Whereas the role of muscle is central and obvious in syndromes
such as sarcopenia and cachexia, which are defined—at
least in part—by loss of muscle mass and strength, the potential
role of muscle in the prevention of obesity is less well appreciated.
The development of obesity results from an energy imbalance
over a prolonged time, which means that energy intake
exceeds energy expenditure. An effect on energy balance can
therefore be achieved by altering either energy intake or energy
expenditure. Total energy expenditure is the sum of resting energy
expenditure (REE), the thermic effect of food, and the energy
expenditure related to activity. Under most circumstances,
REE is the largest component of total energy expenditure (20).
The energy expenditure related to muscle metabolism is the only
component of REE that might vary considerably. The resting
metabolic requirements of splanchnic tissues, brain, and skin
vary little under normal conditions because of relatively constant
tissue mass and protein turnover rates (21). In contrast, large
variations in muscle mass are possible, and the rate of muscle
protein turnover (ie, muscle protein synthesis and breakdown)
mayvary as well. The synthesis and breakdown of muscle protein
are principally responsible for the energy expenditure of resting
muscle. Whereas the precise in vivo energetics of muscle protein
turnover are uncertain, a conservative estimate can be made on
the basis of muscle protein synthesis. The average 24-h (including
response to meal feeding) fractional synthetic rate (FSR) of
muscle protein is 0.075%/h (22). The absolute synthetic rate
can be calculated as the product of the FSR and muscle mass.We
have found the average muscle mass of young, healthy males to
range from 35 to 50 kg (22). In contrast, an elderly woman may
have13 kg muscle. Thus, muscle protein synthesis ranges from
0.23 to 0.90 kg/d, depending on the amount of muscle mass.
Because 4 mol ATP is utilized per mole of amino acids incorporated
into protein (21), and because the hydrolysis of 1 mol ATP
releases 20 kcal energy (23), the energy released per day as a