Glutamine and glutamate supplementation raise milk glutamine concentrations in lactating gilts

Glutamine is the most abundant free -amino acid in the body of most mammals and the majority of stored glutamine is concentrated in skeletal muscle [1,2]. Glutamine and glutamate comprise between 5 and 15% of the amino acid content of most foods and commercial feedstuffs, but since these amino acids are effectively metabolized by the epithelial cells of the small intestine there is little net absorption at normal dietary intake levels [1]. Thus, the large glutamine pool in the body arises from de novo synthesis in the skeletal muscle through the action of glutamine synthetase. Catabolic stress increases the need for glutamine by a number of tissues. This demand is met by increased glutamine release from skeletal muscle. Initially, release of glutamine from the existing intramuscular pool results in a drop in the intracellular glutamine concentration. However, continued release of glutamine requires increased synthesis. An increase in net proteolysis within the muscle cells provides the amino acid substrates for glutamine synthesis [3,4]. We recently determined that, in the horse, lactation represents a mild catabolic state accompanied by a loss of lean body mass and a decrease in muscle glutamine content [5]. Additional evidence supports a similar conclusion for a number of other species, including the pig [6-11].It is well established that during lactation there is increased glutamine utilization since glutamine is the most abundant amino acid in the milk of most species [12-17]. Additionally, enlargement of the intestines during pregnancy and lactation also increases the glutamine requirements of the intestinal epithelial cells [18]. The source of the substrates for increased glutamine synthesis to meet these demands has not been definitively identified, but it is generally thought that amino acids obtained from increased dietary protein intake would suffice [18]. However, our work with the horse suggests that, in addition to dietary amino acids, muscle proteolysi