Basal metabolic rate (BMR) has been reported to decrease during semistarvation and during hypocaloric treatment of obesity. However, it remains controversial whether the decrease is due to change in body composition, or if it represents a downregulation in cellular metabolism. To examine this relationship, we reviewed studies of BMR in children, adolescents, adults, semistarved non-obese, anorexics, and weight-reducing obese. The relationship between BMR and fat-free mass (FFM) of children, lean adolescents, and lean and obese adults consuming sufficient energy could be described by a single line, BMR (MJ/d) = 2.44 + 0.084 FFM (SEE = 0.63, R2 = .80). Obese adolescents demonstrated BMRs greater than predicted and semistarved lean individuals demonstrated BMRs less than predicted by this relationship. Obese individuals demonstrated a reduced BMR during underfeeding, but less so than semistarved lean individuals. The reduction in BMR relative to FFM in semistarved lean individuals could not be explained by disproportionate reductions in body cell mass (BCM).