Amino acids AAs are not only building blocks of protein, but are also signalling molecules as well as regulators of gene expression and the protein phosphorylation cascade. Additionally, AAs are key precursors for syntheses of hormones and low-molecular weight nitrogenous substances with each having enormous biological importance. For example, physiological concentrations of AA metabolites e.g., nitric oxide, polyamines, glutathione, taurine, thyroid hormones, and serotonin are required for cell functions. Growing evidence shows that humans and animals have dietary requirements for all proteinogenic AAs. Mammals, birds and fish also have species- and age-dependent needs for some AA-related substances. However, elevated levels of other products e.g., ammonia, homocysteine, Hsub2subS, and asymmetric dimethylarginine are pathogenic factors for neurological disorders, oxidative stress, and cardiovascular disease. Thus, optimal amounts of AAs and their ratios in diets and circulation are cr