Vegetarian populations tend to be slimmer than meat-eaters, and they experience lower rates of heart disease, diabetes, high blood pressure, and other life-threatening conditions linked to overweight and obesity. The new review, compiling data from 87 previous studies, shows the weight-loss effect does not depend on exercise or calorie-counting, and it occurs at a rate of approximately 1 pound per week.

Rates of obesity in the general population are skyrocketing, while in vegetarians, obesity prevalence ranges from 0 percent to 6 percent, note study authors Susan E. Berkow, Ph.D., C.N.S., and Neal D. Barnard, M.D., of the Physicians Committee for Responsible Medicine (PCRM).

The authors found that the body weight of both male and female vegetarians is, on average, 3 percent to 20 percent lower than that of meat-eaters. Vegetarian and vegan diets have also been put to the test in clinical studies, as the review notes. The best of these clinical studies isolated the effects of diet by keeping exercise constant. The researchers found that a low-fat vegan diet leads to weight loss of about 1 pound per week, even without additional exercise or limits on portion sizes, calories, or carbohydrates.

"Our research reveals that people can enjoy unlimited portions of high- fiber foods such as fruits, vegetables, and whole grains to achieve or maintain a healthy body weight without feeling hungry," says Dr. Berkow, the lead author.

"There is evidence that a vegan diet causes an increased calorie burn after meals, meaning plant-based foods are being used more efficiently as fuel for the body, as opposed to being stored as fat," says Dr. Barnard. Insulin sensitivity is increased by a vegan diet, allowing nutrients to more rapidly enter the cells of the body to be converted to heat rather than to fat.

Earlier this month, a team of researchers led by Tim Key of Oxford University found that meat-eaters who switched to a plant-based diet gained less weight over a period of five years. Papers reviewed by Drs. Berkow and Barnard include several published by Dr. Key and his colleagues, as well as a recent study of more than 55,000 Swedish women showing that meat-eaters are more likely to be overweight than vegetarians and vegans.

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