β-Hydroxybutyric acid, also known as 3-hydroxybutyric acid or BHB, is an organic compound and a beta hydroxy acid with the chemical formula CHCH(OH)CHCOH; its conjugate base is β-hydroxybutyrate, also known as 3-hydroxybutyrate. β-Hydroxybutyric acid is a chiral compound with two enantiomers: -β-hydroxybutyric acid and -β-hydroxybutyric acid. Its oxidized and polymeric derivatives occur widely in nature. In humans, -β-hydroxybutyric acid is one of two primary endogenous agonists of hydroxycarboxylic acid receptor 2 (HCA), a  G protein-coupled receptor (GPCR).