The role of degradation in the acute control of protein balance in adult man: failure of feeding to stimulate protein synthesis as assessed by L-[1-13C]leucin infusion.

The effect of feeding on whole-body protein turnover was measured in six healthy volunteers using the essential amino acid, L-[1-13C]leucine, as a tracer for protein metabolism. Varied lengths of periods of feeding and isotope infusion produced different apparent responses to feeding. When parameters of protein turnover were estimated from 8-hour infusions, the change from post-absorptive in the first four hours to mixed feeding during the final four hours was found to produce positive leucine balance by decreasing degradation from 89.5 +/- 5.0 to 31.7 +/- 7.3 mumol leucine/kg/h (P less than .001), with no apparent change in synthesis. By contrast, when tracer was infused for 24 hours with 12 hours of feeding followed by 12 hours of fasting, the estimate of protein synthesis during feeding was 35% higher than during fasting (P less than .01). However, when tracer infusion during the 12-hour feeding/12-hour fasting protocol was limited to the last four hours of each nutritional period, the estimates of fed and fasted protein synthesis showed no significant difference, 71.3 +/- 6.5 and 66.2 +/- 5.6 respectively, while the calculated rate of protein degradation was 43% lower during feeding (P less than .002). As relatively higher levels of enrichment in plasma leucine were detected in comparable nutritional states following longer infusions, the possibility of significant recycling of label was investigated. Residual tracer was still detectable in both breath and plasma 12 hours after cessation of a 12-hour tracer infusion, supporting the conclusion that significant errors in estimates of protein turnover due to recycling of label arise with prolonged infusions.(ABSTRACT TRUNCATED AT 250 WORDS)