Amino acids (AA) are not only building blocks of proteins but also play roles in cell signaling, gene regulation, and metabolic pathways. They are precursors for hormones and nitrogenous substances with significant biological importance. Physiological levels of AA and their metabolites are essential for normal function, while elevated levels can lead to diseases such as neurological disorders, oxidative stress, and cardiovascular disease. An optimal balance of AA in diet and circulation is crucial for overall health. Some AA, known as functional AA, regulate key metabolic processes necessary for maintenance, growth, reproduction, and immunity. These include arginine, cysteine, glutamine, leucine, proline, and tryptophan. Dietary supplementation with these AA may help alleviate health issues at various life stages, such as fetal growth restriction, neonatal mortality, weaning-related intestinal dysfunction, obesity, diabetes, cardiovascular disease, and infertility. They can also enhance metabolic efficiency, improve muscle growth, milk production, egg and meat quality, and athletic performance, while preventing excessive fat deposition and reducing adiposity. AA have important roles in both nutrition and health. AA are divided into α-AA, which are building blocks of proteins, and non-α-AA, which also play roles in metabolism. Skeletal muscle is the largest reservoir of AA in the body. Research has focused on defining optimal AA requirements in various species under different conditions. The small intestine is a major site for AA catabolism, affecting their entry into the portal circulation and plasma patterns. There is growing interest in the regulatory functions of L- and D-AA in nutrition and physiology, as well as the underlying cellular and molecular mechanisms. Although each AA has unique catabolic pathways, many share common characteristics. Important metabolites include ammonia, CO2, fatty acids, glucose, H2S, ketone bodies, nitric oxide, urea, uric acid, polyamines, and other nitrogenous substances. Complete oxidation of AA carbons requires conversion to acetyl-CoA, which is oxidized to CO2 and H2O via the Krebs cycle and mitochondrial electron transport system. Oxidation of AA is less efficient for ATP production compared to fat and glucose. The efficiency of energy transfer from L-AA to ATP ranges from 29% for methionine to 59%. The text also provides several chemical reactions involved in AA catabolism.Amino acids (AA) are not only building blocks of proteins but also play roles in cell signaling, gene regulation, and metabolic pathways. They are precursors for hormones and nitrogenous substances with significant biological importance. Physiological levels of AA and their metabolites are essential for normal function, while elevated levels can lead to diseases such as neurological disorders, oxidative stress, and cardiovascular disease. An optimal balance of AA in diet and circulation is crucial for overall health. Some AA, known as functional AA, regulate key metabolic processes necessary for maintenance, growth, reproduction, and immunity. These include arginine, cysteine, glutamine, leucine, proline, and tryptophan. Dietary supplementation with these AA may help alleviate health issues at various life stages, such as fetal growth restriction, neonatal mortality, weaning-related intestinal dysfunction, obesity, diabetes, cardiovascular disease, and infertility. They can also enhance metabolic efficiency, improve muscle growth, milk production, egg and meat quality, and athletic performance, while preventing excessive fat deposition and reducing adiposity. AA have important roles in both nutrition and health. AA are divided into α-AA, which are building blocks of proteins, and non-α-AA, which also play roles in metabolism. Skeletal muscle is the largest reservoir of AA in the body. Research has focused on defining optimal AA requirements in various species under different conditions. The small intestine is a major site for AA catabolism, affecting their entry into the portal circulation and plasma patterns. There is growing interest in the regulatory functions of L- and D-AA in nutrition and physiology, as well as the underlying cellular and molecular mechanisms. Although each AA has unique catabolic pathways, many share common characteristics. Important metabolites include ammonia, CO2, fatty acids, glucose, H2S, ketone bodies, nitric oxide, urea, uric acid, polyamines, and other nitrogenous substances. Complete oxidation of AA carbons requires conversion to acetyl-CoA, which is oxidized to CO2 and H2O via the Krebs cycle and mitochondrial electron transport system. Oxidation of AA is less efficient for ATP production compared to fat and glucose. The efficiency of energy transfer from L-AA to ATP ranges from 29% for methionine to 59%. The text also provides several chemical reactions involved in AA catabolism.