WHOLE-BODY PROTEIN-SYNTHESIS IN HEALTHY ADULT HUMANS - (CO2)-C-13 TECHNIQUE VS PLASMA PRECURSOR APPROACH

Different methods for the estimation of whole body protein synthesis (PS) in healthy adult humans were simultaneously compared in seven young adult subjects (6 males, 1 female) fed for 6 days a diet providing 1 g protein.kg(-1).day(-1) and similar to 188 kJ.kg(-1).day(-1). A 24-h intravenous tracer study with L-[1-C-13]leucine was performed starting at 6 P.M. on day 6. During fasted (6 h) and fed (6 h) steady states, PS was estimated using an approach based on (CO2)-C-13 excretion (PSexcr) and on urinary nitrogen excretion data (corrected for changes in body urea pool). Simultaneously, we used the conventional two-pool model and plasma [C-13]ketoisocaproate enrichment for estimating PS. The latter mean estimates of PS were significantly higher than PSexcr during fasting [861 +/- 58 (SD) vs. 663 +/- 160 mg protein.kg(-1).6 h(-1); P < 0.01] and feeding (985 +/- 63 vs. 779 +/- 127 mg protein.kg(-1).6 h(-1); P < 0.01) and were much less variable. In hourly small-meal feeding, urinary nitrogen excretion was not a reliable index of body protein oxidation when measured over short periods of 6 h, thereby introducing a lack of precision in PSexcr. We suggest that application of the (CO2)-C-13 technique to measure PS in humans is limited by the need for relatively prolonged experimental periods of urine collection and tracer infusion within a given physiological state.