Simple Summary L-Glutamate (Glu) and L-glutamine (Gln) are abundant amino acids in feedstuffs and ruminants. Dietary Gln is extensively utilized by ruminal microbes, but dietary Glu undergoes little catabolism by these microbes because they do not take up extracellular Glu due to the lack of the necessary transporters. Microbial proteins and dietary Glu exit the rumen into the abomasum and then the small intestine, where proteins undergo hydrolysis to release amino acids (including Glu and Gln) and small peptides for transport into enterocytes. Most dietary Gln escapes the underdeveloped rumen of preruminants, instead entering the abomasum and the small intestine. Within the enterocytes, Glu and Gln are extensively oxidized to provide ATP and are actively used to synthesize glutathione and other amino acids (alanine, ornithine, citrulline, arginine, proline, and aspartate), whereas Gln and aspartate are essential for purine and pyrimidine syntheses. Under normal feeding conditions, all diet- and rumen-derived Glu and Gln are extracted by the small intestine and, therefore, do not enter the portal circulation. De novo synthesis plays a crucial role in maintaining the homeostasis of Glu and Gln in the whole body but may be insufficient for maximal growth performance, production (e.g., lactation and pregnancy), and optimal health in ruminants. Dietary supplementation with Glu or Gln can safely improve the digestive, endocrine, and reproduction functions of ruminants and thus augment health and production parameters.Abstract Although both L-glutamate (Glu) and L-glutamine (Gln) have long been considered nutritionally nonessential in ruminants, these two amino acids have enormous nutritional and physiological importance. Results of recent studies revealed that extracellular Gln is extensively degraded by ruminal microbes, but extracellular Glu undergoes little catabolism by these cells due to the near absence of its uptake. Ruminal bacteria hydrolyze Gln to Glu plus ammonia and, intracellularly, use both amino acids for protein synthesis. Microbial proteins and dietary Glu enter the small intestine in ruminants. Both Glu and Gln are the major metabolic fuels and building blocks of proteins, as well as substrates for the syntheses of glutathione and amino acids (alanine, ornithine, citrulline, arginine, proline, and aspartate) in the intestinal mucosa. In addition, Gln and aspartate are essential for purine and pyrimidine syntheses, whereas arginine and proline are necessary for the production of nitric oxide (a major vasodilator) and collagen (the most abundant protein in the body), respectively. Under normal feeding conditions, all diet- and rumen-derived Glu and Gln are extensively utilized by the small intestine and do not enter the portal circulation. Thus, de novo synthesis (e.g., from branched-chain amino acids and alpha-ketoglutarate) plays a crucial role in the homeostasis of Glu and Gln in the whole body but may be insufficient for maximal growth performance, production (e.g., lactation and pregnancy), and optimal health (particularly intestinal health) in ruminants. This applies to all types of feeding systems used around the world (e.g., rearing on a milk replacer before weaning, pasture-based production, and total mixed rations). Dietary supplementation with the appropriate doses of Glu or Gln [e.g., 0.5 or 1 g/kg body weight (BW)/day, respectively] can safely improve the digestive, endocrine, and reproduction functions of ruminants to enhance their productivity. Both Glu and Gln are truly functional amino acids in the nutrition of ruminants and hold great promise for improving their health and productivity.
