Changes in protein, carbohydrate, and fat metabolism with aging: Possible role of insulin

Age is associated with modifications of body composition, i.e., an increase in body fat mass and a decrease in protein mass. Because insulin controls substrate disposal and production, these changes could theoretically be rel ated to changes in either insulin action or secretion on the various substr ates. On the basis of available evidence, insulin action on whole-body amin o acid and protein metabolism seems not to be impaired in the aged. Decreas ed synthesis of contractile and mitochondrial proteins in muscle, associate d with decreased gene expression, was described in humans. Decreased physic al activity apparently represents an important factor responsible for decre ased muscle protein synthesis and mass in the elderly. Exercise in the elde rly may acutely revert these changes, although its chronic effects are stil l uncertain. In addition, the possible interaction between insulin and exer cise in the maintenance of muscle mass needs to be specifically investigate d in aged people. Higher free fatty acid (FFA) absolute flux and oxidation rates were observed in healthy elderly subjects in both the fasting state a nd following hyperinsulinemia, but not when normalized over fat mass. This suggests that FFA kinetics reflect the established changes in fat mass, ins ulin sensitivity on glucose metabolism is usually normal in the aged, despi te subtle impairments in insulin secretion, hepatic uptake, and onset of ac tion. Finally, data support the operation of the Randle cycle (i.e., invers e relationships between fat and glucose oxidation) in the elderly.