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Significant weight loss decreases lean body mass, which makes it difficult to sustain a lower bodyweight. Metabolic rate is proportional to lean weight. Losing muscle mass during weight loss reduces metabolic rate, which promotes weight gain. A study from the U.S. Army Research Institute of Environmental Medicine in Massachusetts found that reductions in insulin-like growth factor 1 (IGF-1) might be responsible for loss of lean mass during weight reduction. IGF-1 works with growth hormone to increase protein synthesis, reduce protein breakdown, and promote fat and carbohydrate metabolism. Test subjects were given three meal treatments at three different times, each of which lasted 48 hours. Diets were low-calorie (1,000 calories), high-carbohydrate, or high-fat/low-carb. The low-calorie diet decreased IGF-1, but diets with increased amounts of fats or carbs had no effect on the hormone. Low-calorie diets depress IGF-1, which decreases lean mass and promotes gaining lost weight. (Metabolism Clinical and Experimental, 62: 179-187)