Leucine kinetics at graded intakes in young men: Quantitative fate of dietary leucine

To explore leucine metabolism in relation to leucine intake, five young adult men received an L-amino acid diet that supplied 40, 30, 20, and 10 mg leucine · kg^-1 · d^-1 for 6 d. A stable-isotope-tracer infusion study was then conducted for 5 h while subjects received an intragastric infusion of the test diet. Primed, constant infusions of L-[1-¹³C]leucine (intragastric) and L-[²H₃]leucine (intravenous) were given simultaneously. A final infusion study was conducted in subjects in the postabsorptive state after an additional 2 d with the 10-mg diet. Estimates were made of leucine flux and oxidation, rates of uptake and release of absorbed leucine by the splanchnic region, and leucine balance. The rate of appearance of dietary leucine in the systemic circulation (Leu(d)) decreased (p < 0.01) between the 40- and 10-mg diets. At the latter intake, splanchnic uptake was ~ 37% of absorbed leucine. The correlation between Leu(d) and plasma leucine concentration was highly positive. A leucine intake of ~ 40 mg · kg^-1 · d^-1 was close to that required to maintain leucine balance under these conditions.