Orotic acid is a minor dietary constituent. Indeed, until it was realized that it could be synthesized by humans, orotic acid was known as vitamin B-13. The richest dietary sources of orotic acid are cow's milk and other dairy products as well as root vegetables such as carrots and beets. Dietary intake probably contributes to a basal rate of orotic acid excretion in urine because fasting decreases excretion by ~50%. However, it is now apparent that most urinary orotic acid is synthesized in the body, where it arises as an intermediate in the pathway for the synthesis of pyrimidine nucleotides. Orotic acid is converted to UMP by UMP synthase, a multifunctional protein with both orotate phosphoribosyltransferase and orotidylate decarboxylase activity. The most frequently observed inborn error of pyrimidine nucleotide synthesis is a mutation of the multifunctional protein UMP synthase (UMP synthase deficiency or orotic aciduria). This disorder prevents the conversion of orotic acid to UMP, and thus to other pyrimidines. As a result, plasma orotic acid accumulates to high concentrations, and increased quantities appear in the urine. Indeed, urinary orotic acid is so markedly increased in individuals harboring a mutation in UMP synthase that orotic acid crystals can form in the urine. The urinary concentration of orotic acid in individuals suffering from orotic aciduria can be of the order of millimoles per millimole creatinine. By comparison, the urinary level in unaffected individuals is ~ 1 mol/mmol creatinine. Orotic aciduria is characterized by megaloblastic anemia and orotic acid crystalluria that is frequently associated with some degree of physical and mental retardation. These features respond to appropriate pyrimidine replacement therapy and most cases appear to have a good prognosis. When present in sufficiently high levels, orotic acid can act as an acidogen and a metabotoxin. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of orotic acid are associated with at least seven inborn errors of metabolism, including argininemia, LPI syndrome (lysinuric protein intolerance), hyperornithinemia-hyperammonemia-homocitrullinuria (HHH), OTC deficiency, citrullinemia type I, purine nucleoside phosphorylase deficiency, and orotic aciduria. Orotic acid is an organic acid. Abnormally high levels of organic acids in the blood (organic acidemia), urine (organic aciduria), the brain, and other tissues lead to general metabolic acidosis. Acidosis typically occurs when arterial pH falls below 7.35. In infants with acidosis, the initial symptoms include poor feeding, vomiting, loss of appetite, weak muscle tone (hypotonia), and lack of energy (lethargy). These can progress to heart abnormalities, seizures, coma, and possibly death. These are also the characteristic symptoms of the untreated IEMs mentioned above. Many affected children with organic acidemias experience intellectual disability or delayed development. In adults, acidosis or acidemia is characterized by headaches, confusion, feeling tired, tremors, sleepiness, and seizures.