1. The variability between normal individuals in the efficiency of postprandial protein utilization (PPU), a determinant of the apparent protein requirement, was examined in relation to the relative responses of protein synthesis and proteolysis to protein feeding by means of [1-13C]leucine turnover and balance studies.2. Twenty-five healthy adults were infused intravenously with L-[1-13C]leucine continuously for 9 h. This was started in the postabsorptive state (PA, 3 h) and followed by low-protein feeding (LP, 3 h), and then by isoenergetic high-protein feeding (HP, 3 h). This allowed protein intake to be varied against a constant postprandial insulin level so that the extent of any amino-acid-mediated responses which were additional to those exerted by insulin could be investigated. Leucine oxidation, O, and balance (intake-oxidation), protein synthesis, S, and degradation, D, were calculated from plasma [1-13C]alpha-ketoisocaproic acid enrichment and 13CO2 excretion.3.PPUprotein, calculated as change in leucine balance/change in intake (HP-LP), varied from 0.58 to 0.99 (mean=0. 81+/-0.10), independently of age or sex. PPUprotein varied directly with the inhibition of D and inversely with the increase in leucine concentration and stimulation of O and S.4. Efficient PPU, as demonstrated by the top quintile of individuals categorized in terms of PPUprotein, involves maximal inhibition of D by protein feeding with minimal increases in free amino acid concentrations, O and S. Lesser inhibition of D and greater stimulation of S and O characterized the lower, less efficient quintile. This indicates that the efficiency of protein utilization in individuals, and a component of their apparent protein requirement, is determined by the sensitivity of the insulin-mediated inhibition of proteolysis to amino acid supply.