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Hereditary Fructose Intolerance


Matt Demczko

, MD, Sidney Kimmel Medical College of Thomas Jefferson University

Last full review/revision Apr 2020| Content last modified Apr 2020

Hereditary fructose intolerance is a carbohydrate metabolism disorder that is caused by a lack of the enzyme needed to metabolize fructose. Very small amounts of fructose cause low blood sugar levels and can lead to kidney and liver damage. Fructose intolerance disorders occur when parents pass the defective genes that cause these disorders on to their children.

  • Fructose intolerance is caused by the lack of an enzyme needed to break down fructose.
  • Typical symptoms include low blood sugar, sweating, confusion, and kidney damage.
  • The diagnosis is made by analyzing a sample of liver tissue.
  • Treatment includes avoiding fructose in the diet and, when needed, taking glucose tablets.

Fructose is a sugar present in table sugar (sucrose), honey, and many fruits. Fructose is also present in sorbitol (a sugar substitute).

Children who have one of these disorders are missing one of the enzymes needed to break down (metabolize) fructose. As a result, a by-product of fructose accumulates in the body, blocking the formation of glycogen and its conversion to glucose for use as energy.

There are different types of inherited disorders. In hereditary fructose intolerance, both parents of the affected child carry a copy of the abnormal gene. Because usually two copies of the abnormal gene are necessary for the disorder to occur, usually neither parent has the disorder. (See also Overview of Hereditary Metabolic Disorders.)

Symptoms of Hereditary Fructose Intolerance

Ingesting more than tiny amounts of fructose or sucrose causes low blood sugar levels (hypoglycemia), with sweating, confusion, and sometimes seizures and coma. Affected children who continue to eat foods containing fructose develop kidney and liver damage, resulting in jaundice, vomiting, mental deterioration, seizures, and death.

Chronic symptoms include poor eating, failure to thrive, digestive symptoms, liver failure, and kidney damage. For most types of this disorder, early diagnosis and dietary restrictions started early in infancy can help prevent these more serious problems.

Fructokinase deficiency, which is a type of hereditary fructose intolerance, does not cause any symptoms.

Diagnosis of Hereditary Fructose Intolerance

  • Analysis of liver sample

Doctors base the diagnosis of hereditary fructose intolerance on the person's symptoms after consuming fructose and on a chemical analysis of a sample of liver tissue. The analysis helps doctors determine that the enzyme is missing.

People who may carry the gene that causes this disorder can undergo blood tests to determine whether they are carriers. Carriers are people who have an abnormal gene for a disorder but who do not have symptoms or visible evidence of the disorder.

Treatment of Hereditary Fructose Intolerance

  • Eliminating fructose, sucrose, and sorbitol from the diet
  • For hypoglycemia, glucose

Treatment of hereditary fructose intolerance involves excluding fructose (generally present in sweet fruits), sucrose, and sorbitol from the diet.

Severe attacks of hypoglycemia are relieved by glucose given by vein. Milder attacks are treated with glucose tablets, which should be carried by anyone who has hereditary fructose intolerance.

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