Increased expenditure of energy at rest has been considered a contributing factor to the negative energy balance and weight loss that occur in patients with human immunodeficiency virus (HIV) infection. However, the true determinant of energy balance is not resting but total energy expenditure. We sought to determine the contribution of total energy expenditure to weight changes in patients with HIV-associated wasting. We performed 51 assessments of energy metabolism in 27 men with HIV infection at different stages of disease, including periods of both rapid and slow weight loss. Resting energy expenditure was measured by indirect calorimetry, total energy expenditure by the doubly-labeled-water technique, and energy intake by recording the weight of food consumed. The results were compared with the rate of weight loss or gain. The mean (+/- SD) total energy expended by the HIV-infected men was 2750 +/- 670 kcal per day, no more than that expended by normal men. There was a significant positive relation between total energy expenditure and the rate of weight change (r = 0.61, P < 0.001); thus, during rapid weight loss, total energy expenditure was reduced to 2180 +/- 580 kcal per day (P = 0.009), primarily because of reduced physical activity. During rapid weight loss, the negative energy balance (-850 +/- 580 kcal per day) was primarily the result of the reduction in energy intake, to 1330 +/- 610 kcal per day; intake correlated strongly with the rate of weight change (r = 0.84, P < 0.001). In patients with HIV infection, total energy expenditure is reduced during episodes of weight loss. Reduced energy intake, not elevated energy expenditure, is the prime determinant of weight loss in HIV-associated wasting.
