The Mediterranean diet study draws a flawed analysis

Dear Doctor: For years now, I’ve been trying to eat more fruits and vegetables, less meat and a whole lot of olive oil and mixed nuts. Now I’m seeing that one of the studies supporting this has been retracted. Can I go back to hamburgers and fries?

Dear Reader: The Mediterranean diet has been the focus of all sorts of attention in the decades (yes, decades) since it was first introduced. Based on the eating habits of people living in southern Italy, Spain and Greece in the 1940s and ‘50s, it’s just as you outlined — a fruit- and vegetable-forward diet with lean proteins, whole grains, and plenty of nuts, legumes and olive oil. First publicized in the mid-1970s, the diet really entered the public consciousness in the 1990s. That’s when a spate of studies looking into the eating plan got a lot of ink, and diet books devoted to the concept began to hit best-seller lists.

In 2013, a large study in Spain randomly assigned 7,447 individuals to either the Mediterranean diet or to a low-fat diet. All were overweight, and all had risk factors for heart disease, such as smoking and diabetes. Although the study participants assigned to the Mediterranean diet faithfully followed the plan, those assigned to the low-fat diet tended to return to their normal eating habits. As a result, the study turned out to be as much a referendum on the modern diet as it was about the Mediterranean plan. After five years, researchers concluded that the Mediterranean diet lowered heart disease by 30 percent.

But did it? According to a recent analysis of the study by a scientist in England, it’s impossible to know. That’s because of two extremely important words in the description of the 2013 study — randomly assigned. For the results of these types of studies to be free from bias, participants must be randomly assigned to the treatment option they will follow. Without absolute randomization, you can’t be confident the resulting data is the effect of the treatment. And as you’ve probably already guessed, the English scientist discovered that the 2013 study did not, in fact, use absolute randomization. In some cases, entire villages were assigned to the Mediterranean diet. Although those participants all followed the same food rules, they also shared other important environmental factors that could affect their health outcomes. This was sufficient to derail the claim of randomization for the 2013 study, and to merit both a retraction and a rewrite by the New England Journal of Medicine in June.

The original takeaway of the 2013 study was that several thousand people had fewer heart attacks and strokes because they followed the Mediterranean diet. The new conclusion is several thousand people had fewer heart attacks and strokes and they followed the Mediterranean diet. In this study, the diet can no longer be cited as the specific reason for the health benefits.

Regarding your question about burgers and fries, the answer is moderation. As an occasional treat? Probably fine. But as a dietary staple? Please don’t.