Adaptation of body protein metabolism in adult and aging man.
Aging causes a selective loss in lean body mass, muscle being most extensively eroded. This presentation is an analysis of the consequences of this age-related loss for protein and amino acid metabolism. The use of stable isotopes allows us to measure the dynamics of protein metabolism in the intact human subject, while the output of 3-methylhistidine permits estimates of muscle turnover to be made which show a smaller reduction in body protein turnover per kg body weight in elderly subjects. This is due to the reduced contribution by muscle protein turnover, which decreases from 30 per cent of total body protein turnover in the young adult to 20 per cent in the elderly, and can be entirely accounted for by the reduced mass of muscle, the rate of turnover per unit muscle weight being unchanged. The consequences of the changing proportions of major tissues with age has been examined for its impact on amino acid metabolism. The interaction of various organs in order to ensure metabolism of different amino acids is described. After a meal containing abundant protein, the liver selectively degraded the excess of most essential amino acids, while allowing the three branched-chain essential amino acids to pass through, so that in consequence they account for more than 50 per cent of the amino acid outflow from the splanchnic area. During this period of absorption muscle takes up amino acids, over 70 per cent being the branched-chain amino acids where they are catabolised, the nitrogen becoming available for formation of non-essential amino acids for muscle protein synthesis. The release of large amounts of glutamine from muscle is dependent on branched-chain amino acid degradation, whereas the release of large amounts of alanine is regulated by availability of pyruvate from glucose. The study of amino acid metabolism in elderly compared with young adults and its relationship to the declining mass of muscle with age is still in its initial stages. Metabolism of glycine at different levels of protein intake fails to demonstrate any significant effects of aging on metabolism of this amino acid. On the other hand, the flux of leucine shows a significant reduction in fasting elderly female subjects. This observation should be followed up by studying leucine kinetics following a meal of meat, in order to determine whether the reduced mass of muscle in the elderly will restrict the capacity to metabolise branched-chain amino acids. It has been concluded that the extensive age-related change in muscle mass should be explored for its impact on the nutritional requirements for branched-chain amino acids, their metabolism in health and disease, and their therapeutic uses at different ages.
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USDA Human Nutrition Research Centre on Aging, Tufts University, Boston, Massachusetts, USA.