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

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.