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Fructose intolerance refers to both hereditary fructose intolerance and fructose malabsorption. Fructose malabsorption is a condition in which intestinal cells needed to absorb the fructose are deficient. Previously, fructose malabsorption was incorrectly referred to as fructose intolerance. Hereditary fructose intolerance results in an inability of the body to digest fructose, because the appropriate liver enzymes are not able to break down fruit sugar.

Hereditary fructose intolerance is a condition that makes it difficult for the body, mainly the liver and kidney, to use fructose and fructose-containing foods. The sugar is taken up by the body, but in attempting to use the sugar, toxic substances are produced, which lead to serious illness. Transmitted as an autosomal recessive trait, hereditary fructose intolerance results from a deficiency in the enzyme fructose-1-phosphate aldolase. The enzyme operates at only 1% to 10% of its normal biological activity, thus preventing rapid uptake of fructose by the liver after ingestion of fruit or foods containing cane sugar.

In HFI fructose is not properly metabolised due to a deficiency of the enzyme, aldolase B. Normally, once fructose is absorbed it is transformed into fructose-1-phosphate by an enzyme called fructose-1-phosphate aldolase. Aldolase B in turn finishes the partially converted fructose to glucose (for energy). Lack of aldolase B results in the accumulation of fructose-1-phosphate in the liver, kidneys and intestine. This inhibits glycogen breakdown and glucose synthesis, thereby causing severe hypoglycaemia (low sugar levels) following ingestion of fructose.

Hereditary Fructose Intolerance Symptoms:

Symptoms of fructose intolerance include severe abdominal pain, vomiting, and hypoglycemia following ingestion of fructose or other sugars metabolized through fructose-1-phosphate. Prolonged fructose ingestion in infants leads ultimately to hepatic and/or renal failure and death. Patients develop a strong distaste for sweet food, and can avoid a recurrence of symptoms chronic course of the disease by remaining on a fructose intolerance diet (fructose- and sucrose-free diet). Hereditary fructose intolerance may be relatively mild or a very severe disease. In the severe form, even eliminating fructose and sucrose from the diet may not prevent progressive liver disease.

Hereditary Fructose Intolerance Incidence:

The worldwide incidence rate of hereditary fructose intolerance remains unknown due to the difficulty of diagnosis. A recent study in the UK using DNA fructose intolerance testing indicated an incidence rate of 1 in 22,000 where the range would be between 1 in 12,000 to 1 in 58,000. Hereditary fructose intolerance is (or seems) more common in North-West Europe and in areas with many people of North-West European descent (USA, Australia). However data among other populations are scarce, and there may be other populations with similar or higher incidences.

Hereditary Fructose Intolerance Diagnosis:

Diagnosis of hereditary fructose intolerance is difficult and (in small children) dangerous. The only definitive way to ascertain if one is suffering from hereditary fructose intolerance is to have one of two tests:

• An enzymatic assay to determine aldolase activity. The aldolase is obtained from patient liver tissue in an invasive surgical procedure called a liver biopsy.
• A fructose intolerance test. Fructose is injected intravenously under controlled conditions where acute glucose, fructose, and phosphate levels are monitored.

A newer, non-invasive DNA test is readily becoming more available. While the DNA test is not diagnostic because negative results do not guarantee that one does not have HFI, positive results, along with clinical fructose intolerance symptoms, are strong indicators of disease. A presumptive diagnosis can be made.

Hereditary Fructose Intolerance Treatment:

Treatment of hereditary fructose intolerance involves following a strict fructose intolerance diet, which consists of exclusion of fructose and sucrose (cane sugar or table sugar) from the diet. Otherwise, treatment is supportive as the patient’s progress is monitored. Treatment of individual complications follows mainstream medical guidelines. For example, some patients can take medication to lower the level of uric acid in their blood and thereby decrease their risk for gout.

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