Glutamine in animal science and production.

With its many proposed metabolic roles, glutamine would seem to have major potential in normal animal production systems as well as during situations involving adverse challenges. In practice, however, responses to glutamine supplementation have been inconsistent. Thus, during lactation and growth studies in ruminants, both positive and null effects on production responses have been reported. Similarly, therapeutic responses to glutamine supplementation during various digestive tract disorders have been inconsistent in both pigs and ruminants. This is despite a proven involvement in the nucleic acid biosynthesis necessary to support cell proliferation. In sheep, at least, glutamine may exert a protective effect against hepatic amino acid (AA) oxidation, particularly for methionine. This may offer anabolic potential because methionine is the first limiting AA in a number of animal feedstuffs. Glutamine is also important in control of metabolic acidosis, but, in contrast to rodents, the main site of production seems to be extra-hepatic. In the immune system, while lymphocyte proliferation is glutamine-dependent, intracellular concentrations are low (in contrast to other tissues, such as muscle and liver). Instead, glutamate is accumulated, but the majority of this (approximately 65%) is derived in vivo from plasma glutamine. In sheep, endotoxin challenge elevates the plasma flux of glutamine, with a corresponding decrease in plasma concentration. At the same time, both the glutamate accumulation and fractional rate of protein synthesis within lymphocytes are enhanced. These lymphocyte responses, however, are not altered by an AA supplement that contains glutamine. Overall, although glutamine obviously plays important metabolic roles within the body, supplementation does not appear to provide consistent beneficial or therapeutic effects, except during certain catabolic situations. Glutamine availability, therefore, does not seem to be a limitation in many challenge situations. Rather, glutamine may signal alterations in nutrient demands among organs and a better understanding of this role may increase understanding of where modulation of glutamine status would be beneficial.