Forearm and leg amino acid metabolism in the basal state and during combined insulin and amino acid stimulation after a 3‐day fast

Aim: Fasting is characterized by a progressive loss of protein, but data on protein kinetics are unclear and few have studied the effects of re‐feeding. The present study was designed to test the hypothesis that a combined infusion of insulin and amino acids after fasting would induce compensatory increases in protein synthesis and reductions in protein breakdown at the whole body level and in muscle. Methods: We included 10 healthy male volunteers and studied them twice: (1) in the post‐absorptive state and (2) after 72 h of fasting. Amino acid kinetics was measured using labelled phenylalanine and tyrosine, whole body energy expenditure was assessed and urea nitrogen synthesis rates were calculated. Results: After fasting we observed an increase in arterial blood concentration of branched chain amino acids and a decrease in gluconeogenic amino acids (P < 0.05). Isotopically determined whole body, forearm and leg phenylalanine fluxes were unaltered apart from a 30% decrease in phenylalanine‐to‐tyrosine conversion (2.0 vs. 1.4 μmol kg^?1 h^?1, P < 0.01). During infusion of insulin and amino acids, amino acid concentrations increased. Conclusion: Our data indicate that after a 72‐h fast basal and insulin/amino acid‐stimulated regional phenylalanine fluxes in leg and forearm muscle are unaltered. During fasting concentrations of gluconeogenic amino acids decrease and hepatic and/or renal phenylalanine‐to‐tyrosine conversion decreases. Thus, as opposed to glucose and lipid metabolism, fasting does not induce insulin resistance as regards amino acid metabolism.