Three inborn errors are known in the pathway of fructose metabolism depicted in Fig. 9.1. Essential fructosuria is a harmless anomaly characterised by the appearance of fructose in the urine after the intake of fructose-containing food. In hereditary fructose intolerance (HFI), fructose may provoke prompt gastrointestinal discomfort and hypoglycaemia upon ingestion, symptoms that may vary from patient to patient and depend on the ingested dose. Fructose may cause liver and kidney failure when taken persistently, and its intake becomes life threatening when it is given intravenously. Fructose-1,6-bisphosphatase (FBPase) deficiency is also usually considered an inborn error of fructose metabolism although, strictly speaking, it is a defect of gluconeogenesis.