Circadian variations in plasma and erythrocyte concentrations of glutamate, glutamine, and alanine in men on a diet without and with added monosodiumglutamate

Variations in plasma and erythrocyte concentrations of glutamate, glutamine . and alanine during the day were studied in 10 healthy men fed ordinary Ta iwanese meals, first without end, 1 week later, with monosodium glutamate ( MSG) added. MSG at a level of 15, 40, and 45 mg/kg (total, 100 mg/kg/d) was added, respectively, to the breakfast, lunch, and dinner meals. Heparinize d blood samples were collected over 24 hours with 1- to 3-hour intervals. I n both trials, plasma glutamate concentrations increased significantly afte r lunch and dinner. Although the circadian variations of plasma glutamate w ere small (between 32 and 53 mu mol/L), the levels nevertheless varied sign ificantly as a function of the time of day in both trials. Considering that the dietary intake of glutamate was high when MSG was added, the low plasm a glutamate concentration over 24 hours indicates that glutamate is activel y metabolized. On the other hand, the concentrations of erythrocyte glutama te (507 to 631 mu mol/L) and glutamine (427 to 613 mu mol/L) did not show a significant postprandial increase or circadian variation. Nevertheless, th e concentration of plasma glutamine (539 to 657 mu mol/L) varied significan tly as a function of time in both trials. The plasma concentration of alani ne (274 to 494 mu mol/L) increased significantly after each meal and decrea sed significantly from 2:00 to 5:00 AM in both trials. Both plasma and eryt hrocyte alanine concentrations varied significantly as a function of time. These results show that the substantial amount of MSG intake had no apparen t effect on the circadian variation profiles of blood glutamate, glutamine, and alanine. Copyright (C) 1999 by W.B. Saunders Company.