Although weight loss ameliorates many of the metabolic abnormalities associated with obesity, there has been reluctance to prescribe weight loss in obese, older individuals because of the fear that it will cause debilitating loss of muscle mass and impair physical function. To gain insight into the mechanisms responsible for the weight loss-induced changes in muscle mass, we measured the rate of muscle protein synthesis (by using stable isotope labeled tracer methodology) during basal, postabsorptive conditions and during mixed meal ingestion in eight obese, older adults: (i) before weight loss therapy, (ii) ∼3 months after starting the weight loss intervention (i.e., during the active weight loss phase), when subjects had lost ∼7% of their initial body weight, and (iii) after they had lost ∼10% of their body weight and maintained this new body weight for ∼6 months (∼12 months after starting the weight loss intervention). The basal muscle protein fractional synthesis rate (FSR) was not affected by weight loss. Mixed meal ingestion stimulated the rate of muscle protein synthesis, and the anabolic response (i.e., increase in the protein synthesis rate above basal values) was greater (P<0.05) during negative energy balance and active weight loss at 3 months (0.033 0.012%per hour, mean s.e.m.) than during weight maintenance before and at 12 months of weight loss therapy (0.003 0.003 and 0.008 0.012%per hour, respectively). We conclude that during dietary calorie restriction and weight loss in older adults, the rate of muscle protein synthesis is not impaired. Thus, the loss of muscle mass must be mediated predominately by adverse effects of dietary calorie restriction on muscle proteolysis.