Fructose Intolerance

Fructose intolerance is a condition in which the body has difficult digesting fructose and fructose-containing foods. It is treated by complete elimination of fructose and sucrose from the diet.

Sugars and sweeteners

The second type of fructose intolerance is dietary fructose intolerance (DFI), also called fructose mal-absorption. DFI is quite common—according to the U.S. National Library of Medicine's Genetics Home Reference center, fructose malabsorption may affect up to 40% of people in the Western hemisphere. Its exact incidence is also difficult to evaluate since many people show no symptoms at all and its cause is not precisely known. It seems to be caused by the lack of special cells (epithelial cells) on the surface of the intestine that usually aid in digestion. As a result, the body is not able to absorb fructose efficiently.

Fructose is a monosaccharide, or simple sugar, that has the same chemical formula as glucose, the main source of energy for the body, but a different molecular structure. It is found in all fruits, in some vegetables, and in honey. Fructose and other sugars are carbohydrates, which are important sources of energy for the body. The main types of sugars found in beverages and foods are lactose, maltose, dextrose, corn syrup, and high fructose corn syrup.

The HFI Laboratory at Boston University has prepared a list of sugars and their tolerance. Sweeteners that are not generally tolerated include:

Foods not allowed in a fructose and sucrose-free diet include:

Other foods that must be avoided include catsup, sauces that contain sugar, chile piquin with lemon, all regular soft drinks and sodas, jams, jellies, marmalades, maple syrup, canned or bottled fruits, and corn syrup.

Foods that are allowed on a fructose-free diet include:

Other permitted foods include coffee, tea, diet soft drinks that use artificial sweeteners, cocoa, salt, pepper, and other spices.

Fructose, sucrose, and sorbitol are used in many manufactured foods to such an extent that very few processed foods are allowed in the diet. Sugar is used in many other less obvious products such as canned food, bottled sauces, flavorings, and even prescription drugs—sucrose and sorbitol are also often used in medications as bulking agents or to improve the taste. Sorbitol is very often used as an artificial sweetener, especially in diabetic foods and drinks, which should accordingly be avoided. Isomalt and lycasin, alternative sweeteners that are predominantly used in confectionary sweets, also contain sorbitol. Glucose can be used as an alternative sweetener and as a source of energy.

Only vegetables that contain predominantly starch are allowed in the diet. New potatoes have a higher fructose content than old potatoes. Cooked vegetables have a lower fructose content and should be selected rather than raw vegetables, as the cooking process results in a loss of free sugars. Vegetables provide a good source of fiber that may be otherwise difficult to obtain. Fiber is usually found in fruits, vegetables, and whole grains. Wholemeal flour contains more fructose than white flour because both germ and bran contain sucrose, so whole-grain foods, such as brown rice and whole-grain pasta, contain more sucrose than the refined products.

The symptoms of both types of fructose intolerance are gastrointestinal distress, flatulence, bloating, diarrhea, fatigue, vomiting, low iron, and other nutrient deficiencies. Dietary fructose intolerance has also been associated with mood disturbances and depression. The function of a fructose-free diet is to eliminate dietary sugar intake so as to alleviate these symptoms.

Clinical intolerance to fructose was initially described in 1956. Some 4–5 years later, the defect in aldolase B enzyme in the liver was demonstrated, and hereditary fructose intolerance (HFI) became clinically recognized. The rapid early progress in the understanding of this disorder may be due to the fairly clear symptoms associated with ingestion of fructose, which are difficult to miss. In many young infants, the age of onset of symptoms leads to the diagnosis. Genetic counseling may be of value to prospective parents with a family history of fructose intolerance. Other tests that may be run include a blood sugar test, kidney function tests, liver function tests, a uric acid blood test, and urinalysis.

Although they have different causes, both HFI and DFI are treated by dietary adjustments. Complete elimination of fructose and sucrose from the diet is the only effective treatment for HFI. DFI treatment also involves a fructose-free diet, with the treating physician allowing some concessions in mild cases. A strict fructose-free diet involves exclusion of any beverage or food containing fructose, sucrose, or sorbitol. Some patients may find a threshold level and can eat some fructose without getting symptoms. Close dietary monitoring is important for good outcomes and should include at least semiannual visits to a biochemical geneticist (for HFI) and monthly meetings with a registered dietitian.

Eating out is one of the most challenging parts of maintaining a fructose-free diet. This is because restaurant employees have little time to check food contents on the labels of the ingredients used by the kitchen to prepare menus. Some suggestions for eating at restaurants include:

Sugar is often an ingredient in foods that the consumer is not aware of, and not only in restaurants. High fructose corn syrup (HFCS) is present in products such as soft drinks, fruit drinks, sports drinks, baked goods, candies, jams, yogurts, condiments, canned and packaged foods, and other prepared and sweetened foods. Also, potatoes may provide a significant amount of fructose if prepared in a certain way. For this reason, the guidance of a registered dietitian is required for the treatment of fructose intolerance.

Absolute elimination of fructose and glucose from the diet produces good outcomes in most people with fructose intolerance. Medical researchers unanimously agree that symptoms of dietary fructose intolerance can improve in patients willing to adhere to a low fructose diet. Research performed at the University of Innsbruck in Austria showed that fructose- and sorbitol-reduced diets in subjects with fructose malabsorption did not only reduce gastrointestinal symptoms but also improved mood and early signs of depression. Improvement of the signs of depression were more pronounced in females than in males. For a rapidly diagnosed and treated infant, the outcome for a normal state of health is excellent. In the absence of substantial liver damage, life expectancy is normal.

There is no known prevention for fructose intolerance.

See also CSIRO total wellbeing diet .