Fructose intolerance is a disorder of fructose metabolism. Three different varieties of this metabolic disorder are distinguished:

• Fructosuria / Fructosaemie
• Intestinal Fructose Intolerance or Fructose Malabsorption
• Hereditary Fructose Intolerance (HFI)

Fructosuria:

Ineffectiveness or deficiency of the enzyme fructokinase leads to increased concentrations of fructose in the blood, this condition is called fructosaemia. Due to the filtering processes that occur in the kidneys, fructose is then excreted in the urine, which is called fructosuria. This disorder occurs in one out of 50.000 individuals and normally does not require treatment. In most cases it is accidentally discovered.

Intestinal Fructose Intolerance:

Intestinal fructose intolerance or fructose malabsorption is acquired fructose intolerance. However, it may also be a hereditary condition. The acquired disorder may be caused by chronic stress, e.g. by prolonged malnutrition, environmental pollution, constant stress or long-term use of medication.

Fructose contained in food is normally metabolized by certain enzymes and then absorbed through the lining of the small intestine from where it reaches the blood. If this transport system is affected, fructose is insufficiently absorbed by the small intestine. The remaining fructose stays in the bowel and is later broken down in the large intestine with the help of bacteria. This produces excess gas that leads to flatulence, stomach rumbling, watery diarrhoea, constipation, irritated bowel syndrome and sometime colic. Irritations due to fructose malabsorption may be temporary or even last a lifetime.

Treatment consists in a diet that is low in fructose or fructose-free.
This usually leads to symptom remission within a few weeks. If symptoms should not be relieved, further diagnostic clarification is recommended.

Hereditary Fructose Intolerance:

Hereditary fructose intolerance is a rare, hereditary disease of fructose metabolism. A genetic defect in affected individuals causes decreased activity of the enzyme fructose-1-phosphate-aldolase-B. This condition occurs in one out of 20.000 newborn babies. Symptoms usually appear during the first few months when the baby begins to take solid foods that contain fructose, such as fruit, vegetables, juices or honey. Symptoms consist in irritations of stomach and bowels and in low blood sugar levels (hypoglycaemia) that lead to sickness, vomiting, trembling, sweating, pale skin, lethargy and convulsions.

If fructose intake continues it leads to serious lesions of the liver and kidneys. This development is progressive. Possible liver dysfunctions include enlarged liver and spleen (hepatosplenomegalia), hepatitis, oedema, ascites (abdominal dropsy) and serious coagulation disorders. Kidney lesions are often indicated by the excretion of protein with urine.

Treatment consists of immediate and total elimination of fructose from the diet. This includes all natural forms of fructose as well as foodstuffs that contain sucrose and sorbitol. In most cases, babies develop a strong aversion to foods that contain fructose. The diet must be changed immediately to prevent serious lesions. Symptoms are usually alleviated within a few days. Growing age often leads to slightly elevated tolerance. Moderate intake of fructose with food may then become possible.

If symptoms should not remit, further diagnostic clarification is recommended.

Which foods contain fructose?

Fructose is contained in foods in its free form or in compounds. All foods that contain such compounds must be eliminated in a fructose-free diet. This includes e.g. fruits and fruit juices, jam, honey, dried fruits, vegetables, bread and pastry with sugar, breakfast cereals such as cornflakes, muesli, oats and nuts, sausages that contain sugars, lemonade, vinegar, alcohol, products for diabetics, canned goods and instant meals that contain sugars, spices and spice mixes with sugars. If the product is labelled "sugar free" it may still contain sorbitol. Therefore, care must be taken. This also applies for a number of pharmaceutical products that are often coated with sugars.