Built around vegetables, fruits, whole grains, olive oil, nuts, and other minimally processed foods, the Mediterranean diet has long been linked to a range of potential health benefits.
But could those benefits be demonstrated in a large-scale scientific study?
Researchers in Spain set out to explore that question, enrolling more than 7,000 people in an experiment that began in the late 2000s.
Highlights
- More than 7,000 people took part in the Mediterranean diet study, which ended roughly five years into its scheduled duration.
- It found that the eating pattern, rich in healthy fats, was linked to a lower risk of major cardiovascular events.
- The study was later retracted after a British anesthesiologist identified irregularities in its randomization process.
The participants were divided into different groups, with some following a Mediterranean-style eating plan rich in healthy fats, while others followed a different diet for comparison.
The research was expected to continue for several years, but roughly five years in, scientists made the unexpected decision to end it early.
The reason stemmed from an ethical dilemma that emerged as the study progressed.
The Mediterranean diet showed a clear advantage in reducing serious cardiovascular outcomes
The study involved men and women between 55 and 80 years of age who were considered at high risk of heart complications because they were either overweight, smokers, or had diabetes.
Participants were assigned to one of three groups: a Mediterranean diet supplemented with extra-virgin olive oil, the same eating pattern supplemented with a mix of nuts, including hazelnuts, almonds, and walnuts, or a comparison group advised to eat less fat.
The researchers tracked major cardiovascular events, including strokes and fatal heart attacks, among the participants.
During the trial, 96 participants in the olive oil group and 83 in the nut group experienced one of these events, compared with 109 in the control group.
Overall, the Mediterranean diet groups showed roughly a 30% lower risk of major cardiovascular issues than the comparison group.
These results created an ethical dilemma.
With one dietary approach appearing to offer greater protection against serious cardiovascular problems, researchers faced a question over whether it was appropriate to keep the trial running and continue assigning participants to the comparison group.
It was eventually agreed that the study should end early because the benefits associated with the Mediterranean diet had become sufficiently apparent.
Paleoanthropologist Daniel Lieberman recently referenced the study while appearing on Chris Williamson’s Modern Wisdom podcast.
“You know, I probably, I wouldn’t say it’s [the Mediterranean diet] a clear winner, but it probably has the most evidence in its favor of any diet that’s ever been studied,” he said.
The promising findings later came under scrutiny when questions were raised about how the study had been conducted
The study may have remained largely unquestioned had it not been for a statistical analysis by John Carlisle, an anesthesiologist in England who had developed a knack for identifying unusual patterns in clinical experiment data.
In 2017, he examined thousands of randomized controlled trials, looking for patterns that appeared unlikely to occur if participants had genuinely been assigned to groups at random.
The Mediterranean diet study was among the papers that caught his attention.
Carlisle’s analysis did not prove that anything had gone wrong with the study, but it flagged discrepancies that warranted a closer examination.
His findings soon caught the attention of the New England Journal of Medicine, which had published the study.
The journal contacted the researchers and asked them to examine the concerns surrounding the trial.
That review sent the researchers back to the way participants had originally been assigned to the different groups.
What they found revealed that the randomization process had not been carried out exactly as the original paper had described, setting the stage for a much bigger problem with the study.
The investigation uncovered deviations in the randomization process that eventually led to the study’s retraction
As the researchers examined their records, they found that the assignment process had deviated from the protocol for a substantial number of participants.
At one study site, people from the same household had been placed in the same dietary group rather than being individually randomized. At another site, an entire village had been assigned to a particular intervention.
Problems were also identified with the use of a randomization table at a third location.
Altogether, the researchers determined that these issues affected 1,588 of the study’s total participants.
That mattered because the original paper had presented the trial as a randomized study in which participants were individually assigned to the different dietary approaches.
Because the original description no longer accurately reflected how part of the trial was conducted, the researchers and the New England Journal of Medicine, which published the study, retracted the paper in 2018.
However, the story did not end with the retraction.
Rather than discarding the data entirely, the researchers changed the language used to describe the findings.
The revised analysis still linked the Mediterranean diet to fewer major cardiovascular events
A new analysis was published alongside the retraction, with the researchers retaining the overall finding that participants following the Mediterranean diet experienced fewer major cardiovascular events.
The wording, though, became more cautious.
While the original paper said the diet “reduced” the risk, the updated version described it as being “associated with” a lower risk, reflecting the limitations created by the problems with participant assignment.
Carlisle praised the journal’s response.
“I think that the NEJM editorial team responded very maturely to my paper,” he said.
“They took the possibility of a problem seriously and acted quickly and thoroughly.”
"I love the Mediterranean diet," a netizen said
