Digestive Disorders > Intolerances > Fructose Intolerance

Intestinal fructose intolerance: How does fructose malabsorption occur?

Dr. med. Andre Sommer

Dr. med. Andre Sommer

When most people talk about so-called intestinal fructose intolerance they are talking about fructose malabsorption, which is very common worldwide, and should not be confused with the rare hereditary fructose intolerance.

With fructose malabsorption, the fructose is not absorbed sufficiently by the intestine into the body, but is metabolized by intestinal bacteria. Typical symptoms of fructose malabsorption include bloating, abdominal pain, and diarrhea, which do not occur in all of those affected.

The symptoms can be alleviated by a change in diet with (at least) an initial reduction in fructose-containing foods. Fructose is not only found in many types of fruit, but also in juices, vegetables, products such as honey and increasingly as a sweetener in beverages and finished products.

In this article, we take a closer look at the origin and treatment of intestinal fructose intolerance. Here, nutrition plays an important role, since many affected people wonder, “What else can I eat?”

How does intestinal fructose malabsorption develop?

If you consume fructose-containing foods, the fructose is usually removed from the chyme in the small intestine and transferred to the bloodstream through the intestinal mucosa via special transporters, known as GLUT5 transporters. The capacity of this transporter is reduced in those affected, so only little or no fructose (under 25 grams) can be absorbed. This can exist from birth or occur later. The fault can also be permanent or only occur for a certain time.

Studies have shown that every person can absorb different amounts of fructose. The limit in healthy people is probably around 35-50 grams per day. Amounts in excess of this are not absorbed (i.e. malabsorbed). This fraction of fructose is largely absorbed by bacteria in the large intestine and metabolized to gases (hydrogen and carbon dioxide) and short-chain fatty acids. This process is also called fermentation. Carbon dioxide is particularly responsible for flatulence, while the short-chain fatty acids lead to diarrhea (osmotic diarrhea).

Does everyone with fructose malabsorption have symptoms?

No, not all people with fructose malabsorption show characteristic symptoms, so doctors only refer to fructose intolerance if symptoms occur. Healthy people can fail to absorb a certain amount of fructose when the amount of fructose ingested is considerable.

The fermentation of substances in the large intestine is a natural process and is even required in small quantities. The appearance of symptoms also depends on the location and the amount of bacteria in the intestine. If fermentation processes already occur in the small intestine, for example, this is much less tolerated.

What are the symptoms of fructose malabsorption?

Sufferers very often show symptoms similar to those of irritable bowel syndrome (IBS).

In the case of fructose malabsorption, a distinction can be made between symptoms that appear immediately after consumption and accompanying symptoms that may be the result of prolonged fructose malabsorption.

The following symptoms typically occur about 30–90 minutes after consuming fructose-containing foods or drinks:

  • Bloating
  • Flatulence, meteorism / bloating
  • Mushy stool
  • Pain, nausea, gastrointestinal cramps
  • Diarrhea

Both the intestinal gases and the stool are often malodorous due to the fermentation processes.

What are the side effects of intestinal fructose intolerance?

Similar to IBS, one often sees psychological changes in those with fructose intolerance. Patients with fructose malabsorption are more likely to experience depression. Affected people with fructose malabsorption show lower concentrations of folic acid than usual, starting from 35 years of age. It is believed that fructose malabsorption changes the intestinal flora, which is important for the absorption of folic acid in the body. The same effects could be found with zinc.

Small intestinal bacterial overgrowth can also occur as a result of prolonged fructose intolerance. Fructose malabsorption not only leads to increased bacterial activity in the large intestine, it also promotes the transfer of bacteria into the small intestine.

How is fructose malabsorption diagnosed?

Those affected who seek a doctor because of their symptoms will often already show indications of a fructose intolerance in their medical history. Many sufferers have already tried an elimination diet and report to their doctor that the symptoms improve when omitting foods containing fructose.

In order to make it easier to distinguish it from other diseases with similar symptoms, a simple diagnostic test is recommended: the hydrogen breath test (H2 breath test). Hydrogen is measured in the exhaled air, which is produced by bacteria in the intestine and then absorbed into the bloodstream. The more gas the bacteria produce, the greater the likelihood of fructose malabsorption.

How does the hydrogen breath test work with fructose intolerance?

You have to be sober for several hours before the test so as not to falsify the results. Then 25 grams of fructose are dissolved in 250 milliliters of water and drunk by the test person. The hydrogen content of the breathing air is measured at measuring intervals of 20 minutes over a total of 180 minutes. Clinical symptoms and a 20ppm increase from baseline indicate fructose malabsorption.


Keep in mind: In addition to other disruptive factors, colonization of the small intestine or antibiotic therapy can lead to incorrect results of the hydrogen breath test. That is why the breath test is often carried out once with fructose and glucose, which enables the diagnosis of colon deficiency in the small intestine. The lactulose breath test is also used for diagnostics and can show whether the intestine is capable of producing hydrogen at all. In this way, false negative results from other tests can be avoided.


Since there are similar symptoms in IBS, the hydrogen breath test is also part of irritable bowel diagnosis.

What other diseases can be hidden by fructose intolerance symptoms?

The most common diseases associated with the same symptoms are lactose intolerance and sorbitol intolerance. A distinction can only be made by elimination attempts or by breath tests.

Irritable bowel symptoms manifest themselves in a similar way to fructose malabsorption, which is why advanced irritable bowel diagnostics should be carried out especially in the case of negative breath tests.

More rarely, food allergies cause symptoms. As a rule, in the case of a real food allergy, an immediate reaction occurs immediately after the allergenic food has been consumed. This includes a furry feeling in the mouth, tongue burning or swelling of the lips, which is also called "oral allergy syndrome".

