A rapidly occurring compensatory decrease in physical activity counteracts diet-induced weight loss in female monkeys.

To study changes in energy balance occurring during the initial phases of dieting, 18 adult ovariectomized female monkeys were placed on a low-fat diet, and available calories were reduced by 30% compared with baseline consumption for 1 mo. Surprisingly, there was not significant weight loss; however, daily activity level (measured by accelerometry) decreased soon after diet initiation and reached statistical significance by the 4th wk of dieting (18 +/- 5.6% decrease, P = 0.02). During a 2nd mo of dieting, available calories were reduced by 60% compared with baseline consumption, leading to 6.4 +/- 1.7% weight loss and further suppression of activity. Metabolic rate decreased by 68 +/- 12 kcal/day, with decreased activity accounting for 41 +/- 9 kcal/day, and the metabolic activity of the weight lost accounting for 21 +/- 5 kcal/day. A second group of three monkeys was trained to run on a treadmill for 1 h/day, 5 days/wk, at 80% maximal capacity, leading to increased calorie expenditure of 69.6 +/- 10.7 kcal/day (equivalent to 49 kcal/day for 7 days). We conclude that a diet-induced decrease in physical activity is the primary mechanism the body uses to defend against diet-induced weight loss, and undertaking a level of exercise that is recommended to counteract weight gain and promote weight loss is able to prevent the compensatory decrease in physical activity-associated energy expenditure that slows diet-induced weight loss.