils. Italy presented an opportunity to study a country that boasts one of the great traditional world cuisines. It is also a place that has extreme regional differences in diets, with heavy intakes of meat to the north and a larger reliance on olive oil, fish, and legumes to the south.
Greece offered an opportunity to study populations that eat a high-fat, though low-saturated fat, diet. Investigators here were able to study the effects of a diet consisting largely of monounsaturated fats from olive oil and polyunsaturated fats from fish, grains, vegetables, and legumes. Finland, in contrast, offered the opportunity to study the health effects of a high total and saturated fat diet. Here, rates of coronary artery disease surpassed all other countries, and typical rural lunches consisted of fatty meats wrapped in equally fat-laden dark hunks of bread.
The Dutch diets were a bit more moderate in fat and higher in fruits and vegetables than those of the Finns and provided a moderate comparison.
The U.S. study focused on railroad workers because of their high total and saturated fat diet combined with varying degrees of work-related physical activity. Japan presented an opportunity to document a traditional low-fat diet that previously had not been largely studied. Its diet was also higher in sodium than the other patterns of eating studied and offered a unique opportunity for comparison.
What Researchers Discovered
Data from the Seven Countries Study revealed significant variations in the total dietary fat intake of the participating populations—diets ranged from 9% to 40% total fat—as well as fourfold to fivefold differences in the incidence of CVD. Strong positive correlations existed across study populations between serum cholesterol levels and blood pressure and heart disease risk. The incidence of heart disease was not, however, strongly related across cultures to smoking habits or levels of physical activity. Smoking, in fact, was a minor risk factor in Greece, Italy, and Japan—the three countries with the lowest rates of both all-cause mortality and heart disease. In contrast, in those countries with higher rates of heart disease, smoking was strongly associated with increased risk for both cardiovascular and noncardiovascular deaths. Also, levels of usual physical activity and measures of resting heart rate were more predictive of death in European populations than in the U.S. or Asian cohorts.
Although serum cholesterol was the most reliable predictor of CVD across cultures, there were some surprises. In Finland, for example, heart attack rates (the highest in all the countries studied) were greater than predicted by mean cholesterol values alone. And in Crete, the rates of heart attack were actually less than predicted, given the average serum cholesterol levels measured. These unexpected results caused researchers to conclude that elevated serum cholesterol levels, although a strong indicator, do not always predict CVD risk. Subsequent investigations into the diets of people living in Finland, Japan, and Greece have led to some of the most interesting hypotheses regarding the epidemiology of heart disease yet proposed. These hypotheses, in turn, formed the foundation of the dietary recommendations that were to be popularized as the Mediterranean diet.
The 1950s Cretan Diet
Residents of rural Crete in the 1950s ate significant amounts of olive oil, olives, fish, fruits, vegetables (especially wild greens), and nuts. They consumed moderate amounts of wine and cheese and small quantities of meat, milk, and eggs. Foods comprising the core of their diets provided ample amounts of many beneficial nutrients, including fiber, antioxidants, vitamins E and C, selenium, phytochemicals, and omega-3 fatty acids.
The Cretan residents had the lowest rates of CVD of all populations observed in the Seven Countries Study, followed closely by rural Japanese. This statistic is interesting because Cretans had one of the highest-fat diets (37% of calories from fats), while the Japanese had the lowest (9% of calories from fats). Most fats in the Cretan diet came from olive oil, but Cretans also consumed large amounts of fish (more than the Japanese) containing omega-3 fatty acids.
The men also ate substantial amounts of wild plants, including purslane, a succulent green that is a good source of alpha-linolenic acid (ALA). Other sources of ALA in the Cretan diet included walnuts, a variety of green vegetables, legumes, and figs. Sources of protein other than fish included free-range meats and chickens—animals that were never fed grain but lived on purslane, grasses, insects, worms, and figs.
Because animals in Crete consumed foods high in ALA, their meat products, milk, and eggs became good sources of omega-3 fatty acids.
The Seven Countries Study researchers attributed the low incidence of heart disease in Crete to a diet based on large amounts of monounsaturated (olive) oil and low quantities of saturated fat. In contrast, the high rates of heart disease in Finland were attributed to very high intakes of both total and saturated fat. In fact, scientists concluded after a 20-year follow-up that 81% of the difference among populations in coronary deaths could be explained by average saturated fatty acid intake alone.
Researchers did not, however, explain why a high total-fat diet in Crete was more protective against CVD than a low total-fat diet in Japan. In the 1980s, a new group of scientists began to address this concern. They determined that the rural Cretan diet contained omega-3 fatty acids and/or ALA—both protective against heart disease—in every meal. Figs and walnuts, both sources of ALA, were common snacks. Meals generally featured green vegetables or animal products with ALA and/or omega-3 fatty acids. Even noodles made with local eggs contained these healthy fats. The Japanese cohort, in contrast, also consumed ALA and omega-3 fatty acids daily (from canola oil, soybeans, soybean oil, and fish) but not in as large amounts.
