Circadian variations in plasma and erythrocyte glutamate concentrations inadult 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 gl utamate (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 an d 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. O ne 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 br eakfast, lunch and dinner, respectively). Both plasma and whole-blood sampl es were analyzed for free amino acids. Unlike large neutral amino acids, wh ich experienced high peak plasma concentrations at 2100-2300 h, the circadi an variations in plasma GLU concentrations were small, varying between 33 a nd 48 mu mol/L on days in which no MSG was fed, and between 32 and 53 mu mo l/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 e arly in the morning (P < 0.05). Calculated erythrocyte GLU concentrations v aried between 500 and 640 mu mol/L, with or without MSG addition to the mea ls. The rather low plasma GLU concentrations over the 24-h period, despite high dietary intake of MSG, indicate that dietary MSG is metabolized very r apidly.