Incompletely absorbed (malabsorbed) fructose, fructo-oligosaccharides (chains of fructose molecules), including fructans and inulin, and galacto-olicosaccharides (chains containing galactose, glucose and often fructose molecules), such as raffinose or stachyose, are fermented by the flora in the lower bowel resulting in the formation of gas and chemical substances, such as short-chain fatty acids. The activity of the transport proteins determining the absorption of fructose, such as GLUT5, may be genetically determined or possibly changed by inflammation or stress. The threshold of fructose malabsorption varies individually and widely. Most individuals will show malabsorption of sugars, including fructose, when very large quantities are consumed, but the malabsorption generally does not result in symptoms. In pre-disposed individuals the increased production of gas, short-chain fatty acids and availability of osmotically active compounds due to malabsorption give rise to the symptoms of fructose intolerance. The intolerance often only becomes apparent in adulthood. This common and benign form of fructose intolerance must be distinguished from the rare and potentially dangerous hereditary fructose intolerance (HFI) (see Hereditary fructose intolerance).
Fructose and fructose oligosaccharides are consumed in fruit, vegetables and grains in daily amounts of at least 20 to 60g, depending on the local diet, and in high fructose corn syrup (HCFS)-containing snacks or drinks. Some sports and fruit drinks contain up to 50g of fructose per 1000ml. Fructose consumption has increased markedly in the last decades and is implicated in the rapid increase in childhood obesity, the metabolic syndrome and certain liver disease.
Frequency in population and natural history
- Approximately 30% of healthy adults show malabsorption of fructose doses below 50g, but less than 10% have symptomatic intolerance. There appear to be no major racial differences.
- Up to 70% of patients with Irritable Bowel Syndrome have fructose intolerance due to malabsorption.
- Commonly appears in adults and may be triggered by stress or inflammation.
These include bloating, abdominal cramps and pain, diarrhea and constipation, increased intestinal sounds and gas production, reflux (e.g. acid taste in mouth, heartburn) and nausea or vomiting. These symptoms resemble those of functional bowel disease. Up to 70% of patients with Irritable Bowel Syndrome have intolerance of normal amounts of fructose.
Depression may be more common in both adults and children with fructose malabsorption, and can improve with strict dietary reduction in fructose intake. This may be due to lowered tryptophan levels in the blood.
Zinc and folic acid blood concentrations have been reported to be decreased in a proportion of fructose intolerant.
Testing and diagnosis
Self-diagnosis of fructose intolerance is often difficult due to the presence of fructose not only in fruit, but also in vegetables and grains. In case of suspicion of severe fructose intolerance in children a careful history should be taken and specific genetic tests performed to exclude hereditary fructose intolerance (HFI) before performing the fructose breath test. This is recommended to prevent serious reactions to the ingested fructose.
Reduction of the intake of fructose and fructo-oligosaccharides to individually tolerated levels will rapidly lead to symptom relief in most individuals. Best results are obtained if generally all poorly absorbed, short-chain carbohydrates (FODMAPs) are reduced. The identification of fructose and fructan content is not intuitive and requires specialist advice to ensure adequate vitamin, fruit and vegetable intake. Generally the foods identified as ‘healthy’ contain fructose or fructans and replacement with other equally healthy alternatives or the addition of supplements constitute the dietician’s challenge. Sorbitol and xylitol consumption should be reduced to a minimum, due to the high degree of concurrent intolerance and symptom exacerbation (see Sorbitol intolerance). As concurrent glucose consumption increases fructose absorption and hence tolerability, foods with high glucose content are better tolerated and glucose taken just before meals may reduce symptoms. Small amounts of fructose eaten after meals are better tolerated than a single dose on an empty stomach.
See Food tables for a listing of fructose and glucose food content.
- Food allergies may co-exist with sugar intolerances. Generally tell-tale signs of possible additional allergies are skin rashes and itching, sinusitis or asthma and hay fever.
- Lactose and fructose intolerance co-exist in approximately 20-30% of individuals.
- Sorbitol, xylitol and fructose intolerances very commonly co-exist and may exacerbate each other (see Sorbitol intolerance).
Inulin-type fructans (chains consisting mainly of fructose molecules) are natural components of food, including wheat, onion, banana, garlic, leek and chicory. They are non-digestible, fermented in the colon, and classified as dietary fiber and prebiotic. The fermentation produces large amounts of gas, often resulting in bloating, flatulence and abdominal pain or cramps. Animal and emerging human research have demonstrated the following inulin effects: modification of composition and activities of the gut flora, improved stool production, enhanced absorption of calcium and other minerals, regulation of gastrointestinal endocrine peptides, increased immunity and resistance to infections, improved lipid homeostasis and the likelihood of reducing the risk of intestinal infections, functional bowel diseases, colon cancer, osteoporosis and obesity. Inulin-type fructans are thus functional food ingredients that are eligible for enhanced function claims, but, as more human data become available, risk reduction claims will become scientifically substantiated.
Links to literature
Hereditary fructose intolerance (HFI)
In distinction to the fructose malabsorption described above, HFI is a severe fructose intolerance due to genetic defects (aldolase B) mainly affecting children at the time of introduction of sugars, but also manifesting in adults. Besides the same symptoms as fructose malabsorption (see above), metabolic disturbances, such as hypoglycemia, and permanent liver and kidney damage can ensue. In infants the disease may be lethal due to seizures and coma. There is even sensitivity to the fructose component of sucrose, household sugar, as well as to infusions containing fructose. All forms of sucrose and fructose, and probably also sorbitol, should be strictly avoided. A positive family history of sugar intolerance or an aversion against sweets / candies is a useful clue. Diagnosis is by careful history taking, blood sampling for metabolic, liver and kidney disease, and, specifically, genetic testing. At present not all forms of HFI can be identified using genetic blood tests.
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