Sarcopenia is defined as the age-associated decline in skeletal muscle mass (1). It is associated with weakness and poor functional capacity in older people (2). The etiology of sarcopenia is multifactorial and potential causes of loss of muscle include decreased physical activity, decreases in anabolic hormone level, decreased protein intake, poor nutritional status, and inflammation that may be secondary to increases in body fatness. In addition, hospitalization and bed rest may contribute to accelerated sarcopenia. Increased body fatness in older people has strong influence on functional capacity and may be a more powerful predictor of late-life disability than sarcopenia. If obesity results in reduced functional capacity in older people, voluntary weight loss by energy restriction is associated with increased mortality. This increased in risk may be caused by losses in skeletal muscle and bone mineral contents when weight loss is accomplished without exercise. We have demonstrated that consumption of the current Recommended Dietary Allowance for protein of 0.8 g/kg/d by older people results in a loss of muscle mass (3). Decreased energy intake to lose weight may result in an inadequate dietary protein intake and an accelerated loss of muscle. While it is now well known that resistance exercise training can increase muscle mass, strength and functional capacity even in very old people, the effects of bed rest in elderly people are less well understood. Our laboratory has explored the effects of 10 days of complete bed rest in healthy older people (mean age 67 years). We have reported (4) that even though these subjects consumed the recommended dietary allowance for protein during the 10 days of bed rest they experienced a decrease in nitrogen balance that was accompanied by a large loss of muscle mass and function. The fractional synthetic rate (FSR) of skeletal muscle protein was measured using a 24-hour infusion of 15N-phenyalanine and muscle biopsies. The 10 days of bed rest resulted in a loss of almost 1 kg of muscle from the legs and substantial decreases in VO2peak (-15.1%), strength (-19%), and a 30% reduction in FSR. This loss of muscle mass and function resulted in an overall decrease in physical activity after the bed rest period (5). This 10-day period of bed rest also results in a substantial increase in insulin resistance as measured by a euglycemic glucose clamp that may the cause of the decrease in skeletal muscle protein synthetic rate. Incorporating 15 minutes of walking each substantially attenuates the losses of muscle mass and function. Episodic period of inactivity in elderly people due to illness, depression, or hospitalization may result in losses in muscle mass and function that may be so large that recovery without aggressive rehabilitation becomes problematic.