The Mediterranean diet, focused on fruits, vegetables, whole grains, olive oil, and nuts, has been associated with various health benefits. In a significant study initiated in Spain in the late 2000s, over 7,000 participants were recruited to investigate its effects. Participants were divided into groups that followed either a Mediterranean diet enriched with healthy fats or a control diet, while researchers monitored cardiovascular outcomes over several years.
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After about five years, researchers made the unexpected decision to terminate the study early due to ethical concerns. The diet demonstrating significant benefits in reducing serious cardiovascular outcomes raised questions about the ethics of continuing the trial with a comparison group receiving less beneficial dietary guidance.
The study involved men and women aged 55 to 80 who were at high risk for heart disease due to factors such as obesity, smoking, or diabetes. Participants were randomly assigned to three groups: a Mediterranean diet supplemented with extra-virgin olive oil, one supplemented with nuts like hazelnuts, almonds, and walnuts, and a control group that was instructed to decrease fat intake. Over the course of the study, 109 major cardiovascular events occurred in the control group, compared to 96 in the olive oil group and 83 in the nut group, indicating approximately a 30% lower risk of significant cardiovascular issues for those on the Mediterranean diet.
As these results became evident, scientists questioned whether it was justifiable to assign participants to a diet that appeared less beneficial. The consensus was to conclude the study early to avoid denying potential benefits to participants in the control group.
The findings received attention from paleoanthropologist Daniel Lieberman, who remarked on the Mediterranean diet’s strong evidence base in comparison to other diets. However, later scrutiny arose when John Carlisle, an anesthesiologist, analyzed the data and observed irregular patterns that prompted further investigation.
His concerns led the New England Journal of Medicine, which published the original study, to request a review from the researchers. This revealed discrepancies in the randomization process, as some participants had been assigned to groups improperly, affecting 1,588 individuals. Due to these issues, the journal retracted the original paper in 2018.
Despite the retraction, researchers did not discard the data. A revised analysis was published, still concluding that the Mediterranean diet was associated with a lower incidence of major cardiovascular events, though the language shifted to reflect the methodological concerns.
Carlisle acknowledged the journal’s response, praising their prompt and thorough handling of the situation.