Regulation of whole-body leucine metabolism with insulin during mixed-meal absorption in normal and diabetic humans

To determine the effects of insulin on dietary and endogenous leucine metabolism, five normal subjects, seven insulin-insufficient insulin-dependent (IDDM) diabetic patients, and five diabetic patients controlleed with continuous subcutaneous insulin infusion (CSII) were studied before and for 8 h after ingestion of a chemically defined elemental test meal (10 cal/kg) containing crystalline amino acids. L-[1-¹⁴C]leucine was included in the meal to trace the entry and oxidation of the dietary leucine. Total (meal + endogenous) entry of leucine into the circulation was estimated with a constant infusion of [²H₃]leucine. Postabsorptive and meal-related increases in the plasma leucine concentration were greater (P < .05) in the insulin-insufficient IDDM than in the normal subjects but returned to near-normal values with CSII. Baseline leucine flux was ~40% greater in the insulin-insufficient IDDM than in normal subjects (2.17 ± 0.17 vs. 1.55 ± 0.15 μmol·kg^-1·min^-1, respectively; .05 < P < .01) but were near normal during CSII treatment (1.85 ± 0.25 μmol·kg^-1·min^-1). Furthermore, total leucine entry during meal absorption was greater in the insulin-insufficient IDDM (1.41 ± 0.10 mmol·kg^-1·8 h^-1) than in either normal (0.96 ± 0.08 mmol·kg^-1·8 h^-1, P < .01) or IDDM subjects during CSII treatment (1.09 ± 0.11 mmol·kg^-1·8 h^-1, P < .05). Fractional oxidation (~40-50%) and entry of dietary leucine were similar in all three groups. The calculated rate of leucine entry from endogenous proteins over the 8 h of study in insulin-insufficient IDDM (1.04 ± 0.12 mmol·kg^-1·8 h^-1) was significantly greater than in normal subjects (0.64 ± 0.07 mmol·kg^-1·8 h^-1, P < .01) or in IDDM subjects during CSII treatment (0.76 ± 0.10 mmol·kg^-1·8 h^-1, P < .05). However, during meal absorption, the calculated rate of leucine entry from endogenous protein was significantly suppressed below baseline only in the normal subjects. In summary, during absorption of a mixed meal, 1) one-third of the leucine entering the peripheral plasma space is derived from the diet in normal humans; 2) the rate of absorption and oxidation of dietary leucine in IDDM is normal regardless of adequate of insulin therapy; and 3) postprandial hyperleucinemia in poorly controlled diabetic patients is at least partly the result of inadequate suppression of the entry rate of leucine from endogenous protein, suggesting persistence of excessive rates of proteolysis, which is at least partly corrected by intensified insulin treatment. Thus, insulin-mediated suppression of endogenous proteolysis is an integral part of protein anabolism during meal absorption.