Leucine oxidation in its excess in growing rats: Proposal of the method to determine safe intake level for leucine

Abstract

Animals have ability to adapt to various dietary intake levels of protein and amino acids (AA) by controlling enzymatic activities. Therefore, we hypothesized that leucine (Leu) intakes within its catabolic capacity are safe levels, and investigated Leu‐oxidizing capacity in growing rats.Male F344 rats (N=88) were accustomed to 9h of feeding of diets containing graded doses of additional Leu (0–30%) for 14 days. After 15h of fasting period, rats were given diets in which parts of additional Leu were replaced with L‐[1–13C]Leu. Breath CO2, plasma Leu and plasma . α‐ketoisocaproate (KIC) after 6h feeding were analyzed using IRMS, AA analyzer and GCMS, respectively. Rates of Leu oxidation (decarboxylation of KIC) were calculated from the rates of 13CO2 production and 13C‐enrichments of plasma KIC. Plasma Leu concentrations and rates of Leu oxidation increased depending on the dosage of additional Leu. However, higher Leu load (20–30%) did not increase the oxidation rates further. Correlation analysis of these two parameters resulted in two linear regression segments that met at a breakpoint (0.8 mM of plasma Leu), suggesting saturation of its catabolic pathway above the breakpoint. The breakpoint occurred at 5 to 6% of additional Leu, which was less than no observed adverse effect level (NOAEL) of Leu in growing rats. Thus, determination of metabolic capacity would be one of keys to estimates safe intake level of Leu.