The use of doubly labeled milk protein to measure postprandial muscle protein synthesis rates in vivo in humans

Nicholas A. Burd, Naomi M. Cermak, Imre W K Kouw, Stefan H. Gorissen, Annemie P. Gijsen, Luc J C Van Loon

Research output: Contribution to journalArticle


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-[1-13C]phenylalanine and L-[1- 13C]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-2H5]phenylalanine and L-[1-13C]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-2H5]phenylalanine, 2) ingestion of intrinsically L-[1-13C]phenylalanine-labeled casein, and 3) constant infusion of L-[1-13C]leucine in combination with the ingestion of intrinsically L-[1-13C]leucine-labeled casein. Postprandial MPS was increased (P < 0.05) after protein ingestion (70% above postabsorptive values) based on the L-[1-13C]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-2H5]phenylalanine (P = 0.2). Comparisons of postprandial MPS based on the primed continuous infusion of L-[1-13C]leucine or the ingestion of intrinsically L-[1-13C]phenylalanine- labeled casein protein demonstrated differences compared with the primed continuous infusion of L-[ring-2H5]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.

Original languageEnglish (US)
Pages (from-to)1363-1370
Number of pages8
JournalJournal of Applied Physiology
Issue number11
StatePublished - Dec 1 2014
Externally publishedYes


  • Dietary protein
  • Hypertrophy
  • Nutrition
  • Stable isotopes

ASJC Scopus subject areas

  • Physiology
  • Physiology (medical)

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