Figure 1. Amino acids are degraded to compounds that can be metabolized to CO2 and H2O, or used in gluconeogenesis. Indeed the oxidative degradation of amino acids produces 10 – 15% of total metabolic energy in animals. The standard amino acids are degraded to one among the seven metabolic intermediates (pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate, acetyl-CoA or acetoacetate). Thus, amino acids may be divided into two groups, on the basis of their catabolic pathways: 1) gluconeogenic amino acids, which are catabolized to pyruvate, α-ketoglutarate, succinyl-CoA, fumarate or oxaloacetate, and are glucose precursors; 2) ketogenic amino acids, which are catabolized to acetyl-CoA or acetoacetate, and, thus, may be transformed into fatty acids or ketone bodies. Some amino acids are precursors both of carbohydrates and ketone bodies. Because mammals have no metabolic pathway which allows a net transformation of acetyl-CoA or acetoacetate to gluconeogenic precursors, no net synthesis of carbohydrates is possible from lysine and leucine, exclusively ketogenic amino acids. BCAAs, branched-chain amino acids.