Glutamine is a potentially limiting amino acid for milk production in dairy cows: a hypothesis.

Recently, extensive research has been focused on glutamine because of its key position between energy and protein metabolism. Evidence is growing that glutamine is essential in situations of metabolic stress; practical application of this knowledge can already be found in parenteral nutrition of severely ill patients. Furthermore, glutamine is claimed to increase muscle protein synthesis. Glutamine and its counterpart, glutamic acid, are the most abundant amino acids in milk protein. Nevertheless, the role of nonessential amino acids (NEAA) in milk protein synthesis in high-yielding dairy cows has been practically neglected during the past 20 years. Evaluating current literature on glutamine metabolism in ruminants with emphasis on data related to milk protein production, we conclude the following: (1) Ruminants have a relatively low glutamine synthetase capacity compared with monogastric species, reflected in relatively low plasma glutamine levels; (2) The uptake of glutamine by the mammary gland is effectively 100% of the arterial supply; (3) Milk production in high-yielding dairy cows represents a metabolic stress comparable to fasting or acidosis; and (4) Responses of plasma and tissue glutamine pools in conditions of "metabolic stress," including high milk production, resemble those of most essential amino acids (EAA). Therefore we hypothesize that glutamine, although regarded as a NEAA, limits milk protein synthesis in high-yielding dairy cows.