title: Forearm and leg amino acid metabolism in the basal state and during combined insulin and amino acid stimulation after a 3-day fast.
abstract: OBJECTIVE
  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 mumol kg(-1) h(-1), P < 0.01). During infusion of insulin and amino acids, amino acid concentrations increased.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Anaesthesia and Intensive Care, Aarhus University Hospital, Aarhus, Denmark. jakobgjedsted@dadlnet.dk
authors.forename: J
authors.initials: J
authors.lastname: Gjedsted
authors.forename: L
authors.initials: L
authors.lastname: Gormsen
authors.forename: M
authors.initials: M
authors.lastname: Buhl
authors.forename: H
authors.initials: H
authors.lastname: Nørrelund
authors.forename: O
authors.initials: O
authors.lastname: Schmitz
authors.forename: S
authors.initials: S
authors.lastname: Keiding
authors.forename: E
authors.initials: E
authors.lastname: Tønnesen
authors.forename: N
authors.initials: N
authors.lastname: Møller
mesh_terms: D000328:Adult; D000596:Amino Acids / Q000378:metabolism* / Q000494:pharmacology; D005215:Fasting / Q000378:metabolism*; D005542:Forearm / Q000502:physiology*; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D007866:Leg / Q000502:physiology*; D008297:Male; D010649:Phenylalanine / Q000378:metabolism; D011849:Radioactive Tracers; D012039:Regional Blood Flow; D014443:Tyrosine / Q000378:metabolism
chemical_list: D000596:Amino Acids; D007328:Insulin; D011849:Radioactive Tracers; D014443:Tyrosine; D010649:Phenylalanine