Internal comparison between deuterium oxide (D2O) and L-[ring-13C6] phenylalanine for acute measurement of muscle protein synthesis in humans

Stable isotope tracer methodologies are becoming increasingly widespread in metabolic research; yet a number of factors restrict their implementation, such as, i.v infusions, multiple cannulae, tissue samples, and significant cost. We recently validated the sensitivity of the orally administered stable isotope tracer deuterium oxide (D2O) for quantifying day-to-day changes in muscle protein synthesis (MPS). This method is less invasive, restrictive, and more cost-effective than traditional amino acid (AA) tracer techniques. In the present study, we hypothesized the sensitivity of our analytical techniques (GC-Pyrolysis-IRMS) would permit D2O-derived measurements of MPS over much shorter periods (i.e., hours) usually only possible using AA-tracer techniques. We recruited nine males (24 ± 3 year, BMI: 25 ± 3 kg·m(-)²) into an internally controlled comparison of D2O versus (13)C AA-tracers. The day before the acute study subjects consumed 400 mL D2O, and on the study day, received a primed (0.3 mg·kg(-1)) continuous (0.6 mg·kg·h(-1)) i.v infusion of L-[ring-(13)C6]-phenylalanine to quantify MPS under both: (1) basal [postabsorptive] and; (2) stimulated [postprandial] that is, consumption of 20 g EAA, conditions. Measures of MPS yielded indistinguishable technique differences with respect to EAA, (13)C: 0.065 ± 0.004 to 0.089 ± 0.006%·h(-1) (P < 0.05) and D2O: 0.050 ± 0.007 to 0.088 ± 0.008%·h(-1) (P < 0.05) with qualitatively similar increases. Our findings reveal that acute measurement of MPS, usually only possible using AA-tracers, are feasible over shorter periods with orally administered D2O when used in tandem with GC-Pyrolysis-IRMS. We conclude that this D2O approach provides a less invasive, cost-effective, and flexible means by which to quantify MPS acutely over several hours.