title: The use of doubly labeled milk protein to measure postprandial muscle protein synthesis rates in vivo in humans.
abstract: We aimed to determine the impact of precursor pool dilution on the assessment of postprandial myofibrillar protein synthesis rates (MPS). A Holstein dairy cow was infused with large amounts of L-[1-(13)C]phenylalanine and L-[1-(13)C]leucine, and the milk was collected and fractionated. The enrichment levels in the casein were 38.7 and 9.3 mole percent excess, respectively. In a subsequent human experiment, 11 older men (age: 71 ± 1 y, body mass index: 26 ± 0.1 kg·m(-2)) received a primed constant infusion of L-[ring-(2)H5]phenylalanine and L-[1-(13)C]leucine. Blood and muscle samples were collected before and after the ingestion of 20-g doubly labeled casein to assess postprandial MPS based on the 1) constant tracer infusion of L-[ring-(2)H5]phenylalanine, 2) ingestion of intrinsically L-[1-(13)C]phenylalanine-labeled casein, and 3) constant infusion of L-[1-(13)C]leucine in combination with the ingestion of intrinsically L-[1-(13)C]leucine-labeled casein. Postprandial MPS was increased (P < 0.05) after protein ingestion (∼70% above postabsorptive values) based on the L-[1-(13)C]leucine tracer. There was no significant stimulation of postprandial MPS (∼27% above postabsorptive values) when the calculated fractional synthesis rate was based on the L-[ring-(2)H5]phenylalanine (P = 0.2). Comparisons of postprandial MPS based on the primed continuous infusion of L-[1-(13)C]leucine or the ingestion of intrinsically L-[1-(13)C]phenylalanine-labeled casein protein demonstrated differences compared with the primed continuous infusion of L-[ring-(2)H5]phenylalanine (P > 0.05). Our findings confirm that the postprandial MPS assessed using the primed continuous tracer infusion approach may differ if tracer steady-state conditions in the precursor pools are perturbed. The use of intrinsically doubly labeled protein provides a method to study the metabolic fate of the ingested protein and the subsequent postprandial MPS response.
authors.affiliation: NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University, Maastricht, The Netherlands.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Burd
authors.affiliation: NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University, Maastricht, The Netherlands.
authors.forename: Naomi M
authors.initials: NM
authors.lastname: Cermak
authors.affiliation: NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University, Maastricht, The Netherlands.
authors.forename: Imre W K
authors.initials: IW
authors.lastname: Kouw
authors.affiliation: NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University, Maastricht, The Netherlands.
authors.forename: Stefan H
authors.initials: SH
authors.lastname: Gorissen
authors.affiliation: NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University, Maastricht, The Netherlands.
authors.forename: Annemie P
authors.initials: AP
authors.lastname: Gijsen
authors.affiliation: NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University, Maastricht, The Netherlands l.vanloon@maastrichtuniversity.nl.
authors.forename: Luc J C
authors.initials: LJ
authors.lastname: van Loon
mesh_terms: D000368:Aged; D002247:Carbon Isotopes; D002364:Caseins / Q000378:metabolism*; D006801:Humans; D007930:Leucine / Q000097:blood; D008297:Male; D009124:Muscle Proteins / Q000096:biosynthesis*; D009210:Myofibrils / Q000378:metabolism; D010649:Phenylalanine / Q000097:blood
chemical_list: D002247:Carbon Isotopes; D002364:Caseins; D009124:Muscle Proteins; D010649:Phenylalanine; D007930:Leucine
keywords: dietary protein; hypertrophy; nutrition; stable isotopes