title: Leucine supplementation chronically improves muscle protein synthesis in older adults consuming the RDA for protein.
abstract: OBJECTIVE
  Protein-energy supplementation is routinely employed to combat muscle loss. However, success is often compromised by increased satiety, poor palatability, high costs and low compliance.
  
  
  METHODS
  For 2-weeks we supplemented meals of older individuals with leucine (4 g/meal; 3 meals/day; days 2-14). Metabolic studies were performed prior to (Day 1) and following (Day 15) supplementation. Leucine was not provided on metabolic study days. Venous blood and vastus lateralis muscle biopsies were obtained during a primed constant infusion of L-[ring-(13)C(6)] phenylalanine. Mixed muscle fractional synthesis rate (FSR), body composition and markers of nutrient signaling (mTOR, 4E-BP1 and p70S6K1 phosphorylation) were measured before and after a low protein/carbohydrate simulated meal.
  
  
  RESULTS
  The meal modestly increased FSR on Day 1 (postabsorptive: 0.063 ± 0.004 vs. postprandial: 0.075 ± 0.006%/h; p = 0.03), however, two weeks of leucine supplementation increased postabsorptive FSR (p = 0.004) and the response to the meal (p = 0.01) (postabsorptive: 0.074 ± 0.007 vs. postprandial: 0.10 ± 0.007%/h). Changes in FSR were mirrored by increased phosphorylation of mTOR, 4E-BP1 and p70S6K1 (p ≤ 0.1). No change in fat free mass was observed (p > 0.05).
  
  
  CONCLUSIONS
  In older adults, leucine supplementation may improve muscle protein synthesis in response to lower protein meals.
authors.affiliation: Department of Internal Medicine, University of Texas Medical Branch, Galveston, TX 77555, USA.
authors.forename: Shanon L
authors.initials: SL
authors.lastname: Casperson
authors.forename: Melinda
authors.initials: M
authors.lastname: Sheffield-Moore
authors.forename: Susan J
authors.initials: SJ
authors.lastname: Hewlings
authors.forename: Douglas
authors.initials: D
authors.lastname: Paddon-Jones
mesh_terms: D000368:Aged; D000596:Amino Acids / Q000097:blood; D001786:Blood Glucose; D001823:Body Composition / Q000187:drug effects; D004040:Dietary Carbohydrates / Q000008:administration & dosage; D004044:Dietary Proteins / Q000008:administration & dosage* / Q000378:metabolism; D019587:Dietary Supplements; D005260:Female; D006801:Humans; D007328:Insulin / Q000097:blood; D007930:Leucine / Q000008:administration & dosage*; D008297:Male; D062407:Meals; D009124:Muscle Proteins / Q000096:biosynthesis*; D018482:Muscle, Skeletal / Q000737:chemistry; D018673:Nutrition Policy; D009751:Nutritional Requirements; D010649:Phenylalanine / Q000032:analysis / Q000378:metabolism; D019518:Postprandial Period; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism
chemical_list: D000596:Amino Acids; D001786:Blood Glucose; D004040:Dietary Carbohydrates; D004044:Dietary Proteins; D007328:Insulin; D009124:Muscle Proteins; D010649:Phenylalanine; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases; D007930:Leucine