Protein intake affects erythrocyte glutathione synthesis in young healthy adults in a repeated-measures trial

In 2005, the Institute of Medicine advised using methods other than nitrogen balance (NB) for determining protein requirements. Since then, protein requirements using indicator amino acid oxidation (IAAO) have been published and are higher than NB. Glutathione (GSH), a tripeptide of cysteine, glutamate, and glycine, is a principal antioxidant that can be used as a functional indicator of protein adequacy.

The aim of this study was to measure changes in erythrocyte GSH kinetics [fractional synthesis rate (FSR) and absolute synthesis rate (ASR)] in healthy adults following a range of protein intakes at and above the current recommendations.

Sixteen healthy adults [8 males and 8 females, aged 25.6 ± 0.9 y (mean ± SEM)] were studied at 4 of 6 protein intakes ranging from 0.6 to 1.5 g⋅kg^-1⋅d^-1. Erythrocyte GSH kinetics were assessed during a 7-h infusion of [U-¹³C₂-¹⁵N]glycine following 2 d of adaptation to each protein intake. Blood and urine tests were performed to measure oxidative stress markers, plasma homocysteine, triglycerides, plasma amino acid concentrations, 5-L-oxoproline (5-OP), and urinary sulfate. The protein intake that maximized GSH synthesis was determined using mixed-effect change-point regression in R. Primary and secondary outcomes were analyzed using linear mixed-effects and repeated-measures analysis of variance with Tukey's post hoc test.

The protein intake that maximized GSH FSR at 78%⋅d^-1 was 1.0 g⋅kg^-1⋅d^-1 (95% confidence interval: 0.63, 1.39). GSH ASR was significantly lower at 0.6 and 0.8 g⋅kg^-1⋅d^-1 than at 1.5 g⋅kg^-1⋅d^-1 (2.03 and 2.17, respectively, compared with 3.71 mmol⋅L^-1⋅d^-1). Increasing the protein intake led to increased urinary sulfate but did not affect erythrocyte GSH concentration, plasma oxidative stress markers, triglycerides, homocysteine, or 5-OP.

A protein intake of 1.0 g⋅kg^-1⋅d^-1 maximized GSH synthesis, which is in agreement with earlier IAAO-derived protein requirements of 0.93 to 1.2 g⋅kg^-1⋅d^-1. These findings suggest that recommendations based on NB (0.66 g⋅kg^-1⋅d^-1) may underestimate protein needs for adequate health. This trial was registered at clinicaltrials.gov as NCT02971046.