1. Body composition changes with age and their consequences

Aging is associated with a decrease in skeletal muscle size and strength. LOSS of muscle mass with age in humans has been demonstrated both indirectly and directly. The excretion of urinary creatinine, reflecting muscle creatine content and total muscle mass, decreases by nearly 50% between the ages of 20 and 90 (TZANKOFF and NORRIS, 1978). Muscle and non-muscle mass can be calculated from total potassium and nitrogen; about 60% of the body's potassium is found in skeletal muscle, and the ratio of nitrogen is higher in muscle than in non-muscle lean tissue (COHN et al., 1978). Using total body K and N. Cohn and co-workers determined that skeletal muscle protein is reduced and non-muscle protein is maintained with advancing age.

Computerized tomography of individual muscles shows that, after age 30, there is a decrease in cross-sectional areas of the thigh along with decreased muscle density associated with increased intramuscular fat. These changes are more pronounced in women (IMAMURA et al., 1983). Muscle atrophy may result from a gradual and selective loss of muscle fibers. The number of muscle fibers in the midsection of the vastus lateralis of autopsy specimens is lower by about 110000 in elderly men (age 70-73) than in young men (age 19-37), a 23% difference (LEXELL et al., 1983). The decline is more marked in Type 11 muscle fibers, which fall from an average 60% of total fibers in sedentary young men to below 30% after the age of 80 (LARSSON, 1983), and is significantly associated with age-related decreases in strength (r=0.54, p<0.001).

Declining muscle mass is associated with a number of age-related changes. For example, declining skeletal muscle mass is closely associated with the age-related decrease in basal metabolic rate (TZANKOFF, 1978). As activity decreases, energy requirements are decreased in the elderly. Unfortunately, energy intake does not decrease to the same extent as energy requirements, with a resultant increase in body fat content with advancing age (EVANS and MEREDITH, 1989). Reduced muscle mass and activity are likely to be an important cause of age-related loss in bone mineral, resulting in osteoporosis (SANDIER, 1989).

Muscle atrophy and weakness are more prevalent among elderly individuals who develop hip fractures than those of similar age who do not (ANIANSSON et al., 1984). We have shown that resistance training is an effective way to increase both the size and strength of muscles in the elderly (FRONTERA et al., 1988; FIATARONE et al., 1990). Not only can resistance training increase muscle size, but a recent report indicated that long-term resistance training may prevent age-associated changes in histochemical fibre-type distribution, myosin heavy chain isoforms and tropomyosin isoforms (KLITGAARD et al., 1990a; b).

Regularly performed submaximal exercise by elderly men and women can result in important improvements in aerobic and muscle oxidative capacity (MEREDITH et al., 1989a), glucose tolerance and insulin sensitivity (HUGHES et al., in review) and has been demonstrated to prevent the onset of type II diabetes. Because of these positive outcomes seen in the elderly, increased and vigorous activity is recommended for men and women at all ages (EVANS and MEREDITH, 1989; EVANS, ROSENBERG and THOMSON, 1991; FIATARONE and EVANS, 1990; FRONTERA and EVANS, 1986). This review will focus on the interaction between exercise, aging, and protein and energy metabolism.