title: Habituation to low or high protein intake does not modulate basal or postprandial muscle protein synthesis rates: a randomized trial.
abstract: Muscle mass maintenance is largely regulated by basal muscle protein synthesis rates and the ability to increase muscle protein synthesis after protein ingestion. To our knowledge, no previous studies have evaluated the impact of habituation to either low protein intake (LOW PRO) or high protein intake (HIGH PRO) on the postprandial muscle protein synthetic response.
  
  
  
  We assessed the impact of LOW PRO compared with HIGH PRO on basal and postprandial muscle protein synthesis rates after the ingestion of 25 g whey protein.
  
  
  
  Twenty-four healthy, older men [age: 62 ± 1 y; body mass index (in kg/m2): 25.9 ± 0.4 (mean ± SEM)] participated in a parallel-group randomized trial in which they adapted to either a LOW PRO diet (0.7 g · kg-1· d-1; n = 12) or a HIGH PRO diet (1.5 g · kg-1· d-1; n = 12) for 14 d. On day 15, participants received primed continuous l-[ring-2H5]-phenylalanine and l-[1-13C]-leucine infusions and ingested 25 g intrinsically l-[1-13C]-phenylalanine- and l-[1-13C]-leucine-labeled whey protein. Muscle biopsies and blood samples were collected to assess muscle protein synthesis rates as well as dietary protein digestion and absorption kinetics.
  
  
  
  Plasma leucine concentrations and exogenous phenylalanine appearance rates increased after protein ingestion (P < 0.01) with no differences between treatments (P > 0.05). Plasma exogenous phenylalanine availability over the 5-h postprandial period was greater after LOW PRO than after HIGH PRO (61% ± 1% compared with 56% ± 2%, respectively; P < 0.05). Muscle protein synthesis rates increased from 0.031% ± 0.004% compared with 0.039% ± 0.007%/h in the fasted state to 0.062% ± 0.005% compared with 0.057% ± 0.005%/h in the postprandial state after LOW PRO compared with HIGH PRO, respectively (P < 0.01), with no differences between treatments (P = 0.25).
  
  
  
  Habituation to LOW PRO (0.7 g · kg-1· d-1) compared with HIGH PRO (1.5 g · kg-1· d-1) augments the postprandial availability of dietary protein-derived amino acids in the circulation and does not lower basal muscle protein synthesis rates or increase postprandial muscle protein synthesis rates after ingestion of 25 g protein in older men. This trial was registered at clinicaltrials.gov as NCT01986842.
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Center+, Maastricht, Netherlands.|Top Institute Food & Nutrition, Wageningen, Netherlands; and.
authors.forename: Stefan Hm
authors.identifier: 0000-0003-3737-9053
authors.initials: SH
authors.lastname: Gorissen
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Center+, Maastricht, Netherlands.|Top Institute Food & Nutrition, Wageningen, Netherlands; and.
authors.forename: Astrid Mh
authors.initials: AM
authors.lastname: Horstman
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Center+, Maastricht, Netherlands.
authors.forename: Rinske
authors.initials: R
authors.lastname: Franssen
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Center+, Maastricht, Netherlands.
authors.forename: Imre Wk
authors.initials: IW
authors.lastname: Kouw
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Center+, Maastricht, Netherlands.
authors.forename: Benjamin T
authors.initials: BT
authors.lastname: Wall
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Center+, Maastricht, Netherlands.|Top Institute Food & Nutrition, Wageningen, Netherlands; and.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Burd
authors.affiliation: Top Institute Food & Nutrition, Wageningen, Netherlands; and.|Division of Human Nutrition, Wageningen University, Wageningen, Netherlands.
authors.forename: Lisette Cpgm
authors.initials: LC
authors.lastname: de Groot
authors.affiliation: NUTRIM School of Nutrition and Translational Research in Metabolism, Maastricht University Medical Center+, Maastricht, Netherlands; l.vanloon@maastrichtuniversity.nl.|Top Institute Food & Nutrition, Wageningen, Netherlands; and.
authors.forename: Luc Jc
authors.initials: LJ
authors.lastname: van Loon
mesh_terms: D015502:Absorptiometry, Photon; D000368:Aged; D001786:Blood Glucose / Q000378:metabolism; D015992:Body Mass Index; D018753:Diet, Protein-Restricted; D004044:Dietary Proteins / Q000008:administration & dosage*; D005215:Fasting; D006801:Humans; D007328:Insulin / Q000097:blood; D007930:Leucine / Q000097:blood; D008297:Male; D008875:Middle Aged; D009124:Muscle Proteins / Q000096:biosynthesis*; D018482:Muscle, Skeletal / Q000378:metabolism*; D010649:Phenylalanine / Q000097:blood; D019518:Postprandial Period; D014176:Protein Biosynthesis; D000067816:Whey Proteins / Q000008:administration & dosage / Q000032:analysis
chemical_list: D001786:Blood Glucose; D004044:Dietary Proteins; D007328:Insulin; D009124:Muscle Proteins; D000067816:Whey Proteins; D010649:Phenylalanine; D007930:Leucine
keywords: habitual protein intake; muscle protein synthesis; protein digestion; stable-isotope tracers; whey protein