Free and protein-bound glutamine have identical splanchnic extraction in healthy human volunteers

The objectives of the present study were to determine the splanchnic extrac tion of glutamine after ingestion of glutamine-rich protein (N-15-labeled o at proteins) and to compare it with that of free glutamine and to determine de novo glutamine synthesis before and after glutamine consumption. Eight healthy adults were infused intravenously in the postabsorptive state with L-[1-C-13]glutamine (3 mu mol.kg(-1).h(-1)) and L-[1-C-13]lysine (1.5 mmol. kg(-1).h(-1)) for 8 h. Four hours after the beginning of the infusion, subj ects consumed (every 20 min) a liquid formula providing either 2.5 g of pro tein from N-15-labeled oat proteins or a mixture of free amino acids that m imicked the oat-amino acid profile and contained L-[2,5-N-15(2)]glutamine a nd L-[2-N-15]lysine. Splanchnic extraction of glutamine reached 62.5 +/- 5. 0% and 66.7 +/- 3.9% after administration of N-15-labeled oat proteins and the mixture of free amino acids, respectively. Lysine splanchnic extraction was also not different (40.9 +/- 11.9% and 34.9 +/- 10.6% for N-15-labeled oat proteins and free amino acids, respectively). The main conclusion of t he present study is that glutamine is equally bioavailable when given enter ally as a free amino acid and when protein bound. Therefore, and taking int o consideration the drawbacks of free glutamine supplementation of ready-to -use formulas for enteral nutrition, protein sources naturally rich in this amino acid are the best option for providing stable glutamine.