Fructose, also called fruit sugar, is a type of sugar found primarily in various fruits, vegetables, honey and processed foods. The food industry uses fructose to make products such as cookies and dressings sweeter. In fructose intolerance, fructose cannot be properly absorbed in the intestines. Normally, fructose is absorbed in the small intestine, where it can pass through the intestinal wall with the help of the transport protein GLUT-5. In fructose intolerance, this transport protein is defective, preventing the fructose from leaving the small intestine and entering the large intestine. Once there, intestinal bacteria ensure that fructose is broken down by fermentation. The fermentation process releases gases and attracts moisture, which can cause intestinal problems.
Fructose is a monosaccharide, a singular sugar molecule. Fructose is very similar in structure to glucose, which is also a monosaccharide. Glucose is absorbed in the small intestine by the transport protein GLUT-2. Fructose can attach to glucose and thereby “ride along” with GLUT-2 to pass the intestinal wall. When there is an equal amount of glucose and fructose in the small intestine, the fructose can be absorbed properly. If there is more fructose than glucose in your diet, fructose is left over, called free fructose. This free fructose cannot bind to glucose and eventually ends up in the large intestine. There it starts fermenting which can cause intestinal problems for people with fructose intolerance.
The official term for fructose intolerance is fructose malabsorption, because it involves a reduced absorption capacity of fructose in the intestine. Roughly 30% of Europeans suffer from fructose malabsorption, with the degree of symptoms being very different from person to person. This fructose intolerance may also be related to irritable bowel syndrome (IBS) in which the intestines are more sensitive and may have difficulty digesting certain food components, including fructose.
Some people suffer from a genetic variant of fructose intolerance. In this case the fructose metabolism in the liver is disturbed, whereby aldolase B enzyme activity is absent. This enzyme is essential and necessary for the metabolism of fructose in the liver. Even small amounts of fructose lead to severe liver damage in the genetic variant.
If you are sensitive to fructose, or you are fructose intolerant, then it is more likely that you are also sensitive to polyols. Polyols, also called ‘sugar alcohols’, are naturally found in certain types of foods such as cauliflower and avocado. But they are particularly found as sweeteners in cookies, sweets and soft drinks. Certain types of polyols, for example sorbitol, are converted to fructose in the liver. This means, you may be affected by polyols in your diet if you have a fructose intolerance.
If you are sensitive to fructose, it may be that you are also more sensitive to the fructans. Fructans are oligosaccharides, which are chains of 3-9 carbohydrate molecules attached together. You can find them in onion, garlic, leeks, cabbage and wheat, among other things. When fructans are broken down in the intestines, a molecule of fructose is also split off. If you are fructose intolerant, you may therefore also experience symptoms from fructans in your diet.
Fructose is naturally found in a number of different foods, mainly in fruits and certain vegetables. However, fructose is also found in many processed foods because it is used as a sweetener. Below are common foods that are high in (free) fructose:
- Fruit, such as apple, pear and cherry
- Vegetables, such as asparagus, artichoke and sugar snaps
- Fruit juice, such as apple juice and pear juice
- Dried fruits, such as raisins, figs and dates
- Jams and fruit compotes/sauces such as apple sauce
- Honey and maple syrup
In addition, fructose is often added as fructose syrup to processed products such as soft drinks, sweets, dressings, sauces and breakfast cereals. Check the label if you want to know if a product contains fructose. In the ingredient declaration, it is also referred to as fructose syrup or glucose syrup.
Symptoms of fructose intolerance are caused mainly by the fermentation process in the large intestine. This process attracts moisture and may also release gas. This can cause unpleasant intestinal symptoms. The most common complaints and symptoms of fructose intolerance are:
- Abdominal pain
- Bloated stomach
- Constipation (obstipation)
- Intestinal cramps
People with fructose intolerance are also more likely to feel depressed or down. This is because free fructose can bind to tryptophan. Tryptophan is needed to make the happiness hormone serotonin. When free fructose binds to tryptophan, less serotonin can be produced, which can make you feel down. People with fructose intolerance are therefore also significantly more likely to develop depression. The hormone serotonin also ensures a feeling of fullness, for example during meals. If your body does not produce enough serotonin, you may feel less full and continue to eat.
Most people will continue to suffer from fructose intolerance for the rest of their lives. Fortunately, you can prevent symptoms as much as possible with a few simple tips.
If you know you are fructose intolerant, you can limit the fructose in your diet as much as possible to avoid symptoms. It is important to know how much fructose your intestines can tolerate without developing symptoms. Pay attention to the amount of free fructose in your diet. Certain fruits contain about the same amount of glucose and fructose, so you can eat them without problems because fructose can bind with glucose.
In the low FODMAP app from Monash University you can easily check which types of food are safe and which you should limit to avoid complaints. If you are unsure which foods you can eat or should avoid, contact a dietitian for personal advice and guidance on a fructose-restricted or fructose-free diet.
Certain enzymes can convert fructose into glucose, which can prevent the fermentation of free fructose and the symptoms associated with it. For example, the enzyme Xylose Isomerase can convert fructose into glucose, allowing you to properly digest the fructose in your diet. This enzyme is available in supplement form that you can use with meals that contain free fructose. If you have questions about using supplements for a fructose intolerance, you can always consult a dietitian.
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