Continuous oral stable isotope ingestion to measure whole-body protein turnover

Protein kinetic responses to nutrition and exercise interventions are commonly evaluated using a primed-constant infusion of stable isotope tracers. While this methodology is state-of-the-art, the required preparation at a certified pharmacy makes the utilization of isotope infusion both expensive and logistically cumbersome. Oral tracer ingestion has been used to quantify 24-h whole-body protein status; however, this does not permit examination of acute interventional effects. Ingestion of a priming bolus, followed by continuous ingestion of stable isotope tracer in a 'sip feeding' fashion may provide a more feasible alternative for quantifying acute kinetic responses. Therefore, the purpose of this study was to evaluate the viability of a primed continuous oral sip-ingestion method of stable isotope tracers for the evaluation of whole-body protein kinetics.

In a randomized, crossover design, eight healthy adults (63% female; Age: 29.4 ± 5.8 yrs; BMI: 24.3 ± 2.7 kg/m²) completed two, two-period stable isotope oral ingestion studies, consisting of a 3 h basal fasted period, followed by a 4-h post-ingestion period. After the basal period, subjects ingested either 6.3 g (Low) or 12.6 g (High) of an essential amino acid (EAA) enriched whey protein supplement. The continuous oral sip-feed method was initiated with a primed oral bolus dose of L-[ring-²H₅]phenylalanine, L-[ring-²H₂]tyrosine, and L-[ring-²H₄]tyrosine, followed by oral sip doses of L-[ring-²H₅]phenylalanine, L-[ring-²H₂]tyrosine every 10 min to approximate steady state tracer enrichment. Blood samples were taken throughout the basal and post-meal periods to determine tracer enrichment. Whole-body net protein balance (NB), synthesis (PS), breakdown (PB), and exogenous hydroxylation were calculated for each period. Repeated measure ANOVAs (treatment × time) were used to assess differences in protein kinetics.

Using the sip feed method, NB, PS, and hydroxylation were significantly increased with ingestion of protein (p < 0.05) during the postprandial period, regardless of amount of protein ingested; ΔNB from the postabsorptive to postprandial period was significantly greater for high compared to low protein (p = 0.026; low = 6.2 ± 5.1 g protein·240 min^-1; high = 11.8 ± 3.9 g protein·240 min^-1).

The current study provides preliminary evidence that continuous oral sip-feeding of stable isotope tracer is a feasible method that provides physiologically relevant measures of protein metabolism. Assessments of variance and individual responses revealed high measurement variability with the sip-feed method compared to previously published constant infusion responses, but ΔNB, ΔPS, and ΔPB were comparable. In situations where constant infusion is not feasible, oral sip-feeding could be used as an alternative method for measurement of acute, postprandial protein metabolism.