Splanchnic versus whole-body production of alpha-ketoisocaproate from leucine in the fed state

The extent to which dietary branched-chain amino acids are deaminated by the splanchnic tissues (ie, the liver and gut) in the fed state and released as ketoacids into the systemic circulation is not known. To determine this, we combined the oral (L-[1-13C]-leucine, [13C]-Leu) and intravenous (L-[5,5,5-2H3]leucine, [2H3]-Leu) leucine tracer infusion with the intravenous administration of an independent isotope of the leucine ketoanalog alpha-ketoisocaproic acid (KIC) ([4,5-3H]KIC). The study was conducted during constant administration of a complete mixed meal. We found that 26% +/- 5% of the orally administered leucine was taken up by the splanchnic organs at first pass, whereas 74% +/- 5% appeared in the systemic circulation. The rate of splanchnic KIC release from deamination of dietary leucine accounted for 3% +/- 0.2% of the oral leucine administration rate and 13% +/- 2% of leucine splanchnic uptake (fractional splanchnic deamination). The fraction of whole-body total leucine uptake that was deaminated to KIC was 41% +/- 5% (P < .05 v fractional splanchnic deamination of dietary leucine uptake). We conclude that (1) the release of KIC from leucine deamination within splanchnic tissues constitutes a minimal fraction of first-pass dietary leucine uptake, and (2) splanchnic tissues are relatively less efficient than the whole body in KIC production from leucine deamination.