A significant amount of metabolic energy can come from amino acid metabolism, particularly under conditions of starvation. The metabolism of amino acids occurs through common metabolic intermediates, many of them part of or linked to the Krebs cycle. The intermediates like pyruvate, oxaloacetate, fumarate, succinyl-CoA and alpha-ketobutyrate all can contribute to the net synthesis of glucose through gluconeogenesis. The amino acids that are degraded into these intermediates are called glucogenic. Some amino acids are degraded directly to gluconeogenic intermediates, while others contribute to gluconeogenesis more indirectly, such as through conversion to oxaloacetate through the Krebs cycle. Other common intermediates in amino acid metabolism are acetyl-CoA and acetoacetate. Animals do not have a metabolic pathway to convert these intermediates into glucose. Instead the metabolic fate of acetyl-CoA and acetoacetate is the production of fatty acids or ketone bodies. Amino acids that are metabolized to produce acetyl-CoA and acetoacetate are called ketogenic. Several of the amino acids do not fall cleanly into one group or another, but are both ketogenic and glucogenic. For example, isoleucine metabolism produces both acetyl-CoA, which makes it ketogenic, but it also produces succinyl-CoA, which contributes to glucose production.
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