EFFECTS OF PROTEIN RESTRICTION AND ACUTE REFEEDING ON LEUCINE AND LYSINE KINETICS IN YOUNG MEN

To explore the effects of altered protein intake on the uptake and uti lization of dietary amino acids in human subjects, six healthy male Ma ssachusetts Institute of Technology students were studied during three dietary periods each of 9 days, with tracer infusion studies conducte d on days 8 and 9. During one period the diet provided a generous prot ein intake (1.5 g . kg- 1 . day- 1) and during the other two a low-pro tein intake (0.1 g-kg-1.day-1). Tracer infusions (fed state) were give n while subjects received either a liquid formula, supplying the prior protein intake or (on day 9) a generous intake. Combinations of [H-2( 3)]Leu and [C-13]Leu and of [N-15]Lys and [H-2(3)]Lys were given via c ontinuous intravenous or intragastric infusion for 4 h to estimate leu cine kinetics and the first-pass splanchnic uptake of tracer. Dietary- induced changes in leucine flux and oxidation were similar irrespectiv e of the tracer and/or route of administration and sampled pool (leuci ne or KIC) used for calculation. Estimates of absolute changes in the rates of leucine disappearance via nonoxidative metabolism (protein sy nthesis) and of appearance via protein breakdown depended on the route of [C-13]Leu administration. However, it appears that both altered ra tes of protein synthesis and breakdown contribute to the adaptations r equired in the body nitrogen economy when protein intakes are restrict ed and subsequently resupplied. From the intragastric [C-13]Leu protoc ol, the first-pass splanchnic uptake of tracer was the same (approxima tely 30%) for generous, low-protein, and refed conditions. Hence, it i s tentatively concluded that when protein intake is low the splanchnic region does not remove indispensable amino acids to a disproportionat e extent, permitting a balanced mix of amino acids to reach the periph eral circulation.