Glutamate Supplementation Regulates Nitrogen Metabolism in the Colon and Liver of Weaned Rats Fed a Low-Protein Diet

Background: Glutamate, a nutritionally non-essential amino acid, is a key intermediate in nitrogen metabolism. Despite more studies on its functional role in intestine health, it remains unknown how glutamate regulates nitrogen metabolism in animals fed a low-protein diet. Methods: Herein, we investigated the effects of glutamate supplementation on colonic amino acid transport, barrier protein expression, microbiota alterations, fecal nitrogen emissions, hepatic amino acid transport, and protein synthesis in weaned rats. Results: We found that protein restriction diminished the mucus thickness, reduced goblet cell numbers, and the expression of EAAT3, y+LAT2 in the colon. In contrast, glutamate supplementation reversed these effects, increasing the colon length and enhancing the expression of ZO-1, Occludin, and Claudin-1 in the colon. At the genus level, glutamate increased the abundance of Lactococcus and Clostridia_sensu_stricto_18. Additionally, glutamate supplementation resulted in an increased apparent nitrogen digestibility, reduced the ratio of fecal nitrogen to total nitrogen intake, and increased the ratio of fecal microbial nitrogen to total nitrogen intake. Protein restriction decreased the mRNA level of ATP1A1, EAAT3, SNAT9/2, and ASCT2, and the protein level of p-mTOR, mTOR, p-mTOR/mTOR, and p-p70S6K/p70S6K as well as p-4EBP1/4EBP1 in the liver. These effects were reversed by glutamate supplementation. Conclusions: In conclusion, glutamate supplementation upregulates amino acid transporters and barrier protein expression in the colon, modulates microbiota composition to reduce fecal nitrogen excretion, and enhances amino acid transport and protein synthesis in the liver by activating the mTOR/p70S6K/4EBP1 pathway, which influences nitrogen metabolism in weaned rats fed a low-protein diet.