A highly publicized trial in Spain found that the Mediterranean diet protects against heart disease. Now the original work has been retracted and re-analyzed, with the same result.
The study was a landmark, one of the few attempts to rigorously evaluate a particular diet. And the results were striking: A Mediterranean diet, with abundant vegetables and fruit, can slash the risk of heart attacks and strokes.
But now that trial, published in the New England Journal of Medicine in 2013, has come under fire. The authors retracted their original paper on Wednesday and published an unusual “re-analysis” of their data in the same journal.
Despite serious problems in the way the study was conducted, their conclusions are the same: A Mediterranean diet can cut the risk of heart attacks and strokes by about 30 percent in those at high risk.
Not everyone is convinced. “Nothing they have done in this re-analyzed paper makes me more confident,” said Dr. Barnett Kramer, director of the division of cancer prevention at the National Cancer Institute.
For decades, researchers have noted that people living in some Mediterranean countries have lower rates of heart disease and cancer. Scientists have long suspected that the regional diet — rich in fruits, vegetables, nuts and olive oil, with moderate levels of fat — played a protective role.
But the idea has been hard to prove. It is very difficult to test any diet in a clinical trial. Participants may be reluctant to stick to the prescribed meal plan, for instance, and it can be difficult to monitor them over months or years.
The original study was conducted in Spain by Dr. Miguel A. Martínez-González of the University of Navarra and his colleagues. The trial enrolled 7,447 participants aged 55 to 80 who were assigned one of three diets: a Mediterranean diet with at least four tablespoons a day of extra virgin olive oil; the same diet with an ounce of mixed nuts; or a traditional low-fat diet.
The participants were followed for a median of nearly five years. Dr. Martínez-González and his colleagues reported that there were fewer cardiovascular events in the groups consuming olive oil and nuts.
But last year Dr. Martínez-González found his study on a list of clinical trials whose data seemed suspect, compiled by Dr. John Carlisle of Torbay Hospital in England.
“That was the first hint that there could have been some imperfection,” Dr. Martínez-González said in an interview.
A statistician at the New England Journal of Medicine suggested the researchers look at the methods at each center that recruited participants.
The idea of a randomized trial is to assign treatments — in this case, diets — to participants with the statistical equivalent of a coin toss. That way, the groups being compared should be equivalent, with no group healthier or sicker, or older or younger, than another on average.