Protein and amino acid metabolism and requirements in older persons.

In this short review, some aspects of body protein and amino-acid metabolism during aging in human subjects have been explored. There is a progressive diminution of total body protein with aging, due largely to a decline in the size of the skeletal muscle mass. These changes are accompanied by a shift in the overall pattern of whole body protein synthesis and breakdown, with muscle mass estimated to account for about 30 per cent of whole body protein turnover in the young adult, as compared with a lower value of 20 per cent or less in the elderly subject. Because skeletal muscle mass plays an important role in the response of body protein and amino-acid metabolism to stress, such as infection and trauma, this decline in the contribution of muscle to total body protein metabolism might be a factor responsible for the reduced ability of older people to withstand unfavorable circumstances. The determination of the dietary requirements for individual essential amino acids and for total protein has been discussed, and it is evident that the data are limited and often contradictory. However, elderly individuals are more likely to be affected by various biological, environmental, and social factors, which would generally increase protein needs above those for younger adults. Thus, in practice, the protein needs in the elderly are likely to be higher than those in the young. The decline in energy intake, together with its possible consequences for reducing the efficiency of dietary protein utilization, also will tend to increase the protein need for elderly subjects, relative to that for physically more active young adults. Until more data become available, it is recommended, for food planning purposes, that an appropriate protein allowance would be 12 to 14 per cent of the total energy intake, for mixed protein sources characteristic of the diets of industrialized countries or the more affluent sector of populations in developing countries. Energy intake should be at a level that meets the estimates proposed by FAO/WHO/UNU8 for older persons. Higher levels of protein intake would be required to achieve tissue protein repletion following periods of undernutrition or increased body protein (nitrogen) losses due to stressful stimuli of physical or psychological origin.