Methylglutaric acid is a leucine metabolite. A large amount of methylglutaric acid is identified in urine of patients with deficiency of 3-methylglutaconyl coenzyme A hydratase (PMID 6181239 ). Methylglutaric acid is also found to be associated with 3-hydroxy-3-methylglutaryl-CoA lyase deficiency, another inborn error of metabolism.