A landmark study from 2013 on the heart health benefits of a Mediterranean diet was found to contain a few errors in the execution of its research method.

The authors retracted the paper from the New England Journal of Medicine (NEJM) on June 13.

What is the Mediterranean diet?

Among the many diets that have gained popularity over the years, the Mediterranean diet is a prominent one. Inspired by countries bordering the Mediterranean Sea, it borrows elements from the mid-20th century eating habits of countries such as Greece and Italy.

Fruits, vegetables, olive oil, nuts, and whole grains are encouraged. The diet also restricts the intake of processed meat, added sugar, trans fats etc. and allows for moderate consumption of wine and fish. 

How is it considered beneficial for health?

Research found a lower incidence of heart disease in Mediterranean countries compared to the United States, prompting questions on whether the diet may play a key role. 

In 2013, a study published in the NEJM seemed to provide strong evidence on the heart health benefits. 7,447 participants from Spain (who were at risk for heart disease) were randomly assigned one of three types of diets.

The findings stated that the diet with olive oil reduced the incidence of heart attacks, stroke, and heart disease-related death by 30 percent. "This is good news, because we know how to prevent the main cause of deaths — that is cardiovascular disease — with a good diet," researcher Dr. Miguel Angel Martinez-Gonzalez stated in 2013.

What was the flaw in the research?

John Carlisle, an anesthesiologist from England, analyzed thousands of trials published in the journal. The aforementioned study was one of the few he noticed errors in. Soon, Martínez-González inspected the study in detail and found problems regarding the randomization.

It turned out that 14 percent of the study participants had not been assigned randomly to either a Mediterranean diet or a low-fat diet. Couples who participated in the study were both given the same diet to follow. In one instance, an entire participating village followed the same diet.

Citing "irregularities in the randomization procedures," the authors retracted the paper and replaced it with a corrected version.

What about recommendations based on the retracted study?

Martínez-González explained that the flaw only affected a small part of the trial, as the results largely remained the same after a reanalysis. "We need to tone down the results, but it is just a little bit," he said. The main difference in the new version is that the data accounted for the errors and the conclusion used softer language.

While many believed the credibility of the study was affected, elements of the diet such as high intake of plant-based foods and reduced intake of processed foods are still recommended by medical professionals. 

"We believe the evidence [in favor of the diet] is still strong, but not as strong as a randomized study in which the randomization was executed flawlessly," said Jennifer Zeis, spokeswoman for the NEJM.