Scientists in the 1980s also analyzed blood samples from the original study. They found that the serum cholesterol esters from blood samples of people living in Crete and Japan contained the highest concentrations of ALA. This discovery led to the hypothesis that it was not the olive oil, fish, and wine that primarily lent protection against heart disease to the people of Crete; rather, it was their consumption of high levels of ALA and, likely, omega-3 fatty acids.
Subsequent Research
The Lyon Diet Heart Study put this hypothesis to the test.8 Researchers designed a single-blind trial to test whether a Mediterranean or prudent Western-type diet better protected individuals against the recurrence of heart attack. The Mediterranean group consumed less saturated fat, cholesterol, and omega-6 (linoleic) fatty acids and considerably more ALA, marine, and monounsaturated oils than controls. Scientists studied patients for five years and found striking results after only 27 months: People following the Mediterranean diet had significantly fewer incidences (1.32 per 100 patients per year vs. 5.55) of both fatal and nonfatal infarctions than the group following the prudent Western diet plan. Plasma fatty acids were measured to confirm the diets were being followed, and the plasma ALA levels measured were positively associated with improved prognosis at the end of the study.
The Lyon study found that a Mediterranean-style diet reduced secondary coronary events and deaths by almost 70% compared with controls. Interestingly, reduced risk of heart disease was evident without any concomitant decrease in levels of serum cholesterol. Researchers attributed the diet’s protective effects primarily to the amounts of ALA consumed, and other studies have since confirmed the health-promoting qualities of ALA.
A prospective study on the cardioprotective effects of ALA using the Nurse’s Health Study cohort was conducted roughly 10 years after the Lyon study was completed. Scientists here examined the association between dietary intakes of ALA and the risk of fatal ischemic heart disease (IHD). ALA consumption was negatively associated with IHD risk. The protective effects of ALA were reduced, however, in diets low in vitamin E (antioxidant) and high in trans fatty acid content. Researchers also concluded that the absolute amounts of ALA consumed were more important than the diet’s ratio of ALA to linoleic acid, and other research has agreed that absolute ALA is of primary importance for preventing heart disease. The optimal balance between these two fatty acids is still under debate.
A recent meta-analysis of studies evaluating the cardioprotective effects of ALA and omega-3 fatty acids finds fault with the Lyon, Seven Countries, and other studies, citing too many dietary variables as potential confounding factors.11 Scientists conducting this review concluded that well-designed dietary studies favored marine oils containing omega-3 fatty acids over ALA in the prevention of heart disease. Thus, the definitive answer regarding the health benefits of specific fatty acids is yet to be determined.
Why Were Cretans So Healthy?
The best way to determine why the people of Crete enjoyed good health is not by considering the benefits of one type of fat over another but by assessing their diet in a more holistic fashion. Nutrition scientists prefer to study one dietary variable at a time to determine potential benefit or harm. Humans, however, do not eat individual nutrients; they eat foods, and these foods contain hundreds of nutrients that synergistically affect health. If the Cretan diet is analyzed integrally and in context, then the protective effects of all aspects of the diet must be considered. These include, but are not limited to, the abundance of antioxidants and ALA from wild plants; the high selenium content of the soil; the low saturated fat but high omega-3 fatty acid content of meats and other products from pasture-fed animals; low intakes of trans fatty acids; and the substantial quantities of fish consumed daily.
Cretans certainly consumed a lot of olive oil and fish and drank healthy portions of red wine, but they were also part of a culture and landscape that supported the production and enjoyment of beneficial foods. Their diet cannot be easily reproduced. Procuring 100% pasture-fed beef or eggs from chickens that are truly freeliving is challenging; current laws do not ensure truthful labeling of meats, fish, chicken, and eggs; the availability of free-range and grass-fed products is limited; and costs are often prohibitive. Not many people forage for wild greens, and most will search in vain for purslane at their local grocery store. Yet, eating the Cretan diet is not impossible. Purslane, herbs, and wild greens can be grown in a home garden. Farmers’ markets often offer eggs from freeliving hens and cheeses from the milk of grass-fed cows. Walnuts and dried figs are easily found in most stores as are other sources of healthy fatty acids such as flaxseeds, salmon, or sardines.
With a little effort, many people can follow the basic features of a Cretan diet—plant some purslane, be picky about the hamburger and eggs you buy, and, yes, consume more olive oil, fish, and wine.
— Rita E. Carey, MS, RD, CDE, is a clinical dietitian and diabetes educator at Yavapai Regional Medical Center and the Pendleton Wellness Center in Prescott, Ariz.
[Via today's dietitian]
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