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Circadian Variations in Plasma and Erythrocyte Glutamate Concentrations in Adult Men Consuming a Diet with and without Added Monosodium Glutamate^{1 ,2}

Department of Biochemistry, College of Medicine, National Taiwan University, Taipei, Taiwan

³To whom correspondence should be addressed.

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 µmol/L on days in which no MSG was fed, and between 32 and 53 µmol/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 µmol/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.

KEY WORDS: • glutamic acid • monosodium glutamate • plasma • erythrocytes • circadian variation • diet • humans