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Forearm and leg amino acid metabolism in the basal state and during combined insulin and amino acid stimulation after a 3‐day fast
Author(s) -
Gjedsted J.,
Gormsen L.,
Buhl M.,
Nørrelund H.,
Schmitz O.,
Keiding S.,
Tønnesen E.,
Møller N.
Publication year - 2009
Publication title -
acta physiologica
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.591
H-Index - 116
eISSN - 1748-1716
pISSN - 1748-1708
DOI - 10.1111/j.1748-1716.2009.02009.x
Subject(s) - phenylalanine , amino acid , medicine , endocrinology , metabolism , tyrosine , insulin , gluconeogenesis , protein metabolism , basal (medicine) , leucine , chemistry , biology , biochemistry
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.