Harvard Health Publications

Study supports heart benefits from Mediterranean-style diets

Posted By Anthony Komaroff, M.D. On February 25, 2013

I have been a fan of Mediterranean-style diets, both for my patients and myself, for many years. The results of a Spanish trial being made public today should get everyone interested in this healthy way of eating.

What makes these new results so important is that they come from a randomized controlled trial, what many consider to be the gold standard of medical research. Beginning in 2003, researchers recruited nearly 7,500 Spaniards to take part in the Prevención con Dieta Mediterránea (PREDIMED) trial. All participants were over age 55, and none had been diagnosed with heart disease—though all were at high risk for developing it. One third of the volunteers were counseled to adopt a Mediterranean-style diet and were given one liter a week of extra virgin olive oil. Another third were counseled to adopt a Mediterranean-style diet and were given about an ounce a day of mixed nuts (walnuts, hazelnuts, and almonds). The third group was counseled to adopt a low-fat diet.

The trial was stopped early, after just under five years of follow-up, when the independent board charged with an ongoing review of the results determined that there had been significantly fewer heart attacks, strokes, and deaths from cardiovascular disease in the Mediterranean groups than in the low-fat diet group. The reduction was impressive: three fewer cardiovascular events per 1,000 people per year in the Mediterranean groups. The results are being presented today at the 6th International Congress on Vegetarian Nutrition being held in Loma Linda, California and were published this morning in the New England Journal of Medicine.

Observing the Mediterranean diet

Before this new study, most previous research on the Mediterranean diet involved what are called ecological studies, or observational studies. In the 1950s and 1960s, nutrition research pioneer Ancel Keys and his colleagues conducted ecological studies. They studied eating patterns in 16 different populations in seven countries. The researchers observed that people living in Crete, other parts of Greece, and southern Italy tended to live longer than others in the study and had lower rates of heart disease and some cancers.

Keys was convinced that the regional diet, dubbed the Mediterranean diet, was an important reason for the good health in those populations. Keys and his wife, Margaret, promoted the Mediterranean diet in a series of best-selling books. His efforts on diet and health were highlighted in a cover story in Time magazine.

Although the results of the Seven Countries Study supported the notion that a Mediterranean-type diet could contribute to good health, they couldn’t prove it. Other things that Keys and colleagues didn’t measure, like low rates of obesity or high rates of physical activity or a disease-preventing genetic variant, could have been invisible contributors.

Over the past four decades, observational studies have come to conclusions similar to those of Keys and his coworkers. In observational studies, very large numbers of people are recruited. Careful measurements are made of their diet, exercise pattern, smoking status, weight and very many other variables. The participants report any new health problems that have developed. All of these measurements of lifestyle and health are made repeatedly over many years.

An outstanding example of such observational studies is work done by Dr. Walter Willett and colleagues, here at Harvard School of Public Health and Harvard Medical School, involving hundreds of thousands of people studied over many decades. Their studies, too, have demonstrated that the main elements of a Mediterranean-style eating pattern are linked to lower risk of heart disease and premature death among American men and women. Although their studies collected much more information, over a much longer period of time, than the ecological studies conducted by Keys, Willett’s studies, too, could not rule out that some factors other than the lifestyles of the study participants accounted for the improved health.

The new study published today is very important because the randomization process used in the trial helped create three groups that were generally equal in everything except diet. Designing the study that way meant that any health improvements seen were likely due to the diet.

A flexible blueprint for healthy eating

Rather than specifying a strict diet, this research shows that a flexible healthy eating pattern—no matter what it is called—can promote good health. Here are the guidelines that the volunteers in PREDIMED’s Mediterranean groups were given:

  • olive oil: use abundantly for cooking and dressing dishes
  • vegetables: at least two to three servings
  • fruits: at least two to three daily servings
  • beans: three or more weekly servings
  • fish or seafood: three or more weekly servings
  • nuts or seeds: at least one serving a week
  • choose poultry instead of red meats or processed meats
  • cook at least twice a week with tomato, garlic, and onion
  • moderate alcohol intake (for those who drink alcohol)
  • eat as desired: nuts, eggs, fish or seafood, low-fat cheese, chocolate (only black chocolate, with more than 50% cocoa), and whole-grain cereals.
  • limit or eliminate: cream, butter, margarine, red meat, sugared beverages, pastries, bakery products (such as cakes, donuts, or cookies), premade sweets and desserts, French fries or potato chips, cured or fatty cheeses.

This is a diet of abundance, not restriction. Add in not smoking and daily physical activity and you have a blueprint for protecting, and perhaps even improving, health.

Related Information: Eat, Drink, and Be Healthy

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