Objective: We investigated the effect of glutamine supplementation on plasma glutamine (Gln), glutamate (Glu), and whole-blood glutathione (GSH) concentrations in human volunteers.
Methods: Subjects first adapted to a standard diet with known intakes of protein, total GSH, cysteine, methionine, and total Glu (Glu values include Glu and Gln) for 3 d. Plasma Gln, Glu, and whole-blood GSH levels were then measured at 4-h intervals over 24 h. Supplemental oral Gln (0.3 g x kg(-1) x d(-1)) was ingested for 10 d and then 24-h plasma levels of Gln, Glu, and whole-blood GSH were measured.
Results: The plasma concentrations of Glu (116%; P = 0.006) and Gln (20%; P = 0.046) were significantly higher, whereas concentrations of GSH were significantly lower (37%; P = 0.00091) after oral Gln supplementation.
Conclusion: Oral Gln increases Glu and Gln levels in plasma of healthy subjects but does not increase GSH red cell (whole-blood) levels. Thus, GSH biosynthesis and preservation of GHS stores in red blood cells may involve rate-limiting substrates other than Gln.