Splanchnic bed utilization of enteral alpha-ketoisocaproate in humans

The branched-chain ketoacids (BCKAs) are used as dietary supplements to spa re essential amino acid nitrogen, yet little is known about their absorptio n and utilization in the body. To study the fate of enterally delivered alp ha-ketoisocaproate (KIC), seven healthy adults were infused in the postabso rptive state with [1-C-13]KIC and [phenyl-H-2(5)]phenylalanine intravenousl y (NGI) and with [5,5,5-H-2(3)]KIC by nasogastric tube (NG). After 3.5 hour s, the routes of tracer infusion were switched for an additional 3.5 hours. Each subject received a second infusion study on a different day with the order of tracer infusion reversed. KIC and phenylalanine kinetics and first -pass uptake and disposal of the enteral tracer by the splanchnic bed were calculated from the tracer enrichments measured in plasma KIC, leucine, and phenylalanine and breath CO2. Phenylalanine flux was 39.5 +/- 1.2 mu mol/k g/h during the IV infusion periods. KIC flux was 33.1 +/- 1.8 and 30.4 +/- 1.4 mu mol/kg/h measured with C-13- and H-2(3)-KIC, respectively, and these values were significantly different. The fraction of enterally delivered t racer sequestered by the splanchnic bed on the first pass was 30.9% +/- 2.0 %, 30.0% +/- 1.4%, and 30.7% +/- 2.7% for C-13-KIC, H-2(3)-KIC, and H-2(5)- phenylalanine, respectively. The fraction of infused C-13-KIC tracer recove red as (CO2)-C-13 was 27.1% +/- 1.2% and 24.0% +/- 0.9% during IV and NG in fusion, respectively. From these data, the fraction of ng KIC tracer extrac ted and oxidized on the first pass was calculated to be 5.1% +/- 1.1%. This fraction was greater than that previously reported for leucine extraction and oxidation (2%), but it was still only a small fraction of the overall e xtraction (5/30 = 16%). Because! the only two fates of the KIC tracer extra cted by the splanchnic bed are oxidation or transamination to leucine, the majority (84%) of the KIC tracer was extracted and converted to leucine. Th ese results demonstrate that KIC delivered enterally to postabsorptive huma ns is rapidly extracted and predominantly converted to leucine by the splan chnic bed. This leucine appears to be available for use by both the splanch nic bed and the whole body. Copyright (C) 1999 by W.B. Saunders Company.