ABSTRACT
By serving as substrates for the synthesis of both polypeptides and other nitrogenous substances that have enormous physiological significance, amino acids (AAs) have both nutritional and regulatory roles in humans and other animals. Cell signaling, protein modifications, antioxidative defense, chemical sensing, and epigenetic regulation of transcription are five emerging roles for AAs and their metabolites in organisms. These nutrients are essential to the functions of all cell types and, therefore, whole-body homeostasis, survival, growth, and development. AAs play important roles in preventing metabolic and infectious diseases and in treating certain disorders in digestive, neurological, muscular, reproductive, and cardiovascular systems. Increasing evidence suggests important roles for D-alanine, D-aspartate, and D-serine in neuroendocrine function, as well as for D-alanine in intracellular osmotic regulation in certain aquatic animals. Along with L-glutamate and glycine, the D-AAs modulate the activity of the N-methyl-D-aspartate receptor, which plays critical roles in synaptic plasticity, memory storage, excitotoxicity, neuronal development, and behavior. When diets cannot provide adequate amounts of certain AAs (e.g., arginine, glutamine, glutamate, glycine, proline, and tryptophan), their supplementation can beneficially prevent their deficiency, enhance food intake, protect cells from oxidative stress, optimize immune responses, improve health, modulate physical behavior, and alter body composition. When using AAs in dietary supplementation and medical therapy, their chemical properties, balances, antagonisms, and safety must be taken into consideration to maximize the desired effects and prevent any undesirable outcomes.
