A landmark study on the benefits of the Mediterranean diet for heart health had serious problems with its methods, the study's authors announced this week.
The problems were so critical that the researchers retracted their original paper — a rigorously designed study first published in 2013 in The New England Journal of Medicine that found that following a Mediterranean diet reduced the risk of heart attacks and strokes. In its place, the authors have published a reanalysis of their data in the same journal on June 13, which they say accounts for the methodology problems and comes to the same conclusion as the original.
But in light of the problems with the original study, do doctors still recommend that people follow a Mediterranean diet to protect their hearts?
Some experts say that despite the study's problems, there's already a lot of other research showing the benefits of the Mediterranean diet, and so they'd continue to recommend the diet.
"Although the methodology of this study is somewhat questionable, there still exists a preponderance of data prior to this study which came to the same findings," Dr. Rachel Bond, associate director of the Women's Heart Health Program at Lenox Hill Hospital in New York City, told Live Science. [7 Tips for Moving Toward a More Plant-Based Diet]
But others say that the reanalysis is not enough to make up for the study's methodology problems, and that now, evidence supporting the Mediterranean diet for heart health is weakened.
"This trial was very influential in making physicians and the public believe that there was strong evidence supporting the Mediterranean diet," said Dr. Gregg Fonarow, a professor of cardiovascular medicine at the University of California, Los Angeles. There are still smaller, less rigorous studies that support recommending the Mediterranean diet, but "the strength of those recommendations, and the conviction of making those recommendations, has now lessened," he added.
A Mediterranean diet is one that's high in fruits, vegetables, whole grains, nuts, legumes, olive oil and fish, according to the National Institutes of Health. Much of the research supporting the Mediterranean diet has come from observational studies, which observe populations and measure outcomes, without intervening.
But the 2013 study was different. In that study, called the PREDIMED trial, or Prevención con Dieta Mediterránea, nearly 7,500 participants living in Spain were assigned to follow a Mediterranean diet or a low-fat diet for a median of nearly five years. The researchers found that following a Mediterranean diet was linked with a 30 percent reduction in the risk of heart attack and stroke among people at risk for heart disease during the study period.
The study was thought to have a rigorous design — it was a so-called "randomized controlled trial," which means that participants were randomly assigned to follow one diet or the other. This randomization is important, because it works to cancel out factors that the researchers can't control — if people are randomly assigned to one group or another, then the two groups can be expected to be similar in their characteristics, such as their overall health.
But after the 2013 study was published, the researchers found critical errors in their study that meant that their "randomization" process wasn't always random. In some cases, researchers would assign all of the members of a household to one diet, instead of randomly assigning each member to a diet, according to the New York Times.
Because of these irregularities in the randomization process, the researchers retracted their original paper, and published a reanalysis of their data, the authors said. This reanalysis tried to use statistical methods to account for the problems caused by the faulty randomization. And the researchers found the same result — the Mediterranean diet was tied to about a 30 percent reduction in heart attack and stroke risk.
However, Fonarow told Live Science that, despite this reanalysis, "there remains real, substantial concerns that these results are no longer reliable." The problem with the randomization process raises a "whole host of questions" that the researchers really cannot statistically adjust for, Fonarow said.
Although Fonarow said that following a Mediterranean diet is "not unreasonable," based on previous studies, there remains a need for more rigorous research that takes a look at which diets are optimal for heart health.
"It actually does create an important public health need to conduct rigorous trials that are not compromised … to provide clinicians and patients and the public the answers they really seek about which dietary inventions or patterns are going to be associated with longer life," he said.
The Mediterranean advantage
Dr. Jo Ann Carson, a professor of nutrition at the University of Texas Southwestern Medical Center, and volunteer spokesperson for the American Heart Association, said she would continue to recommend the Mediterranean diet, based on the overall set of information that links the diet with heart benefits.
Carson said that one advantage to the Mediterranean diet is that it gets people to think about their diet more broadly, rather than focusing on a particular food.
"It's beneficial for people to think about how they eat in terms of an overall pattern," Carson told Live Science. "It's not just one food being good or one food being bad. It’s the totality of how the foods we eat fit together into a heart-healthy pattern."
The American Heart Association recommends a healthy diet pattern that emphasizes a variety of fruits and vegetables, whole grains, low-fat dairy products, skinless poultry and nonfried fish, nuts, legumes and nontropical vegetable oils.
Original article on Live Science.