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Leslie Beck’s takeaway on the Mediterranean Diet study retraction

It was the largest randomized controlled trial to assess the long-term effect of following the Mediterranean diet on the primary prevention of cardiovascular disease.

Table setting of mediterranean style food

What happened?

A Mediterranean diet study called PREDIMED (Prevention with the Mediterranean Diet), published in the New England Journal of Medicine (NEJM), one the most reputable journals in the medical world, was retracted, re-analyzed and republished on June 14, 2018.

The original study, published in 2013, found that a Mediterranean diet supplemented with extra virgin olive oil (Med Diet + EVOO) and a Mediterranean diet supplemented with nuts (Med Diet + Nuts) both reduced the risk of heart and stroke by 30% in people at high risk for cardiovascular disease.

The study was a randomized controlled trial, the gold standard of scientific evidence due to its rigorous design that allows it to prove cause and effect. Participants are assigned at random, or by chance, to follow one diet or another. That way, the groups will be similar and the effect of each diet can be compared more fairly and reliably.

The journal retracted the study because it had design flaws. It became known that some of the participants were not “randomly” assigned to their diets, an error that could influence the findings by introducing bias.

The study was reanalyzed taking into account the fact that some participants were not correctly randomized and found similar results as the original study, a 31% lower risk of heart attack and stroke for the Med Diet + EVOO group and a 28% lower risk for the Med Diet + Nuts group. A secondary analysis that removed these participants also had similar results, showing an even a stronger beneficial effect.

A secondary analysis that removed these participants also had similar results, showing even a stronger beneficial effect.

Why is this a big deal?

PREDIMED was a landmark study. It was the largest randomized controlled trial to assess the long-term effect of following the Mediterranean diet on the primary prevention of cardiovascular disease.

The reanalysis provides strong evidence in favour of the Mediterranean diet, but the results are not as robust as those from a randomized controlled trial. Confidence in the republished results has also been weakened by the design errors that were made. The findings will need to be replicated by future randomized controlled trials.

As well, the reputation of the prestigious NEJM has been diminished due the fact it did not do a thorough job of reviewing the data before publication. While the journal did the correct thing by retracting the study and having the researchers reanalyze it, this highly publicized retraction will likely pose a credibility problem for studies going forward, at least for a while. 

Will I continue to recommend the Mediterranean diet?

Yes. While no longer a true randomized controlled trial, I am confident in the findings of the republished PREDIMED study, which underwent much scrutiny.  My recommendation to follow a Mediterranean-style diet is not based on only one study. Rather, it is based on the totality of evidence that has linked the diet with cardiovascular benefits.

Many studies have shown that adherence to a Mediterranean diet lowers blood pressure, LDL cholesterol and triglycerides, improves vascular function, enhances blood glucose control and reduces inflammatory markers. And numerous observational studies have linked a Mediterranean diet with better cardiovascular outcomes.

Another advantage of the Mediterranean dietary pattern is that it gets people to think about their diets more broadly, rather than focusing on one particular food or another.  I strongly believe that the Mediterranean diet is here to stay and that studies will continue to show that adherence to the diet is associated with positive cardiovascular health outcomes, as well as other health benefits.

Finally, this retraction and the headlines that followed are just another signal to read nutrition news with a critical eye. If you are looking for a sustainable, evidence-based approach to healthy eating, working with a registered dietitian will help you make sense of the overwhelming amount of nutrition advice we’re bombarded with, much of it conflicting.

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