In February, an important Spanish study on the Mediterranean diet got more publicity than any diet study in recent memory. Hailed as a “watershed moment,” it had all the trappings of a big deal. It was large, long, well-designed (a randomized, controlled trial), in a prestigious journal (New England Journal of Medicine), and on a trendy subject (the diet is adored by foodies and has been the focus of many positive studies). On top of all that, the study was halted a year early because the results were so clear—a 30 percent reduction in “major cardiovascular events” (that is, heart attacks, strokes and cardiovascular deaths) in high-risk people, compared to a “low-fat diet.” That’s similar to the risk reduction seen with statin drugs. In fact, the findings were so striking that it was deemed unethical not to allow the control group to switch to the beneficial diet.
As impressive as all that is, the study didn’t prove that the Mediterranean diet is better than a low-fat diet, as was widely reported, especially the type of low-fat diet Americans would typically eat. And it left many key questions unanswered.
What did study participants eat?
The traditional diets of Greece, Spain, Italy and other Mediterranean countries include lots of fresh fruits, vegetables, beans and grains, moderate amounts of fish and poultry, limited dairy (mostly yogurt and cheese), and little red meat or sweets. Olive oil, as the main source of fat, and wine, especially red wine, are its most famous features. It has long been proposed that this diet is largely responsible for the lower cardiovascular mortality rate in Mediterranean countries compared to northern Europe or the U.S.
The Spanish study included 7,447 people, ages 55 to 80, who initially had no cardiovascular disease; nearly all were overweight or obese. They all had either diabetes or at least three major coronary risk factors. Notably, more than 80 percent had hypertension, and 70 percent had undesirable cholesterol levels; most were taking drugs to control these disorders.
The study actually compared three diets: Mediterranean with extra olive oil, Mediterranean with extra nuts and a “low-fat” control group. None of the participants were advised to reduce calories. Both Mediterranean groups were counseled to use olive oil freely, eat mostly white meats, drink wine with meals and eat lots of fruits, vegetables, beans and fish. The olive oil group was also specifically told to consume at least 4 tablespoons daily of olive oil, which was provided to them. The nut group was advised to eat about an ounce of nuts a day (½ walnuts, ¼ almonds, ¼ hazelnuts), also provided. The Mediterranean groups got individual and group dietary counseling.
The “low-fat” control group was given advice similar to that of the American Heart Association: eat lots of fruits, vegetables, low-fat dairy products, lean fish and starches (bread, pasta, rice, potatoes); eat little vegetable oil, butter, red meats, commercial baked goods, sweets and nuts.
Heart attacks and strokes: the numbers
The study was halted after participants had been followed for an average of five years, at which point the two Mediterranean diet groups had a 30 percent lower rate of combined cardiovascular events (the primary endpoint) than the control group. In absolute numbers, for every 1,000 people eating a Mediterranean diet for a year, there were three fewer events.
The biggest reduction was in strokes in the Mediterranean diet groups, especially the nut group. There were no statistically significant differences in the rates of heart attacks and deaths among the groups.
One surprising aspect of the study, as seen in the detailed supplemental data, is how similarly the three groups ended up eating, except for the extra olive oil and nuts. After three years, when the researchers realized that the control group was not following the low-fat guidelines, they started giving them periodic counseling, but even then compliance was poor. In large part, that simply shows how hard it is to get people to change the way they eat, or at least to get Spaniards to switch to low-fat eating.
In fact, the control (“low-fat”) group started with and ended with a Mediterranean diet. Notably, they still consumed lots of olive oil—averaging about 3 tablespoons a day, compared to 4 tablespoons in the olive oil group. The control group got nearly as much total fat as the two Mediterranean groups. All started out with fat supplying 39 percent of daily calories; the low-fat group dropped to 37 percent (definitely not low-fat), while the Mediterranean groups rose to 41 percent—“small” differences, the researchers noted. And they ate similar amounts of protein, carbohydrates and fiber.
The intake of major food groups didn’t differ much, either, except the Mediterranean groups ate a little more fish (0.3 extra servings a week) and more beans (0.4 extra servings a week). The control group consumed about 200 fewer calories a day than the Mediterranean groups, but the study did not restrict calories or focus on body weight.
The main differences were in types of fat consumed by the groups, though even these were relatively small. All three groups consumed the same amount of saturated fat (9 percent of daily calories, within American Heart Association guidelines). Not surprisingly, the olive oil group ate somewhat more monounsaturated fat (22 percent of daily calories) than the control group (19 percent), and the nut group consumed somewhat more polyunsaturated fats (8 percent versus 6 percent of daily calories), especially alphalinolenic acid (an omega-3 fat, supplied by the walnuts). All these unsaturated fats have been linked to cardiovascular health.
“While these dietary changes were small, collectively they added up to a significant change in dietary pattern,” according to Ronald Krauss, M.D., director of Atherosclerosis Research at Children’s Hospital Oakland Research Institute, a member of our Editorial Board, and coauthor of the 2013 Wellness Report Controlling Your Cholesterol. Giving added credibility to the new findings, Dr. Krauss says, is the fact that they are consistent with those of the Lyon Heart Study, an important clinical trial on the Mediterranean diet from 1999. That study suggested that alpha-linolenic acid (supplied by canola oil margarine), in particular, can help prevent a second heart attack in people who had already had one.
Should you switch to a Mediterranean diet, particularly if you’re at high cardiovascular risk, like the people in this study?
It’s hard to say, since the study didn’t compare the diet to a typical American diet, and didn’t succeed in comparing it to a heart-healthy low-fat diet. Nor did it look at people who are at lower risk or those who already have known cardiovascular disease.
A Mediterranean diet can be a good choice, but is it the best choice? How about a strict vegetarian diet, various traditional Asian cuisines, or a very-low-fat regimen, such as that promoted by Dean Ornish, M.D., for people with heart disease? This study doesn’t give us an inkling, and those diets have never been rigorously tested in a large trial like this one.
It can’t hurt to add a little olive oil to your diet, though the control group also consumed a fair amount of olive oil and apparently did not benefit. We can’t advise simply adding 4 tablespoons of olive oil (450 calories) to your daily diet, unless it replaces other high-calorie foods. And it’s not clear how important it was that the olive oil supplied in the study was “extra virgin” and thus had a high polyphenol content. There are good reasons to eat nuts—plenty of studies have linked them to a range of health benefits. But do some of the nuts have to be walnuts, as in the study? Perhaps other foods rich in alpha-linolenic acid (such as canola oil and flaxseeds) would be just as beneficial.
Bottom line: We’re still enthusiastic about the Mediterranean diet. Many people find it delicious and satisfying and thus easy to stick to. Its whole-food components, on balance, are certainly more healthful than the diet the typical American consumes. You can’t go wrong by eating an ounce of nuts a day and a little more olive oil. But don’t forget the diet’s other key elements—more fruits, vegetables, beans and fish, especially in place of red meats and processed foods.
Published May 28, 2013