Functional amino acids in swine nutrition and production

Amino acids were traditionally classified as nutritionally essential or nonessential for swine based on nitrogen balance and growth. It was also assumed without much evidence that pigs could synthesize sufficient amounts of all nonessential amino acids to support maximum production performance. Thus, over the past 50 years, much emphasis has been placed on the roles for dietary essential amino acids as building blocks for tissue proteins. Disappointingly, the current version of NRC does not recommend dietary requirements of so called 'nonessential amino acids' by neonatal, post-weaning, growing-finishing, or gestating pigs. However, a large body of literature shows that these nonessential amino acids, particularly glutamine and arginine, play important roles in regulating gene expression at both transcriptional and translational levels in animals. Additionally, both isotopic and digestive studies have established that large amounts of amino acids in the enteral diet are degraded by the small intestine during the first pass. Thus, only 5% of glutamate and aspartate, 30-33% of glutamine, and 60-65% of proline and arginine in the diet enter the portal circulation. Dynamic synthesis of amino acids via inter-organ metabolism depends on essential amino acids and may be suboptimal in pigs at various stages of the life cycle. Amino acids participate in cell signaling via mammalian target of rapamycin, AMP-activated protein kinase, extracellular signal-related kinase, Jun kinase, mitogen-activated protein kinase, and gases (NO, CO and H2S). Exquisite integration of these regulatory networks has profound effects on cell proliferation, differentiation, metabolism, homeostasis, survival, and function. Importantly, recent advances in understanding of functional amino acids are transforming the practice of swine nutrition. Particularly, dietary supplementation