Something fishy went on with a major study about the Mediterranean diet and its health benefits.

It’s surprisingly hard to gather strong scientific evidence on a diet’s long-term health benefits. So, when a study did just this in 2013, it caught a lot of people’s attention.

A randomized trial appeared to show that the Mediterranean diet – rich in oily fish, fresh vegetables, and olive oil – could reduce the risk of cardiovascular diseases, such as heart attacks and strokes, by around 30 percent. The media splashed the study's bold findings everywhere, while the Spanish, Greeks, and Italians all said: “We told you so.”

However, on June 13, the highly prestigious New England Journal of Medicine (NEJM) retracted and republished an amended version of the study due to problems with the way its data was gathered.

Here’s the problem. Around 10 percent of the 7,447 participants in the study were not assigned diets by a fully randomized procedure, according to The New York Times. When researchers assigned one person to the Mediterranean diet, others in the household (such as a spouse) were also allowed in. The researchers appear to have counted these “others” as though they were randomly assigned, even though that’s not strictly true. Since these others were often family members, they most likely share many lifestyle factors, meaning any changes in their health might not necessarily be down to their diet alone.

It might sound like a fairly minor issue, but it’s enough to skew results and conclusions. The republished study now statistically accounts for the mistake, and they’ve stuck to the same conclusions as the original study, albeit with a much softer tone.

“When we discover a problem we work very hard to get to the bottom of it,” Dr Jeffrey M Drazen, editor-in-chief of the NEJM, told The Associated Press. “There’s no fraud here as far as we can tell. But we needed to correct the record."

The retraction of a highly publicized paper is a fairly big deal. Some scientists, such as Hilda Bastian writing for PLOS Blogs, argue that this error means the study is no longer statistically sound and could bring into question some of their conclusions.  

Nevertheless, other scientists say this shouldn’t undermine your faith in the scientific process – quite the opposite. 

“This is a highly unusual step and I am sure that the NEJM took this decision very seriously,” Professor Naveed Sattar, an independent expert in the field from the University of Glasgow, noted.

“It is reassuring to see results remain broadly similar but that said, for some people, this retraction and re-analysis might somewhat influence their view of the robustness of the trial results.

"Overall, this whole process will, I believe, lead to future lifestyle and diet trials to be of higher standards.”

Needless to say, none of this should put you off eating Mediterranean foods. While the evidence of the diet’s benefits might not be as set in stone as it once was, few would argue against a diet filled with vegetables, whole grains, nuts, and oily fish. 

Just go easy on the pizza.