3-hydroxybutyric acid is a straight-chain 3-hydroxy monocarboxylic acid comprising a butyric acid core with a single hydroxy substituent in the 3- position; a ketone body whose levels are raised during ketosis, used as an energy source by the brain during fasting in humans. Also used to synthesise biodegradable plastics. It has a role as a human metabolite. It is a 3-hydroxy monocarboxylic acid, a hydroxybutyric acid and an (omega-1)-hydroxy fatty acid. It is functionally related to a butyric acid. It is a conjugate acid of a 3-hydroxybutyrate.