Mediterranean Diet Study Walks Back Strongest Claim. Here’s What Researchers Got Wrong
Don’t put down the Greek olives and fresh fish just yet, but the widely recommended Mediterranean Diet is short a few nuts.
The New England Journal of Medicine has retracted the initial version of the landmark study published in 2013, and published a revised version that no longer makes the broad claim the diet could help everyone who is at a high risk of cardiovascular disease.
The new conclusion: People in the study who ate a typical Southern European cornucopia of ingredients with an emphasis on extra nuts or extra-virgin olive oil had fewer strokes and heart attacks than those who weren’t put on such a diet. However, the study no longer says this applies generally to reducing such risk for this class of people—it’s applicable to participants but not more broadly without additional research.
It’s unclear whether the study would have been published with this conclusion in 2013. Its publication raised interest once again in this style of eating, which is modeled on Southern European consumption in the 1940s and 1950s, and emerged as a named and recommended concept as early as the 1970s.
The study looked at whether eating a so-called Mediterranean Diet had a beneficial cardiovascular effect. People who live in Southern Europe traditionally eat largely fruits, nuts, vegetables, cereal grains, and olive oil; some fish and poultry; and very little red meat, dairy, processed meat, and sugary treats.
The study looked at nearly 7,500 subjects, all of whom were at a high risk of stroke or heart attack, but hadn’t yet had a cardiovascular incident, over five years. They were assigned to one of three diets.
Researchers concluded in the 2013 report that people in the control group, given only advice on which fats to consume, had more heart attacks and strokes than in either group following a Mediterranean Diet. That remains the outcome in the revised study. (One of those groups received about a liter of olive oil at no cost each week; the other, about 30 grams, or 1 ounce, of free nuts a day.)
However, a British anesthesiologist, John Carlisle, who has dedicated significant effort to reviewing the statistical validity of published research, discovered that the data showed it was implausible that participants were assigned randomly to one of the three diets, as the study claimed. The original researchers re-examined their work, and discovered 1 of the 11 study sites had enrolled 467 participants who went to the same clinic to participate in the same diet, instead of randomly.
The researchers also noted that when later participants in the study were already in a household that had a study subject, they were assigned the same diet, affecting 425 more people. While the study’s review committee approved that change, it wasn’t noted in the 2013 version. This reduced randomness didn’t change the outcomes, but it reduced the strength of the connection between diet and risk.
The original conclusion was: “Among persons at high cardiovascular risk, a Mediterranean diet supplemented with extra-virgin olive oil or nuts reduced the incidence of major cardiovascular events.”
The revised conclusion: “In this study involving persons at high cardiovascular risk, the incidence of major cardiovascular events was lower among those assigned to a Mediterranean diet supplemented with extra-virgin olive oil or nuts than among those assigned to a reduced-fat diet.”
The benefits of this study were already often overstated in mainstream discussions, as the study recruited people at a high risk of cardiovascular disease, and from relatively homogeneous genetic communities in the Mediterranean. In the 2013 version, the researchers called this out specifically, “whether the results can be generalized to persons at lower risk or to other settings requires further research.”
Now, it will need to be scoped even further: If you don’t live in the Mediterranean, following this diet may or may not help reduce your risks.