Fructose Malabsorption – Causes, Symptoms, and Diagnosis

What Is Fructose Malabsorption (FM)?

Human intestine can’t absorb unlimited amount of fructose, but most people can absorb 25-50 g of fructose per sitting (1). By definition, inability to absorb 25 g of fructose per sitting is considered as fructose malabsorption. Unabsorbed fructose may cause diarrhea and bloating.

Fructose malabsorption (FM) was previously known as ‘dietary fructose intolerance’ (DFI), but this term should be avoided to prevent confusion with hereditary fructose intolerance (HFI).

What Is Fructose?

Fructose is a monosacharide or single sugar, also called fruit sugar. Fructose has the same chemical formula as glucose (C6H12O6), but different molecular structure. They both have about the same caloric value, but fructose is sweeter. Fructose is not essential nutrient for human, meaning everyone can live without fructose, so “fructose deficiency” doesn’t exist.

Fructose naturally appears in fruits, some vegetables, wheat, honey, sugar cane, sugar beets, and some other plants.

In commercial foods and drinks, fructose can be naturally present, or is added as a sweetener like sucrose (table sugar containing 50% fructose, 50% glucose), or as high fructose corn syrup (HFCS), somewhere called fructose-glucose syrup.

Foods, high in fructose and sorbitol

Mechanism of FM

Fructose, unabsorbed in small intestine, drags water from intestinal vessels into intestinal lumen, and travels toward the colon, where bacteria break it down to short chain fatty acids and gases (carbon dioxide, hydrogen, methane) that cause diarrhea and bloating.

Individuals with FM may also have impaired absorption of sorbitol, fructans, and short-chain carbohidrates, named FODMAPs (5).

Fructose malabsorption is NOT a food allergy, meaning there’s no production of IgE antibodies or release of histamine. There’s also no typical allergic symptoms like itching or hives.

What Causes FM?

Anyone at any time from the time of weaning to old age can develop FM. Possible causes include (5):

• Inherited or acquired abnormality of fructose transporting protein GLUT-5 (other family members are often affected)
• Overuse of High Fructose Corn Syrup (HFCS) or fruit juices in children (toddler’s diarrhea)
• Small intestinal bacterial overgrowth (SIBO)
• Celiac disease
• Chemotherapy or radiation (damage of small intestinal mucosa)
• Dumping syndrome (rapid stomach emptying).

Fructose Absorption & Glucose-to-Fructose Ratio

In a healthy person at least 25 g or more of fructose can be absorbed at one sitting in the small intestine (glass of orange juice or a can of Coke contain about 15 g of fructose). It is transport protein GLUT5 in the small intestinal mucosa that is necessary for fructose absorption, but no digestive is needed (1).

Glucose enhances absorption of fructose by the principle that one molecule of glucose enables absorption of one molecule of fructose, so, for example, fructose from table sugar (50% fructose, 50% glucose) is generally well absorbed even in persons with FM. Greater the glucose-to-fructose ratio in the food, easier the fructose will be absorbed (2).

Symptoms in FM

Symptoms of fructose malabsorption may show up at any age. They appear within 2-24 or more hours after the fructose-containing meal. The threshold for the symptoms may be as low as 1 g of fructose in some persons with fructose malabsorption, or as high as 20 g in others. Bloating, flatulence, gurgling, abdominal pain, diarrhea, and aversion to sweet-tasting foods are most common symptoms. Less frequently depression, anxiety, fatigue, headache, brain fog, craving for sugar, and weight loss appear.

Since fructose causes diarrhea, it may flush some micronutrients (iron, vitamins) out from small intestine, before they are absorbed, thus resulting in anemia and various nutrient deficiences over the time.

Picture 1. Apples and pears have a high fructose/glucose ratio

Laboratory Findings in FM

Fructose malabsorption was found to be correlated with low levels of folic acid in blood plasma in some cases, supposedly due to reduced amount of intestinal bacteria that produce folic acid (3). Plasma levels of zinc, iron, calcium, vit C and E, triptophane and glutathione may also be lowered, and amylase, lipase and triglicerides may be elevated (4,7). “Fructose deficiency”, on the other hand, doesn’t exist since fructose isn’t an essential nutrient in human.

Fructose Malabsorption or IBS?

Many people with IBS complain that they cannot digest fruits properly, and also have problems with other foods. In one clinical trial, a group of persons with a diagnosis of IBS were tested with hydrogen breath test with fructose, and 30% of them were found to have fructose malabsorption (1,12).

Hydrogen Breath Test with Fructose

Hydrogen breath test with fructose is reliable test for FM with 98% sensitivity and 86% specificity (7).

Procedure. A tested person ingests fructose-rich meal. In FM, fructose won’t be completely absorbed in the small intestine, so it will reach the colon, where bacteria will break it down to hydrogen that will be absorbed into the blood and will reach the lungs. A person is asked to breath into a hydrogen detecting machine, and from amount of expired hydrogen, an extent of fructose malabsorption is evaluated. Test takes about three hours to be completed (1).

How to prepare for the test?

Additional hydrogen breath tests can be done to exclude other causes of diarrhea:

• lactose breath test to exclude lactose intolerance
• lactulose breath test to exclude small intestinal bacterial overgrowth (SIBO);
• sucrose breath test;
• 3C stable radioisotope breath tests for children and pregnant women.

Reducing Substances

Reducing substances in the stool (lactose, fructose, glucose, lactose) is a screening test for carbohydrate malabsorption.

Treatment

Treatment of the cause of FM doesn’t exist at the time, but symptoms of FM may be prevented by low-fructose diet.

Diet in Fructose Malabsorption

The basis of diet in FM is avoiding foods high in fructose, sorbitol, and fructans. Details of low fructose diet. See also Nutrition guide: foods, safe to eat, to try, and to avoid in FM.

Further Reading:

Mucus In Bowel Movement
Find a Causes of Loose Bowel Movements From Medical Symptoms
Bowel Anatomy, Motility, Digestion & Gut Flora
Comparison of Normal and Loose Bowel Movements
Foods to Avoid In IBS
Yellow bowel movement
Green bowel movement
Black bowel movement

References:

1. Fructose malabsorption clinical trial (uihealthcare.com)
2. Table sugar, and fructose-glucose ratio (healthsystem.virginia.edu)
3. Fructose malabsorption and folate deficiency (clinchem.org)
4. Laboratory findings in FM (uihealthcare.com)
5. GLUTE5 (food-info.net)
6. FDA about HFCS (ific.org)
7. Fructose malabsorption and osteoporosis (foodintol.com)
8. Specifity and sensitivity of hydrogen breath test (allergyadvisor.com)
9. Fruits high in sorbitol (fructose.at)
10. Problematic sugars in FM (food-info.net)
11. Diet in fructose malabsorption (healthsystem.virginia.edu)
12. IBS vs fructose (ncbi.nlm.nih.gov)
13. Sorbitol and unintential weight loss (medscape.com)