Fructose Intolerance: Causes, Symptoms and Treatment
Fructose intolerance is the inability of the body to properly absorb or metabolize fructose (fruit sugar), leading to digestive symptoms such as bloating, abdominal pain, and diarrhea.
Things worth knowing about "Fructose Intolerance"
Fructose intolerance is the inability of the body to properly absorb or metabolize fructose (fruit sugar), leading to digestive symptoms such as bloating, abdominal pain, and diarrhea.
What is Fructose Intolerance?
Fructose intolerance describes a condition in which the body cannot properly absorb or process fructose (fruit sugar). Fructose occurs naturally in fruits, vegetables, and honey and is widely added to processed foods as a sweetener. There are two fundamentally different forms of this condition: the common intestinal fructose malabsorption and the rare hereditary fructose intolerance (HFI).
Forms of Fructose Intolerance
Intestinal Fructose Malabsorption
This is by far the more common form. The small intestine is unable to fully transport and absorb ingested fructose. The undigested fructose passes into the large intestine, where it is fermented by gut bacteria, producing gases and short-chain fatty acids that trigger typical digestive symptoms. This form is not life-threatening and can be well managed through dietary adjustments.
Hereditary Fructose Intolerance (HFI)
Hereditary fructose intolerance is a rare, congenital metabolic disorder caused by a deficiency of the enzyme aldolase B in the liver. When fructose is consumed, a toxic intermediate (fructose-1-phosphate) accumulates in the liver, kidneys, and small intestine, potentially causing severe organ damage. This form requires strict medical management.
Causes
The underlying causes differ depending on the form:
- Fructose malabsorption: Impaired function or overload of the transport protein GLUT-5 in the small intestine, responsible for fructose uptake. It can also develop secondarily due to intestinal diseases such as celiac disease or Crohn's disease.
- Hereditary fructose intolerance: A genetically determined defect of the enzyme aldolase B, inherited in an autosomal recessive pattern.
Symptoms
Symptoms vary depending on the form and severity of the condition:
- Bloating and flatulence
- Abdominal pain and cramping
- Diarrhea or loose stools
- Nausea
- Feeling of fullness
- In hereditary fructose intolerance: vomiting, hypoglycemia (low blood sugar), liver damage, and growth disorders in children
Diagnosis
Diagnosis is established through several methods:
- H2 breath test: The most common test for fructose malabsorption. After consuming a fructose solution, the amount of hydrogen exhaled is measured. Elevated levels indicate malabsorption.
- Genetic testing: Analysis of the ALDOB gene is used to diagnose hereditary fructose intolerance.
- Enzyme assay: If HFI is suspected, aldolase B activity can be measured in a liver biopsy.
- Elimination diet: Temporary removal of fructose from the diet followed by careful reintroduction to monitor symptoms.
Treatment
Fructose Malabsorption
Treatment is primarily based on a low-fructose diet. Patients learn which foods are high or low in fructose and how to identify their individual tolerance threshold. The ratio of fructose to glucose in foods is also important, as glucose enhances fructose absorption. Guidance from a registered dietitian is strongly recommended.
Hereditary Fructose Intolerance
For HFI, a strictly fructose-free diet is essential for survival. Fructose, sucrose, and sorbitol must be completely avoided. With a consistent diet, patients can lead a largely normal life and liver function can recover significantly.
Suitable and Unsuitable Foods
Well-tolerated Foods
- Glucose-rich foods (e.g., dextrose)
- Meat, fish, eggs
- Potatoes, rice, oats
- Lactose-free dairy products
- Bananas (in moderation, due to a favorable glucose-to-fructose ratio)
Foods to Avoid
- Apples, pears, mangoes, cherries (high in fructose)
- Honey and agave syrup
- Fruit juices and sweetened beverages
- Industrially processed products containing high-fructose corn syrup
- Dried fruits
References
- Gibson, P.R. et al. - Fructose malabsorption and the bigger picture. Alimentary Pharmacology and Therapeutics, 2007.
- Orphanet - Hereditary Fructose Intolerance. www.orpha.net, 2023.
- World Gastroenterology Organisation (WGO) - Practice Guideline: Diet and the Gut. www.worldgastroenterology.org, 2018.
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