We previously showed that the splanchnic bed of neonates takes up a significantly higher amount (43%) of threonine in contrast to lysine (12%). We questioned whether this same pattern applied in adult humans and found no information in the literature. Therefore, our objective in the current study was to determine the difference in splanchnic first pass disappearance of threonine and lysine in adult humans during the fed state. During two 5-d study periods, 6 healthy men received L-[1-13C]threonine or L-[1-13C]lysine enterally or parenterally randomized to either study d 3 or 5, respectively. The diets were in the form of an amino acid mixture providing a protein intake of 1 g · kg-1 · d-1 and an energy intake of 1.5 times the resting energy expenditure. Blood and breath samples were collected at baseline and plateau for amino acid and 13CO2 enrichment. Splanchnic disappearances in adult males were similar for threonine and lysine. The splanchnic disappearance of threonine as a percentage of intake was 17.9% (8.4 mg · kg-1 · d-1) in the healthy men compared with 18.5% (11.2 mg · kg-1 · d-1) for lysine. We conclude that the threonine requirement of the gut of healthy men consuming a liquid formula diet based on free amino acids is similar to the lysine requirement, which is in contrast to our previous findings for neonates.