Publication Date

2014

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-[1-13C]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.

School/Institute

Mary MacKillop Institute for Health Research

Document Type

Journal Article

Access Rights

ERA Access

Access may be restricted.

Share

COinS