LEUCINE KINETICS ARE DIFFERENT DURING FEEDING WITH WHOLE PROTEIN OR OLIGOPEPTIDES

To determine if the molecular form of nitrogen intake affects protein metabolism during feeding, 12 normal volunteers received, by continuou s nasogastric infusion, a protein or a peptide-based diet. Leucine kin etics (oral [C-13]leucine and intravenous [H-2(3)]leucine) were measur ed during the following three consecutive periods: first carbohydrates and lipids alone, then with either whole casein or oligopeptides in a randomized crossover design, with these two latter periods being ison itrogenous, isocaloric, and of identical amino acid compositions. Leuc ine concentration, turnover, oxidation, and nonoxidative disposal incr eased during nitrogen administration (all P < 0.01) and were higher wi th oligopeptides than with casein (242 +/- 44 vs. 188 +/- 31 mu mol/l; 2.75 +/- 0.45 vs. 2.23 +/- 0.31; 1.14 +/- 0.19 vs. 0.82 +/- 0.22 mu m ol.kg(-1).min(-1), all P < 0.001; 1.64 +/- 0.32 vs. 1.44 +/- 0.33 mu m ol.kg(-1).min(-1), P < 0.05, respectively). Endogenous leucine product ion was less inhibited by oligopeptides than by casein (0.82 +/- 0.41 vs. 0.38 +/- 0.31 mu mol.kg(-1).min(-1), P < 0.001), whereas splanchni c extractions were similar. Finally, leucine balance was more positive with casein than with oligopeptides (P < 0.001). In conclusion, the r esponse of leucine kinetics to feeding is modified by the molecular fo rm of nitrogen intake, with the oligopeptides inducing a higher oxidat ion and protein synthesis and a lesser inhibition of protein breakdown .