In 2013, the New England Journal of Medicine published a landmark study that found that people put on a Mediterranean diet had a 30% lower chance of heart attack, stroke, or death from cardiovascular disease than people on a low-fat diet. It received massive media and public attention when released, and since has been cited by 3,268 other scientific papers. The study had tremendous impact on the field of nutrition and health science.

Yesterday (June 13), however, the journal retracted the study—providing a new reason for skepticism about how effective the now-popular Mediterranean diet really is.

The reasons for the withdrawal are complicated, having to do with the methodology of the study. As Alison McCook of the Retraction Watch blog writes for NPR, this retraction is the result of the work of John Carlisle, a British anesthesiologist and self-taught statistician. Carlisle has spent recent years analyzing over 5,000 published randomized controlled trials (the gold standard of medical science research) to see how likely they were to have actually been properly randomized. In 2017, he reported his results: at least 2% of the studies were problematic.

One was the 2013 NEJM article on the Mediterranean diet.

The path to a pullback

As Cook reports, the lead author of the paper, Miguel Ángel Martínez González, saw Carlisle’s analysis and decided to follow up with a thorough review of the study design. The study was supposed to randomly assign participants to either the Mediterranean diet with a minimum of four extra tablespoons of olive oil a day, the same diet but with at least an ounce of mixed nuts, or a low-fat diet. But Martínez González found that of the approximately 7,500 participants in the study, 14% had not actually been randomly assigned.

Instead, many married couples were assigned to the same group. In one particularly troubling case, a field researcher decided to assign an entire village to a single group, because some residents were complaining that their neighbors were getting free olive oil. The field researcher working never reported the decision.

Martínez González and his team spent a year reanalyzing the data, working with outside experts. The end result is that the study’s overall findings are still accurate in one sense: There is a correlation between the Mediterranean diet and better health outcomes. But in another sense, the paper was entirely wrong: the Mediterranean diet does not cause better health outcomes.

That might seem like a minor difference, but in the world of medical science, it’s incredibly significant, and the change robs the study of much of its original power.

Broader lessons on nutrition

This retraction is another blow to the public perception of the Mediterranean diet as a touchstone for healthy eating, weight loss, longevity, and disease-risk mitigation.

A major study in 2017 found that if you adjusted for income, the diet doesn’t actually improve heart health: Only wealthy people get the cardiovascular benefits of the Mediterranean diet. And though this is far from a well-designed study, it is interesting that UN data show that people in Mediterranean European countries are more likely to be overweight than those living in any other EU country besides the UK and Germany.

In the larger context, the reality is that this is yet another signal to always read nutrition science with skepticism.

Correction: A previous version of this article misspelled Alison McCook’s name.