PROTEIN-REQUIREMENTS AND AGING - METABOLIC DEMAND AND EFFICIENCY OF UTILIZATION

The protein requirements of the elderly were investigated with [C-13]l eucine balance studies of metabolic demand, the efficiency of postpran dial protein utilization (PPU) and the consequent apparent protein req uirement. Ten elderly subjects aged 68-91 years (five men and five wom en) and ten young adult subjects aged 21-31 years (five men and five w omen) were infused with L-[1-C-13]leucine for 9h commencing in the pos tabsorptive state (0-3h), continuing during the half-hourly feeding of low-protein meals (LP; protein 3% energy, 3-6h), and during similar f eeding of isoenergetic higher protein meals (HP; protein 15% energy, 6 -9h). Leucine oxidation and balance were determined from plasma [1-C-1 3]-alpha-ketoisocaproate enrichment and expired (CO2)-C-13 excretion m easured during the 3rd hour of each 3h period. The protein intake duri ng the HP phase was similar to the habitual intake estimated in the su bjects from 24h urinary N excretion, Metabolic demand was defined as e qual to twice the body-protein equivalent of measured postabsorptive l eucine oxidation. The efficiency of PPU was calculated from the increa sed leucine oxidation observed during feeding, and the apparent protei n requirement was defined as metabolic demand/PPU and calculated in re lation to both body weight (BW) and fat-free mass (FFM) determined by densitometry or bioimpedance. Metabolic demand in the young adults was 0.83g protein/kg per d; in both elderly groups it was 36% lower when expressed per kg BW and 30% lower when expressed per kg FFM. The appar ent protein requirement calculated from metabolic demand and PPU was 0 .99g protein/kg per d in the young adults and this was also lower in t he elderly, although this was only significant in the men (0.66g per k g BW, P = 0.013; 0.79g per kg FFM, P = 0.02), The results show that in this group of healthy elderly adults protein requirements as assessed from leucine balance studies were either similar to or less than thos e of younger adults.