Age and aerobic exercise training effects on whole body and muscle protein metabolism

Aging in humans is associated with loss of lean body mass, but the causes are incompletely defined. Lean tissue mass and function depend on continuous rebuilding of proteins. We tested the hypotheses that whole body and mixed muscle protein metabolism declines with age in men and women and that aerobic exercise training would partly reverse this decline. Seventy-eight healthy, previously untrained men and women aged 19-87 yr were studied before and after 4 mo of bicycle training (up to 45 min at 80% peak heart rate, 3-4 days/wk) or control (flexibility) activity. At the whole body level, protein breakdown (measured as [¹³C]leucine and [¹⁵N]phenylalanine flux), Leu oxidation, and protein synthesis (nonoxidative Leu disposal) declined with age at a rate of 4-5% per decade (P < 0.001). Fat-free mass was closely correlated with protein turnover and declined 3% per decade (P < 0.001), but even after covariate adjustment for fat-free mass, the decline in protein turnover with age remained significant. There were no differences between men and women after adjustment for fat-free mass. Mixed muscle protein synthesis also declined with age 3.5% per decade (P < 0.05). Exercise training improved aerobic capacity 9% overall (P < 0.01), and mixed muscle protein synthesis increased 22% (P < 0.05), with no effect of age on the training response for either variable. Fat-free mass, whole body protein turnover, and resting metabolic rate were unchanged by training. We conclude that rates of whole body and muscle protein metabolism decline with age in men and women, thus indicating that there is a progressive decline in the body's remodeling processes with aging. This study also demonstrates that aerobic exercise can enhance muscle protein synthesis irrespective of age.