Fructose intolrance (fructose-malabsorption)

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In many cases, the causes of gastro-intestinal disorders remain concealed. If the patient experiences troubles with the bowels especially after eating fruit, it could be due to an intolerance of fructose (fruit sugar). This type of metabolic disturbance is also referred to as fructose malabsorption and it has gained importance as a cause of intestinal disorders over the years.

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Malabsorption means a disorder in the uptake and transport of nutrients from the inside of the bowel into the blood and lymph vessels. Fructose malabsorption therefore refers to a limited uptake of fructose in the small intestine and – consequently - the blood circulation, caused by a dysfunction of the GLUT5 protein responsible for fructose transport. If this transport mechanism is disturbed, the fructose absorption from food by the mucous membrane of the small intestine cannot happen at all or only in a limited way. It ends up in the colon where it is decomposed by bacteria resulting in fatty acids and intestinal gas causing the typical troubles such as flatulence, diarrhea or constipation as well as abdominal pain or cramps, nausea and vomiting. This defect can occur temporarily or permanently. At the same time, it is important to understand that it is not an allergic reaction.

What is affecting the fructose transport mechanism?

Sorbit(-ol), a sugar alcohol, limits the transportation of fructose. Some types of fruit contain sorbit(-ol), and it is also used by the food industry as a sugar substitute (E420) in sugar-free or low-sugar sweets.

The following may contain sorbit(-ol):

• low-calorie candy and chewing gum

• fruit such as apples, pears, plums

• dried fruit such as prunes, apricots, dates

• fruit jams and jellies

• sweets

• dietary products such as low-sugar jams, chocolate

• beer, wine

The simultaneous consumption of glucose (dextrose) enhances the fructose uptake and can therefore have a positive impact on the system. Glucose can easily be purchased and used as a sweetener. The positive effect of glucose also explains why usually sucrose (household, cane or beet sugar) is well-tolerated in low quantities. When split, sucrose will break down into equal amounts of free fructose and free glucose. This leads to a good fructose uptake as glucose stimulates the remaining fructose transport activity.

Not to be confused with (hereditary) fructose intolerance!

One basically needs to understand the difference between fructose malabsorption and hereditary fructose intolerance (HFI), the latter occurring much more seldom. HFI is caused by a congenital enzyme deficiency that already shows in early infancy when feeding the baby with fruit, vegetables or honey, for instance. In such cases, the body completely absorbs the fructose from food products, but then is unable to metabolize it. This leads to fructose sedimentation in the intestinal wall, kidneys and liver and can lead to hypoglycemia (low blood sugar levels), liver and renal dysfunctions.

What can you do if you suspect suffering from fructose malabsorption?

Already with reducing your consumption of sorbit(-ol) and food products containing fructose you can achieve quite something for yourself. It is not necessary to completely avoid fructose. You can enhance the fructose uptake by consuming glucose (dextrose) at the same time, which is also why there is no need to avoid household sugar (sucrose). Try and avoid diabetic food, eat fructose-containing food products slowly and when drinking fruit juice, try not to drink up at once.

What can we do for you?

If you think you may suffer from fructose malabsorption, you should get in contact with an internal medicine specialist (gastroenterologist). Making a diagnosis is a complex issue as symptoms are not always clear. There may as well be other causes of gastrointestinal disorders. We will first do some basic tests (blood count, inflammatory parameters, ultrasound diagnostics) to try and exclude other potential causes of the symptoms. If these substantiate an initially suspected fructose malabsorption, a breath test is advisable measuring the hydrogen concentration in the exhaled air. The patient then has to drink up a fructose solution and afterwards regularly exhale into a test device. The unabsorbed sugar will be fermented by colon bacteria, leaving hydrogen among other components. Through the intestinal wall, this gas will get into the blood and, with it, into the lungs from where it is exhaled and can be measured. If this confirms a fructose malabsorption, a change in the patient’s nutrition habits will become necessary. We then recommend seeing a nutritionist to jointly set up a nutrition plan in accordance with your individual needs.

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