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Background: The independent effects of diet- or exercise-induced weight loss on the reduction of obesity and related comorbid conditions are not known. The effects of exercise without weight loss on fat distribution and other risk factors are also unclear. Objective: To determine the effects of equivalent diet- or exercise-induced weight loss and exercise without weight loss on subcutaneous fat, visceral fat, skeletal muscle mass, and insulin sensitivity in obese men. Design: Randomized, controlled trial. Setting: University research center. Participants: 52 obese men (mean body mass index [[+ or -] SD], 31.3 [+ or -] 2.0 [kg/m.sup.2]) with a mean waist circumference of 110.1 [+ or -] 5.8 cm. Intervention: Participants were randomly assigned to one of four study groups (diet-induced weight loss, exercise-induced weight loss, exercise without weight loss, and control) and were observed for 3 months. Measurements: Change in total, subcutaneous, and visceral fat; skeletal muscle mass; cardiovascular fitness; glucose tolerance and insulin sensitivity. Results: Body weight decreased by 7.5 kg (8%) in both weight loss groups and did not change in the exercise without weight loss and control groups. Compared with controls, cardiovascular fitness (peak oxygen uptake) in the exercise groups improved by approximately 16% (P [is less than] 0.01). Although total fat decreased in both weight loss groups (P [is less than] 0.001), the average reduction was 1.3 kg (95% CI, 0.3 to 2.3 kg) greater in the exercise-induced weight loss group than in the diet-induced weight loss group (P = 0.03). Similar reductions in abdominal subcutaneous, visceral, and visceral fat-to-subcutaneous fat ratios were observed in the weight loss groups (P [is less than] 0.001). Abdominal and visceral fat also decreased in the exercise without weight loss group (P = 0.001). Plasma glucose and insulin values (fasting and oral glucose challenge) did not change in the treatment groups compared with controls (P = 0.10 for all comparisons). Average improvement in glucose disposal was similar in the diet-induced weight loss group (5.6 mg/kg skeletal muscle per minute) and in the exercise-induced weight loss group (7.2 mg/kg skeletal muscle per minute) (P [is greater than] 0.2). However, these values were significantly greater than those in the control and exercise without weight loss groups (P [is less than] 0.001). Conclusions: Weight loss induced by increased daily physical activity without caloric restriction substantially reduces obesity (particularly abdominal obesity) and insulin resistance in men. Exercise without weight loss reduces abdominal fat and prevents further weight gain., Exercise can decrease body fat even if no weight is lost. This was the conclusion of a study of 52 obese men who were randomly assigned to one of four groups: diet-induced weight loss, exercise-induced weight loss, exercise without weight loss, and no lifestyle changes. Men who exercised improved their cardiovascular fitness and lost fat tissue. Men who dieted lost fat tissue also, but not as much as those who exercised. Men who exercised and lost weight were less likely to have insulin resistance, which is a precursor of type 2 diabetes.