Obesity is recognized as a serious threat to public health. Overweight and obese people are at higher risk for morbidity from numerous chronic disorders such as diabetes, hypertension and cardiovascular disease. This health risk is greater when excess fat is deposited in the abdominal region because this phenotype is a stronger predictor of cardiovascular disease and type 2 diabetes mellitus (formerly known as non–insulin-dependent diabetes) than general obesity. Treatment strategies that reduce abdominal obesity, particularly visceral fat, can enhance the benefits of general weight reduction. Diet restriction is the most common method of obesity reduction. Increased physical activity has generally not been demonstrated to reduce obesity, although a single exercise session is associated with a significant improvement in insulin-stimulated glucose uptake. The question of whether the improved effect of exercise on insulin action is independent of fat loss needs further clarification. Ross and associates performed a randomized, controlled study to determine the independent effect of diet-induced and exercise-induced weight loss on obesity and insulin resistance in moderately obese men. They also evaluated whether exercise without weight loss was associated with reductions in abdominal obesity and insulin resistance.

The study included 101 nonsmoking, sedentary men with a body mass index (BMI) greater than 27 kg per m² and a waist circumference greater than 100 cm, with stable weight and normal glucose tolerance and plasma lipid profiles. All participants followed a weight maintenance diet for a four- to five-week period during which daily energy requirements were calculated. Participants were divided into four groups for the 12-week trial period: (1) control subjects were asked to maintain their body weight; (2) participants in the diet-induced weight loss program were asked to reduce their diet by 700 kcal per day to achieve a weight loss of approximately 0.6 kg per week; [ corrected] (3) participants in the exercise-induced weight loss group were asked to maintain the isocaloric diet and perform exercise that expended 700 kcal per day; and (4) participants in the exercise-without-weight-loss group were asked to maintain body weight. All participants were free-living, ate self-selected foods and attended weekly seminars about proper food selection. Insulin resistance and glucose tolerance were measured in all participants who completed the study.

Both weight loss groups showed a significant reduction in total body fat with a greater reduction occurring in the exercise-induced weight loss group than in the diet-induced weight loss group. The exercise-without-weight-loss group showed a reduction in abdominal and visceral fat. Reductions of visceral fat alone, even without weight loss, were related to improvements in glucose tolerance and insulin sensitivity. Although visceral fat cannot be measured in clinical practice, it is closely related to waist circumference and BMI. The authors conclude that diet and exercise are effective methods of reducing abdominal obesity and decreasing insulin resistance. The reduction in insulin resistance after equivalent diet- or exercise-induced weight loss are similar. Exercise without weight loss is also associated with improvement in glucose uptake. Walking briskly for approximately 60 minutes a day, even without caloric restriction, helps reduce obesity and insulin resistance.

editor's note: Physical activity improves insulin sensitivity in both normal and insulin-resistant people. A single exercise session can increase insulin sensitivity for up to 16 hours. This physical activity favorably affects glucose transport and metabolism, and may also cause positive changes in lipid metabolism and regulation of hepatic glucose output. To obtain maximum sustained benefit, exercise must be regular and should include a wide variety of skeletal muscle groups. Aerobic exercise regimens that include various modes of exercise and require use of large muscle masses (e.g., swimming, power walking, strength training) are probably optimal. The benefits of exercise in people over 55 years of age seem to be lessened, probably because of a decline in muscle mass associated with aging. For further information see Eriksson JG. Exercise and the treatment of type 2 diabetes mellitus. An update. Sports Med June 1999;27(6):381–91.—r.s.