The objectives of the present study were to determine the splanchnic extraction of glutamine after ingestion of glutamine-rich protein (¹⁵N-labeled oat 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-[1-¹³C]glutamine (3 µmol · kg¹ · h¹) and L-[1-¹³C]lysine (1.5 µmol · kg¹ · h¹) for 8 h. Four hours after the beginning of the infusion, subjects consumed (every 20 min) a liquid formula providing either 2.5 g of protein from ¹⁵N-labeled oat proteins or a mixture of free amino acids that mimicked the oat-amino acid profile and contained L-[2,5-¹⁵N₂]glutamine and L-[2-¹⁵N]lysine. Splanchnic extraction of glutamine reached 62.5 ± 5.0% and 66.7 ± 3.9% after administration of ¹⁵N-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 ¹⁵N-labeled oat proteins and free amino acids, respectively). The main conclusion of the present study is that glutamine is equally bioavailable when given enterally as a free amino acid and when protein bound. Therefore, and taking into 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.