Circadian variations in plasma and erythrocyte glutamate concentrations in adult men consuming a diet with and without added monosodium glutamate.

This study evaluated the effect of monosodium glutamate (MSG) ingestion as a component of the diet on the 24-h variations in plasma and whole-blood glutamate (GLU) concentrations in healthy adult men. In the first arm of the study, subjects were given test meals without added MSG for 3 d. Protein and energy intakes of the subjects were 1.5 g and 40 kcal/(kg body weight.d), respectively. On d 3, blood samples were collected over the 24-h period. One week later, the same protocol was repeated, except that 100 mg/(kg body weight.d) MSG was added to the meals (15, 40 and 45 mg/kg body weight to breakfast, lunch and dinner, respectively). Both plasma and whole-blood samples were analyzed for free amino acids. Unlike large neutral amino acids, which experienced high peak plasma concentrations at 2100-2300 h, the circadian variations in plasma GLU concentrations were small, varying between 33 and 48 micromol/L on days in which no MSG was fed, and between 32 and 53 micromol/L on days in which MSG was added to the meals. In both trials, plasma GLU concentration increased (P < 0.01) after lunch and dinner, and decreased early in the morning (P < 0.05). Calculated erythrocyte GLU concentrations varied between 500 and 640 micromol/L, with or without MSG addition to the meals. The rather low plasma GLU concentrations over the 24-h period, despite high dietary intake of MSG, indicate that dietary MSG is metabolized very rapidly.