Histamine intolerance could also be the cause of the symptoms. Similar to fructose malabsorption, symptoms such as flushing of the face or the so-called histamine headache also occur.

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What foods contain fructose?

Fructose is not only found in many types of fruit, but also in juices, vegetables, and increasingly as a sweetener in processed foods.

The table shows a selection of fructose-containing substances

Food Example
Dried fruit

Raisins, figs, dates, dried apricots, plums etc.

Beverages Apple, pear, apricot juice etc., but also beer
Many types of fruit

Apples, pears, plums, melons
Sweeteners Honey
Vegetables

Cabbage, onions, leek, sauerkraut, beans


Note: Raw foods and fiber-rich foods also cause symptoms in those who are sometimes suffering from prolonged fructose intolerance.


In addition to fructose, sugar substitutes and sugar alcohols such as sorbitol, invert sugar, isomalt, and lycasite should also be avoided.

All types of fruit also contain fructose, but often in combination with glucose, such as in bananas. Dextrose is absorbed in the intestine by a transporter that also takes up a fructose molecule in addition to the glucose molecule. Therefore, foods with glucose in which the fructose content is smaller do not cause symptoms if they are consumed moderately.

Replacement for fructose on a fructose-free diet:

  • Glucose
  • Lactose
  • Maltodextrins
  • Sweeteners (mannitol, aspartame, cyclamate, saccharin)

Unsweetened staple foods include:

  • Dairy products
  • Meat
  • Fish
  • Eggs
  • Vegetable oils
  • Margarine

Since it has been found that those affected with irritable bowels also benefit from a low-fructose diet, fructose-containing foods should not be consumed even with the low-FODMAP diet. This form of nutrition has proven particularly effective in the long-term treatment of patients with food intolerance and IBS.

How is fructose malabsorption treated?

In contrast to rare hereditary fructose intolerance, fructose malabsorption does not require lifelong avoidance of fructose. On the contrary, the permanent absence of fructose means that the GLUT5 transporter works even worse in the intestine, or the number of transporters is reduced. The body practically unlearns how to utilize fructose.

Therefore, switching to a fructose-modified diet makes sense. By changing the composition of the food, while increasing the amount of fat and protein, a better absorption of fructose in the small intestine can be achieved.

A three-phase concept is recommended for the change in diet, the aim of which is to increase fructose tolerance again. An individual diet change, especially in the first two phases, should be carried out by a trained nutritionist. In the next few years, technical aids such as smartphones could support patients in changing their diet through intelligent algorithms.

Diet plan for nutritional therapy for fructose intolerance:

Phase Aim Recommendation Duration
First Phase: Elimination As far as possible symptom reduction
  • Low fructose diet by restricting fructose intake
  • Elimination of sugar alcohols (polyols)
  • Support for fructose absorption through the simultaneous intake of glucose
Maximum 2 weeks
Second Phase: Test phase Expansion of the food selection taking into account the food combinations and, if necessary, fat and protein enrichment in order to increase the physiological tolerance of fructose

  • Principles of light whole foods; fructose-modified food
  • Reintroduction of different types of fruit
  • Refraining from sugar alcohols
  • Loosen strict dietary requirements
  • Determination of individual fructose tolerance
Up to 6 weeks
Third Phase: Permanent nutrition Nutritional needs coverage
  • Individual nutritional recommendations that are based on patient specifications regarding number of meals and meal rhythm

Recently there are tablets on the market that can be taken before a meal containing fructose. However, the number of studies on these is still very limited, which is why the classic way of changing your diet is recommended.

How does hereditary fructose intolerance differ from intestinal fructose intolerance?

The following table explains the difference between the rare hereditary form of fructose intolerance and the fructose malabsorption described in this article. The third column shows the fructose overload (i.e. too much fructose).

Hereditary Fructose Intolerance (HFI) Fructose Malabsorption (FM) Fructose overload
Frequency (prevalence)

Germany: 1 : 20,000 Europe / North America: 1 in 3 adults, 2 in 3 small children Every person affected with individual tolerance limit
Cause (pathophysiology) Congenital metabolic disorder with dysfunction of the enzyme fructose-1-phosphate-aldolase (aldolase B), so that there is a harmful accumulation of fructose-1-phosphate in the liver, kidney and small intestine Restricted transport capacity (<25 g) of the fructose through the small intestinal mucosa leads to the flooding of fructose in the large intestine, which is broken down there by bacteria

The natural absorption capacity of fructose (35 g) is exceeded by the current food supply and the composition of the food
Symptoms

Nausea, vomiting, severe hypoglycaemia, transaminase elevation, quick lowering, unclear liver and / or kidney dysfunction Meteorism, flatulence, feeling of fullness, mushy stools, diarrhea, constipation Irregular, recurring soft pulpy stools, diarrhea, flatulence, constipation
Diagnosis 2-5 milliliters of EDTA blood for aldolase B gene mutation analysis (DNA test for mutations A149P, A174D, Y203X, N334K)

H2 breath test with 25 g fructose in 250 ml water (measuring intervals every 20 min over 180 min): clinical symptoms and average increase of 20 ppm compared to the initial value Control of nutritional and symptom logs

Therapy Lifelong disorder that requires strict fructose-free food Temporarily limited moderate fructose reduction of the food with subsequent testing of the individual threshold value (tolerance limit) Avoidance of isolated consumption of foods with high fructose and sugar alcohol content.

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Dr. med. Andre Sommer

Dr. med. Andre Sommer

I’m André, a medical doctor from Berlin. Together with a team of medical doctors, nutritionists and data scientists we empower people to understand digestive issues with our app Cara.

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