The PREDIMED trial is a randomized study conducted in Spain in which a Mediterranean diet, supplemented with either olive oil or mixed nuts, was compared with a low-fat diet, as described by the American Heart Association’s guidelines. Its results have been published over the last year or so, and have been truly impressive: a 28-30% drop in cardiovascular disease in the Mediterranean diet group, compared to the low-fat diet group. In fact, the results were so impressive that the trial was stopped early to give the low-fat group the benefits of the Mediterranean diet.
Not surprising, this study has been reported on by both traditional and alternative media outlets, fueled in part by the growing interest in low-carb, high-fat diets, such as Paleo and Atkins. Low-fat diets now seem to be going out of style, and are routinely disparaged as encouraging sugar and white flour binges that will both raise one’s dietary glycemic index and cause fuzzy-mindedness.
Well, this may make for some interesting headlines, but from my perspective, the idea that PREDIMED actually tested a Mediterranean diet against a low-fat diet is questionable to begin with! And the current fad to summarily disregard low-fat diets overlooks significant evidence from what are true low-fat diet trials that were much better-conducted than the PREDIMED trial, and have much more relevance to both cardiovascular disease and cancer.
Let’s review the PREDIMED trial first. This trial randomized 7,447 people at high risk for cardiovascular disease to either a Mediterranean or low-fat diet. Those receiving the Mediterranean diet were further divided to receive either a liter per week of extra-virgin olive oil or 30 grams a day of mixed nuts, given free to the study subjects. The subjects on the Mediterranean diet were given regular dietary counseling sessions about implementing a good Mediterranean diet, one that included substantial vegetables, fruits, legumes, unrefined grains, poultry, low-fat dairy and fish, and wine, but reduced red and processed meats, soda, full-fat dairy, sweets, and pastries. In general, not bad advice, though I disagree with the poultry and dairy recommendations, encourage caution regarding alcohol consumption, and would prefer “eliminate” to “reduce” when it comes to soda, red and processed meats, and sweets and pastries. For the first 3 years, the low-fat diet group just got pamphlets encouraging them to consume a low-fat diet – an intervention that has historically shown to NOT result in dietary changes. After that, they were invited for counseling sessions like those of the Mediterranean group, but aimed at the American Heart Association’s dietary guidelines, which recommend fat intake be between 25%-35%. The study was ended early, after 4.8 years, as mentioned above. This is sometimes done in randomized trials when one group is getting far better results than the other. But it is a problematic strategy, and tends to inflate the differences between groups.
What were the results of the study? The PREDIMED diet significantly decreased the risk of stroke, although not the incidence of heart attacks, other cardiovascular diseases or deaths from all causes. By the end of the study, the low-fat group was eating a 37% fat diet, while the Mediterranean group was eating a 41% fat diet. Saturated fat was low and similar in the two groups (9%). The additional 4% fat in the olive oil group was probably mostly from olive oil, since it consisted of monounsaturated fat, while the additional 4% fat in the nut group came from a mix of monounsaturated and polyunsaturated fat. There were modest differences between the groups in legume and fish consumption. When considered from this perspective, this is what the study essentially tested: a Mediterranean diet supplemented with olive oil or nuts – both of which are known to be very heart-healthy foods, and are likely responsible for the significant decrease seen in cardiovascular disease in this group–versus a diet that was not a low-fat diet. As I explained earlier, these researchers used the American Heart Association’s guidelines for fat intake – 25%-35% — as the model for the control group, and their fat intake ultimately exceeded these levels. If the Mediterranean diet actually had been compared to a truly low-fat diet, one can’t help but wonder if the outcome might have been quite different. It should also be mentioned that the Heart Association diet is not actually considered low-fat by many in the nutrition world.
Next we’ll discuss a diet that actually is a low-fat diet that has been extensively tested for reduction of cardiovascular disease: the DASH diet.