(P16-037-23) The Influence of Graded Protein Intakes on Erythrocyte Glutathione Synthesis: A Functional Measure of Protein Needs in Healthy Young Adults

Objectives: The primary objective of the study was to measure changes in erythrocyte glutathione (GSH) kinetics in healthy adults following a range of protein intakes above and below the current protein recommendations.

Methods: Sixteen healthy adult men and women (25.6 ± 0.9 y) were studied at 4 out of 6 levels of randomly assigned protein intakes ranging from 0.6 to 1.5 g/kg/d. Participants were habituated to a diet containing adequate calories and intact milk protein at the randomly assigned test level for 3 days. Erythrocyte GSH kinetics (fractional and absolute synthesis rates, FSR and ASR) were determined using the precursor-product method during a 7h infusion of [U-¹³C₂–¹⁵N]glycine on the third day. Additional blood and urine chemistries were performed to measure metabolites related to GSH: urinary sulfate excretion, plasma 5-L-oxoproline (5-OP), and oxidative stress markers (malondialdehyde, MDA; reactive oxygen metabolites, dROMs; and the ratio of GSH to GSH disulfide, GSG:GSSG). A total of 60 infusion studies were completed; 30 in males and 30 in females. The level of protein intake that maximized GSH synthesis was determined using mixed effect change-point regression in R to the GSH FSR data. The 95% CI of the breakpoint was calculated using the parametric bootstrap method. The remaining primary and secondary outcomes were analyzed using linear mixed-effects and repeated measures ANOVA with Tukey’s post-hoc test.

Results: The protein intake that maximized GSH FSR at 78 %/d was 1.01 g/kg/d (95% CI = 0.63 to 1.39 g/kg/d). GSH ASR was significantly lower at 0.6 and 0.8 g/kg/d compared to 1.5 g/kg/d protein intake (2.03 and 2.17 vs 3.71 mmol/L/d). Protein intake did not influence erythrocyte GSH concentration, GSH:GSSG, plasma oxidative stress markers, or 5-OP. Urinary sulfate increased linearly with increasing protein intake.

Conclusions: A protein intake of 1.0 g/kg/d maximizes GSH synthesis in healthy young adults. These findings agree with the IAAO-derived protein requirements of 0.93 to 1.2 g/kg/d however, are almost 30% higher than current protein recommendations of 0.66 and 0.8 g/kg/d which are based on nitrogen balance.

Funding Sources: This study was funded by The Canadian Institute of Health Research.