Authors Berkow and Barnard reviewed 87 studies in the literature regarding vegan and vegetarian diets and weight loss. They reported that, on average, the weight of men and women vegetarians was 3 to 20% lower than that of non vegetarians. Diets that were low to moderate in fat and higher in carbohydrates were more closely associated with lower body weight. Vegetarian diets tend to reduce calories, fat, saturated fat, protein and cholesterol while increasing carbohydrates and fiber. They hypothesize that this difference supports physiologic mechanisms that account for weight loss. They also noted that vegetarians have a lower risk of coronary heart disease, hypertension, and diabetes. They also felt that more calories are burned by vegetarians after their meals than non vegetarians by increasing the sensitivity of insulin so that nutrients could be used, rather than stored as fat.
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Berkow S.E., Barnard N., “Vegetarian diets and weight status.” Nutrition Reviews vol. 64, no. 4 (2006): 175–188.