FIGURE 6-3 VO_2max declines with aging after normalizing for muscle mass, indicating that the decline in endurance capacity depends not only on muscle mass but also on other factors. Source: Rooyackers, O.E., D.B. Adey, P.A. Ades, and K.S. Nair. 1996. Effect of age on in vive rates of mitochondrial protein synthesis in human skeletal muscle. Proc. Natl. Acad. Sci. USA 93:15364-15369. Copyright 1996 National Academy of Sciences, U.S.A.

of muscle mass and increased muscle weakness and fatigability, which results in substantial impairment of muscle function, has been coined sarcopenia of aging and may contribute substantially to morbidity of the elderly by restricting physical activity, increasing the risk of falls and fractures, and causing changes in body metabolism and composition, which results in increased incidence of noninsulin-dependent diabetes mellitus.

It has been reported that in elderly in comparison with young subjects, there is a decline in the synthesis rate of mixed muscle protein—both total and myofibrillar proteins (Welle et al., 1993; Yarasheski et al., 1993). Interestingly, a recent study demonstrated that not only did synthesis of muscle mitochondrial proteins (pivotal to oxidative phosphorylation and ATP generation) decrease in the elderly, but also that this 40-percent decrease in mitochondrial protein synthesis occurred as early as middle age (average age 52 years) (Rooyackers et al., 1996) (Figure 6-4). The decline in mitochondrial protein synthesis was markedly more pronounced than the concomitant 10- to 15-percent decline in synthesis rates of mixed muscle proteins (Rooyackers et al., 1996). These changes were also associated with a decline in cytochrome-c-oxidase activity and endurance capacity (Rooyackers et al., 1996) (Figure 6-5). It is possible that the decline in mitochondrial protein synthesis may cause the impairment of endurance capacity and the more pronounced muscle fatigability in the aging population. In addition, robust ATP production is crucial for synthesis of other muscle proteins. A general decline in synthesis rates of several muscle proteins