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Fructose Intolerance, Genetic Testing

Hereditary fructose intolerance is an inherited disease that manifests itself as an inability to metabolize fructose. Fructose is mainly contained in fruit but also in honey and vegetables.

The affected person suffers from severe abdominal pain, vomiting, and hypoglycemia (decrease in blood glucose) after receiving food containing fructose, sucrose, or sorbitol. In these people, long-term fructose intake can lead to severe liver and kidney damage.

Clinical characteristics

Following dietary exposure to fructose, sucrose, or sorbitol, hereditary fructose intolerance (HFI) is characterized by metabolic disturbances (hypoglycemia, lactic acidemia, hypophosphatemia, hyperuricemia, hypermagnesemia, hyperalaninemia) and clinical findings (nausea, vomiting, and abdominal distress; chronic growth restriction/failure to thrive). While untreated HFI typically first manifested when fructose- and sucrose-containing foods were introduced in the course of weaning young infants from breast milk, it is now presenting earlier, due to the addition of fructose-containing nutrients in infant formulas. If the infant ingests large quantities of fructose, the infant may acutely develop lethargy, seizures, and/or progressive coma. Untreated HFI may result in renal and hepatic failure. If identified and treated before permanent organ injury occurs, individuals with HFI can experience a normal quality of life and life expectancy.

Diagnosis and testing

The diagnosis of HFI is established in a patient with suggestive metabolic disturbances and clinical findings following dietary exposure to fructose, sucrose, or sorbitol and either biallelic pathogenic variants in ALDOB identified on molecular genetic testing. Note: Fructose tolerance testing ("fructose challenge") in the diagnosis of HFI should be avoided because it is dangerous and, when used in the past, could result in death.

The ALDOB gene provides instructions for making the aldolase B enzyme. This enzyme is one of a group of three aldolase enzymes that are responsible for breaking down certain molecules in cells throughout the body. Four identical aldolase B enzymes need to be attached (bound) to each other in a four-enzyme unit called a tetramer to work.

Aldolase B is found primarily in the liver, but it is also present at lower levels in kidney and intestinal cells. Aldolase B is involved in the breakdown (metabolism) of the simple sugar fructose, which is found mostly in fruits and is used in the body for energy. Aldolase B is responsible for the second step in the metabolism of fructose, which breaks down the molecule fructose-1-phosphate into glyceraldehyde and dihydroxyacetone phosphate. To a lesser degree, aldolase B is also involved in the breakdown of the simple sugar glucose.

Potential sources of fructose should be removed immediately if HFI is suspected.

HFI is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for an ALDOB pathogenic variant, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of inheriting neither pathogenic variant.

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