Hydroxybutyric acid is a group of four-carbon organic compounds that have both hydroxyl and carboxylic acid functional groups. They can be viewed as derivatives of butyric acid. The carboxylate anion and the esters of hydroxybutyric acids are known as hydroxybutyrates. β-hydroxybutyric acid is relevant to human health as it is a member of a class of products of fatty acid oxidation referred to as ketone bodies.
There are three isomers, distinguished by the distance between the two functional groups: