HCAR (hydroxycarboxylic acid receptor, also known as GPR109A) is a member of the G protein-coupled receptor family, characterized by seven transmembrane domains. It is primarily expressed in adipocytes, immune cells, and intestinal epithelial cells. HCAR, by recognizing hydroxycarboxylic acid and short-chain fatty acids such as butyric acid, is involved in regulating fat metabolism, anti-inflammatory responses, and intestinal health. Its specific expression and functional regulation are potentially important for the treatment of diabetes, obesity, and inflammatory bowel disease. Mutations or aberrant expression of HCAR are associated with the development of metabolic syndrome and immune-related diseases.