Landmark Mediterranean diet study was flawed. Authors retract paper published in NEJM

We've been hearing about the benefits of a Mediterranean diet for years, and now authors of a major study long cited for suggesting its heart-healthy benefits said the research was flawed.

The original study, published in 2013 in the New England Journal of Medicine suggested a Mediterranean diet can decrease risk of heart attacks and strokes. Authors said in a letter published Wednesday there were "irregularities in the randomization procedures" and they've revised the report.

The error was spotted earlier by John Carlisle of Torbay Hospital in England, who featured the study on a 2017 list of medical reports with questionable data.

The problem is that some of the 7,447 participants in the original study, funded by the Spanish government, hadn't been assigned randomly to follow the Mediterranean diet, primarily focusing on olive oil or nut intake, or a low-fat one referenced in the paper, lead author Miguel Ángel Martínez-González told USA TODAY. In some cases, family members were assigned the same diet, which does not constitute a random sample size. In one case, an entire village participated in the same diet. Martínez-González said his team was aware of this when the study published in 2013, but did not report it.

He stressed this flaw only affected a small part of the trial (about 10 percent of participants), and the conclusions remain the same: A Mediterranean diet can decrease risk of heart attacks and strokes by about 30 percent among those who are at high risk of developing cardiovascular disease.

"We are more than convinced than ever of the robustness of the protection by the Mediterranean diet against cardiovascular disease," Martínez-González said.

Jennifer Zeis, a spokesperson at NEJM, said the journal became aware of the problems last year and worked with the study authors to resolve them.

"Medical professionals and their patients can use the republished information with confidence," she said.

But, some people are skeptical. Donald Hensrud, medical director of the Mayo Clinic Healthy Living Program  who isn't affiliated with the study, told USA TODAY the revision casts doubt on the entirely of the study.

"The journal and the authors should be applauded for reanalyzing the data, but when there are questions like this raised, it damages the credibility to some extent of the overall study," Hensrud said.

It doesn't mean we shouldn't follow the Mediterranean diet, he said. A plant-based diet of real food and olive oil is still regarded as healthy by many medical professionals. But, this 2013 study is the most high-profile research on the benefits on the diet, Hensrud said.

The Mediterranean diet is mainly comprised of plant-based foods, such as fruits and vegetables, whole grains and nuts. People who follow this diet rarely eat red meat and instead opt for fish and poultry. Butter is often replaced with olive oil.

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