Aging, protein requirements, and protein turnover.

Current protein requirements for the elderly derive from 1985 FAO/WHO/UNU recommendations of no change with age in adults: i.e., 0.6 g/kg average and 0.75 g/kg safe allowance. Although concern has been expressed that protein requirements for the elderly may be greater, a review of nitrogen balance data, none of which are entirely satisfactory, indicates little reason for any revision. Furthermore, the 1985 recommendation is generally consistent with reports that the rate of whole-body protein turnover, a commonly assumed determinant of the protein requirement, exhibits minimal change with age per unit fat-free mass. Recent novel tracer studies aimed at evaluating protein requirements and turnover in a systematic way also support the 1985 recommendations. [1-13C]leucine balance studies have allowed measurement of metabolic demand from postabsorptive leucine oxidation and the efficiency of protein utilization from changes in leucine balance with feeding. The apparent protein requirement is metabolic demand divided by efficiency, an indication of protein needs and utilization during a standardized protocol at intakes similar to habitual ones. In healthy, mobile, elderly persons, metabolic demands are reduced by about one-third, with no significant impairment in efficiency of protein utilization. Thus, apparent protein requirements appear to fall with age from 0.98 +/- 0.17 to 0.69 +/- 0.22 g/kg. These changes with age reflect an improved restraint of proteolysis in the postabsorptive state, with little change with age in whole-body protein synthesis. The requirements of frail and immobile elderly and the efficiency of protein utilization of meals as eaten by elderly people remain to be evaluated.