The latest smack-down in the diet wars appears to deal a knock-out blow to the notion that high-fat olive oil and tree nuts -- walnuts, almonds and hazelnuts -- are a no-no for those wishing to improve their health. On the contrary, Spanish researchers concluded that the consumption of extra-virgin olive oil and nuts "were probably responsible for most of the observed benefits" attained by those in the two study groups on a Mediterranean diet.
The findings, released by the New England Journal of Medicine, also add to mounting evidence contradicting a long-held tenet of dieting to improve health: that all calories are equal.
A Mediterranean diet is rich in fatty fish, fruits, vegetables and fatty acids, and almost entirely without red meat. Although many studies have suggested its benefits, the current trial is the first to meet the "gold standard" of biomedical research, with large numbers of people randomly assigned to distinct groups and followed for several years.
The superiority of the Mediterranean diet was substantial: Compared with a group of 2,450 people who were urged to follow a low-fat diet, the 4,997 on a Mediterranean diet supplemented either with nuts (2,454 people) or extra-virgin olive oil (2,543 people) were 30 percent less likely to suffer either a heart attack, stroke or death attributed to cardiovascular disease.
Mediterranean dieters were almost 40 percent less likely than low-fat dieters to have a stroke during the follow-up period, which lasted nearly five years. The superiority of the Mediterranean diet was consistent across virtually all subgroups. Only among a small group of people without hypertension did a low-fat diet show better results.
Male participants ranged in age from 55 to 80, women from 60 to 80. All of them either had Type 2 diabetes or satisfied at least three of the following criteria: They were active smokers, were overweight or obese, had a family history of premature heart disease or had hypertension or worrisome cholesterol readings.
The findings "blow the low-fat diet myth out of the water," said Cleveland Clinic cardiologist Steven Nissen, who was not involved in the research. Nissen, an expert on the effects of drugs and nutrition on cardiovascular risk, called the study "spectacular" and touted the findings as impressive.
Mediterranean dieters were urged to minimize sodas and fats that are in partially solid form and to limit consumption of commercially baked sweets and pastries to no more than three times a week. They also were told they could drink wine in moderation -- about seven glasses a week.
Low-fat dieters were told to avoid nuts and vegetable oils of all kinds (including olive oil), to limit store-bought sweets to fewer than one per week and to remove visible fat from all meats. They were encouraged to eat fruits, vegetables, low-fat dairy products and three or fewer servings of bread, potatoes, pasta or rice each day.
None of the dieters had a calorie limit.
Researchers found that the primary difference between the Mediterranean and low-fat dieters was the nuts-and-olive-oil consumption. But they were wary of ruling out the contribution of other elements of the Mediterranean diet to improved cardiovascular health.
"Perhaps there is a synergy among the nutrient-rich foods included in the Mediterranean diet that fosters favorable changes" in the physiological responses, such as inflammation and insulin insensitivity, that give rise to cardiovascular disease, the researchers wrote.
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