Glossary · Dysbiosis and disease
Fructose intolerance
In one sentence
The term covers two distinct conditions: fructose malabsorption, a common digestive problem, and hereditary fructose intolerance (HFI), a rare genetic metabolic disease caused by deficiency of the enzyme aldolase B.
Also known as: fructose malabsorption · dietary fructose intolerance
What is fructose intolerance?
The same phrase is used for two very different conditions, and telling them apart comes first.
- Fructose malabsorption. The small intestine absorbs fructose mainly through the GLUT5 transporter, which has limited capacity. Glucose helps fructose absorption, so problems are more likely with foods in which free fructose exceeds glucose. Whatever is not absorbed reaches the colon, where the microbiota ferments it, causing wind, bloating, pain or osmotic diarrhoea.
- Hereditary fructose intolerance (HFI). An autosomal recessive metabolic disease caused by variants in the ALDOB gene, which encodes liver aldolase B. Exposure to fructose, sucrose or sorbitol can trigger vomiting, hypoglycaemia and liver and kidney damage. It typically shows up in infancy, when fruit or sugar is introduced into the diet.
How it is assessed
Malabsorption is investigated with a fructose breath test. The 2017 North American Consensus (Rezaie et al., Am J Gastroenterol) recommends a 25 g dose and treats a hydrogen rise of ≥20 ppm above baseline as positive. It advises measuring methane too and excluding SIBO first, as it can give false positives. Recording symptoms during the test is worthwhile: many healthy people do not fully absorb a large fructose load and feel nothing, so a positive result without symptoms carries less weight.
Hereditary fructose intolerance is confirmed by genetic testing of ALDOB, prompted by the clinical and biochemical picture. GeneReviews (NCBI) states that fructose challenge testing should be avoided in HFI because it is dangerous. For that reason, whenever HFI is suspected, no oral fructose load and no fructose breath test should be performed.
Example
A dietitian working with a gastroenterology clinic sees a student with bloating and loose stools after smoothies and dried fruit. There is no childhood history of hypoglycaemia or aversion to sweet foods, which would point towards the hereditary form. The consultant requests a fructose breath test: hydrogen rises by 31 ppm at 120 minutes and the student logs bloating. The result is consistent with fructose malabsorption, and the dietitian plans a short reduction phase followed by structured reintroduction.
Fructose intolerance at Xenogene
Xenogene offers a fructose intolerance breath test that detects fructose malabsorption after a controlled oral load. It measures hydrogen and methane by gas chromatography over about 3 hours, after dietary preparation. The report shows the fermentation curve; within an integrated panel alongside microbiota analysis, Sophia helps the clinician distinguish malabsorption from increased fermentation linked to dysbiosis.
This test is not suitable for diagnosing hereditary fructose intolerance and must not be requested if HFI is suspected. It is available on prescription from a healthcare professional, who interprets the result.
Frequently asked questions
What is the difference between fructose malabsorption and hereditary fructose intolerance?
Malabsorption is a gut transport problem: unabsorbed fructose is fermented in the colon and causes digestive symptoms. Hereditary fructose intolerance (HFI) is a genetic metabolic disease due to aldolase B deficiency, which can cause hypoglycaemia and liver and kidney damage. They are investigated differently and should never be confused.
Which foods are high in fructose?
Fruit juices, honey, fruits with more fructose than glucose (such as apples, pears, mango and watermelon), drinks sweetened with high-fructose syrups and products containing sorbitol, which further reduces absorption. Tolerance is individual, and dietary changes are best planned with a dietitian.
Is fructose malabsorption permanent?
Tolerance varies between people and is usually managed with a temporary reduction followed by gradual reintroduction to find each person's threshold. Hereditary fructose intolerance, by contrast, is lifelong and requires avoiding fructose, sucrose and sorbitol under specialist supervision.
Related terms
- Breath test
A breath test is a non-invasive investigation that measures gases in exhaled air, mainly hydrogen and methane, after a sugar is swallowed, to assess carbohydrate malabsorption or small intestinal bacterial overgrowth.
- SIBO
SIBO (small intestinal bacterial overgrowth) is the presence of excessive numbers of bacteria in the small intestine, accompanied by digestive symptoms such as bloating, wind, abdominal pain or diarrhoea.
- Microbiota
The microbiota is the community of living microorganisms (bacteria, archaea, fungi, protozoa) that inhabits a specific site of the body, such as the gut, mouth, skin or vagina.
- Dietary fibre
Dietary fibre is the group of plant carbohydrates, plus lignin, that the human small intestine cannot digest or absorb. It reaches the colon, where it adds bulk to stools and is partly fermented by the gut microbiota.
Educational content for healthcare professionals and the general public. It does not replace assessment by a healthcare professional: ordering and interpreting any test is the responsibility of the requesting clinician.
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