The Mediterranean diet, known for its emphasis on vegetables, fruits, whole grains, olive oil, nuts, and minimally processed foods, has been linked to various health benefits. In Spain, researchers began a large-scale study in the late 2000s with over 7,000 participants to further investigate these potential benefits. Participants, aged 55 to 80 and at high risk for heart complications, were divided into groups following a Mediterranean diet supplemented by either extra-virgin olive oil or mixed nuts, while a control group was advised to consume less fat.
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The study tracked significant cardiovascular incidents, such as strokes and heart attacks. Results showed that 96 participants in the olive oil group and 83 in the nut group experienced such events, compared to 109 in the control group. Overall, participants on the Mediterranean diet had approximately a 30% lower risk of major cardiovascular issues than those on the control plan.
However, about five years into the study, researchers decided to terminate it early due to an ethical concern. With clear evidence showing greater protection against serious cardiovascular problems from the Mediterranean diet, they faced the dilemma of whether it was moral to continue assigning participants to the less beneficial control group.
Paleoanthropologist Daniel Lieberman mentioned the study on Chris Williamson's Modern Wisdom podcast, noting that while the Mediterranean diet might not be definitively the best, it has substantial evidence supporting its health effects.
Despite the promising findings, the study faced scrutiny after anesthesiologist John Carlisle raised questions about its methodology in 2017. He identified unusual patterns in the data from numerous trials, including this study, prompting a closer investigation by the New England Journal of Medicine (NEJM).
Upon review, researchers discovered that the assignment of participants did not adhere strictly to the stated randomization protocol. In some instances, individuals from the same household were assigned to the same group rather than being individually randomized, and in other cases, entire villages were grouped together. These issues affected 1,588 participants.
Due to the discrepancies in participant assignment from what was originally reported, the NEJM retracted the study in 2018. Following this, the researchers published a revised analysis while retaining the conclusion that the Mediterranean diet was linked to a decrease in major cardiovascular events. The new findings were described in more cautious terms, stating that the diet was “associated with” rather than “reduced” the risk of cardiovascular issues.
Carlisle praised the NEJM's prompt and thorough response to the concerns he raised, emphasizing the importance of addressing possible issues in scientific research.