Fig 1. A simplified overview of amino acid metabolism linked to glycolysis, fasting and gluconeogenesis and a linked model of isotope variation in carboxyl groups of amino acids.

(A) Glycolysis oxidizes glucose to CO₂ via intermediates such as pyruvate and acetyl CoA with final oxidation in the citric acid cycle. Amino acids are synthesized from intermediates in the glycolysis network (non-essential amino acids (NEAAs) are shown in bold). During fasting, lipids are broken down for energy and can also be used to synthesize glucose in gluconeogentic pathways [11,12] (B) Observed δ¹³C_CARBOXYL can vary depending on the net balance between new inputs and losses via decarboxylation. Inputs from new synthesis and diet generally lower δ¹³C_CARBOXYL of amino acids, while fractionation during decarboxylation increases values in the residual pool. Transamination is the most common mechanism by which amino acids are degraded in mammals, but does not involve bond changes at amino acid carboxyl positions, so will have little effect on δ¹³C_CARBOXYL.