Acute effects of resistance exercise on muscle protein synthesis rate in young and elderly men and women.

Muscle mass and function are improved in the elderly during resistance exercise training. These improvements must result from alterations in the rates of muscle protein synthesis and breakdown. We determined the rate of quadriceps muscle protein synthesis using the in vivo rate of incorporation of intravenously infused [13C]leucine into mixed-muscle protein in both young (24 yr) and elderly (63-66 yr) men and women before and at the end of 2 wk of resistance exercise training. Before training, the fractional rate of muscle protein synthesis was lower in the elderly than in the young (0.030 +/- 0.003 vs. 0.049 +/- 0.004%/h; P = 0.004) but increased (P < 0.03) to a comparable rate of muscle protein synthesis in both young (0.075 +/- 0.009%/h) and elderly subjects (0.076 +/- 0.011%/h) after 2 wk of exercise. In the elderly, muscle mass, 24-h urinary 3-methylhistidine and creatinine excretion, and whole body protein breakdown rate determined during the [13C]leucine infusion were not changed after 2 wk of exercise. These findings demonstrate that, during the initial phase of a resistance exercise training program, a marked increase in quadriceps muscle protein synthesis rate occurs in elderly and young adults without an increase in the rate of whole body protein breakdown. In the elderly, this was not accompanied by an increase in urinary 3-methylhistidine excretion, an index of myofibrillar protein breakdown.