Removal of glutamate from the diet enhanced postprandial leucine oxidation in the gut: evidence for nutritional significance of dietary glutamate to spare dietary leucine

Abstract

Glutamate (Glu), one of the most abundant amino acids in food proteins, plays crucial roles in the gut as major energy source and so on. Recently, we found that postprandial net‐release of branched chain amino acids (BCAAs) from the gut decreased in rats fed Glu‐free diet. Therefore, the present study aimed to clarify roles of dietary Glu on BCAA metabolism using [1‐13C]leucine (Leu).Amino acid defined diet containing 4% Glu (control diet) in accordance with NRC recommendation and diet in which all Glu was replaced with cornstarch (Glu‐free diet) were prepared using [1‐13C]Leu. Fisher strain male rats aged at 9 wks were given control or Glu‐free diets (2g/rat) after 12h fasting. Postprandial breath samples were collected for 7h to monitor 13CO2 production, and arterial and portal blood were collected 1, 2, 3 and 9h after the meal to measure concentrations of 13CO2 and [1‐13C]Leu.Postprandial net release of 13CO2 in expired air increased in rats fed Glu‐free diet by 20%. This increase was accompanied with the increase in portal 13CO2 appearance. Portal‐arterial differences in plasma [1‐13C]Leu concentration indicated decrease in postprandial release of ingested Leu from the gut in rats fed Glu‐free diet. These results suggest that dietary Glu substantially increases availability of ingested Leu for whole body by reducing its oxidation in the Gut. This is the first evidence suggesting sparing of Leu by dietary Glu.