The New York Times made big news with reports that the New England Journal of Medicine study on the beneficial effects of the Mediterranean diet showed it could dramatically reduce the rates of heart attack and stroke. But this study has major issues that bear directly on whether or not physicians should make new recommendations about dietary intake of fats like olive oil, or whether patients should adopt the diet as a whole. Let’s talk about the trial.
First of all, this is a randomized, controlled trial, in which 7447 men and women between 55 and 80 years of age who had major risk factors for cardiovascular disease such as diabetes, obesity, smoking, hyperlipidemia etc., were divided evenly between 3 groups, one which received recommendations on a “low fat” diet, and two in which there was extensive counseling on the Mediterranean combined with either a ready free supply of extra-virgin olive oil, or alternatively a variety of nuts.
The primary end points being studied was the combined number of heart attacks, strokes, and death, and over the course of about 5 years of study about 288 such events occurred. If you combine all three of these end points together, and evaluate their frequency between the groups you find 96 of these end points occurred in the “Mediterranean diet with extra-virgin olive oil” or 3.8% of the group, 83 occurred in the Mediterranean diet with nuts for 3.4% of that population, and 109 in the control group for 4.4% of the controls.
But before anyone takes these results to heart, we have to recognize major flaws with the study design, and the populations that comprised these three groups. First, the rate of primary events was surprisingly low for such a high risk group, and because the study was stopped early, absurdly for “ethical reasons”, the number of events is quite low. For the life of me I can’t think of what that ethics committee was thinking. These results are not that dramatic. Further, the “low fat” diet was very ineffectually enforced or counseled, to the point that midway through the study the authors revised the protocol to include more counseling sessions. Evaluating the supplementary data, specifically table S7, you see this control group was in no way on a low fat diet. They still were consuming 37-39% of their calories from fat! “Low fat” should have 10-15% of calories from fats, so basically, everyone ignored the diet. Further, all of the groups consumed a similar amount of total fat, mono and poly-unsaturated fats, and even a used olive oil as their main culinary fat. All groups consumed (see table S5) a similar amount of red meat (forbidden from all diets), butter, soda, baked goods, etc. The places where there seemed to be more dramatic differences were in olive oil consumption (about 50% of controls had > 4 tbsp a day, vs 80% of the “nuts” group and about 90% of the “extra-virgin olive oil group), wine consumption (modest at about 30% in diet groups vs 25% in “low fat” control), nuts (crazy high at 90% in nuts group, vs 40% and 20% in “olive oil” and “low fat”, as well as modest elevation of the amount of fish, fruits and vegetables in the Mediterranean groups. Further, some of these differences, such as the consumption of alcohol, fruits and vegetables, was higher in the Mediterranean groups at baseline (notice no mean change in table S6) so the groups may have started out in a different place.
What does this mean? First of all, we have to reject the notion that this study compared Mediterranean diet to “low fat” diet. This was a study of basically no diet intervention versus increasing your intake of fish, nuts and/or olive oil. Otherwise, there didn’t appear to be compliance with the negative suggestions of the Mediterranean diet, to decrease red mean intake, baked goods, dairy, etc. The participants basically took the recommended items and increased them in their diets, but didn’t exclude any of the “discouraged” items. This is very interesting, but to call it the “Mediterranean diet” is misleading. In reality, it’s diet supplementation with olive oil, nuts and fish.
Second, the final results, while they sound impressive (30% reduction in combined primary end points!) are actually not as important as some of the less-emphasized findings. For this we have to evaluate the secondary endpoint, which happens to be the one we really care about – all cause mortality. They could not show a difference in mortality! So while you might be less likely to have a heart attack or stroke, you’re no less likely to die. This is why I’m so confused they ended the study early. This is really the only end point that matters, and it was unchanged at the interval at which the ethics committee decided this study had to be stopped for efficacy. Why did they do this? The evidence is suggestive that with more participants, the Mediterranean diet + olive oil might have diverged a bit and shown a benefit compared to the do nothing “low fat” control, but this didn’t reach significance.
What have we learned? Compared to other Spanish folks between the ages of 55 and 80, all with cardiovascular risk factors, those that added olive oil, nuts, and fish to their diet had fewer cardiovascular events, but no difference in their mortality compared with people that did nothing to change their diet.
Why did this make the front page of the New York Times? Let’s show a little bit more critical analysis of findings, and not just swallow the PR.