Galactonic acid is a sugar acid that is a metabolic breakdown product of galactose. Galactose dehydrogenase is responsible for converting galactose to galactonolactone, which then spontaneously or enzymatically converts to galactonic acid. Once formed, galactonic acid may enter the pentose phosphate pathway. Galactonic acid is increased in red blood cells of galactosemic patients, due to a galactose-1-phosphate uridyltransferase (GALT) deficiency. When present in sufficiently high levels, galactonic 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 galactonic acid are associated with at least two inborn errors of metabolism, including galactosemia and galactosemia type II. Galactonic 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, liver abnormalities (jaundice), kidney abnormalities, seizures, coma, and possibly death. These are also the characteristic symptoms of untreated galactosemia. Many affected children with organic acidemias experience intellectual disability or delayed development. High levels of galactonic acid in infants are specifically associated with hepatomegaly (an enlarged liver), cirrhosis, renal failure, cataracts, vomiting, seizure, hypoglycemia, lethargy, brain damage, and ovarian failure.