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Fructose Metabolism Disorders

By Lee M. Sanders, MD, MPH, Associate Professor of Pediatrics, Stanford University

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Deficiency of enzymes that metabolize fructose may be asymptomatic or cause hypoglycemia.

Fructose is a monosaccharide that is present in high concentrations in fruit and honey and is a constituent of sucrose and sorbitol. Fructose metabolism disorders are one of the many carbohydrate metabolism disorders.

Fructose 1-phosphate aldolase (aldolase B) deficiency

This deficiency causes the clinical syndrome of hereditary fructose intolerance. Inheritance is autosomal recessive; incidence is estimated at 1/20,000 births. Infants are healthy until they ingest fructose; fructose 1-phosphate then accumulates, causing hypoglycemia, nausea and vomiting, abdominal pain, sweating, tremors, confusion, lethargy, seizures, and coma. Prolonged ingestion may cause cirrhosis, mental deterioration, and proximal renal tubular acidosis with urinary loss of phosphate and glucose.

Diagnosis of fructose 1-phosphate aldolase deficiency is suggested by symptoms in relation to recent fructose intake and is confirmed by enzyme analysis of liver biopsy tissue or by induction of hypoglycemia by fructose infusion 200 mg/kg IV. Diagnosis and identification of heterozygous carriers of the mutated gene can also be made by direct DNA analysis. (Also see testing for suspected inherited disorders of metabolism.)

Short-term treatment of fructose 1-phosphate aldolase deficiency is glucose for hypoglycemia; long-term treatment is exclusion of dietary fructose, sucrose, and sorbitol. Many patients develop a natural aversion to fructose-containing food. Prognosis is excellent with treatment.

Fructokinase deficiency

This deficiency causes benign elevation of blood and urine fructose levels (benign fructosuria). Inheritance is autosomal recessive; incidence is about 1/130,000 births.

The condition is asymptomatic and diagnosed accidentally when a non-glucose reducing substance is detected in urine.

Deficiency of fructose-1,6-biphosphatase

This deficiency compromises gluconeogenesis and results in fasting hypoglycemia, ketosis, and acidosis. This deficiency can be fatal in neonates. Inheritance is autosomal recessive; incidence is unknown. Febrile illness can trigger episodes.

Acute treatment of fructose-1,6-biphosphatase deficiency is oral or IV glucose. Tolerance to fasting generally increases with age.