The impact of disuse and high-fat overfeeding on forearm muscle amino acid metabolism in humans

Abstract

CONTEXT: Anabolic resistance is mechanistically implicated in muscle disuse atrophy. OBJECTIVE: Assess whether anabolic resistance is associated with reduced postprandial amino acid uptake or exacerbated by excess lipid availability. DESIGN, SETTING, PARTICIPANTS AND INTERVENTIONS: Twenty men underwent 7 days of forearm immobilization while consuming a eucaloric (CON; n=11) or high-fat overfeeding (HFD; n=9; 50% excess energy as fat) diet (parallel design) within our Nutritional Physiology Research Unit. MAIN OUTCOME MEASURES: Pre- and post- immobilization we measured forearm muscle cross sectional area (aCSA), and postabsorptive and postprandial (3 h post ingestion of a liquid, protein rich, mixed meal) forearm amino acid metabolism using the arterialised venous-deep venous balance method and infusions of L-[ring-2H5]phenylalanine and L-[1-13C]leucine. RESULTS: Immobilization did not affect forearm muscle aCSA in either group, but tended to reduce postabsorptive phenylalanine (P=0.07) and leucine (P=0.05) net balances equivalently in CON and HFD. Mixed meal ingestion switched phenylalanine and leucine net balances from negative to positive (P<0.05), an effect blunted by immobilization (P<0.05) and to a greater extent in HFD than CON (P<0.05). Pre-immobilization, meal ingestion increased leucine rates of disappearance (Rd; P<0.05), with values peaking at 191% (from 87±38 to 254±60 µmol.min-1.100 mL forearm volume-1) and 183% (from 141±24 to 339±51 µmol.min-1.100 mL-1) above postabsorptive rates in CON and HFD, respectively, with meal induced increases not evident post-immobilization in either group (P>0.05). CONCLUSIONS: Disuse impairs the ability of a protein-rich meal to promote positive muscle amino acid balance, which is aggravated by dietary lipid oversupply. Moreover, disuse reduced postprandial forearm amino acid uptake; however, this is not worsened under high-fat conditions.