Intolerance to fructose – a carbohydrate present in many fruits but also vegetables – was discovered by two Swedish doctors at the end of the 1960s. They, in fact, had the idea of prescribing a fructose-free diet to patients suffering from long-term and unexplained bloating, colic-like pain, and diarrhea. Result: the symptoms disappeared. The two doctors then administered fructose to patient volunteers and, using a breath test, quantified the amount absorbed by the body. In the intolerant group, the amount of fructose passing into the blood was found to be significantly lower than in the control group. Obviously, fructose was poorly absorbed by the body.
Symptoms of Fructose Intolerance
Fructose intolerance is more common than we think: it affects almost 30% of the population. However, in around half of the people affected, it has no significant consequences. On the other hand, the other half suffer from bloating, stomach aches, diarrhea, flatulence, nausea or abdominal cramps. We speak of fructose malabsorption when symptoms appear after a meal containing less than 25 grams of fructose – which represents a little less than two large apples or the equivalent of two glasses of fruit juice (0.2 liters each ).
That said, even healthy people have trouble eating more than 50 grams of fructose per meal. Beyond that, they often present the same symptoms as those who are fructose intolerant. The importance of the phenomenon is probably linked to the increasing consumption of foods rich in fructose – such as smoothies and other sugary drinks from fast food restaurants – which often exceeds the normal absorption capacity of the intestine. The problem is all the more significant as manufacturers add fructose to many food products, particularly prepared meals. The average consumption of fructose now stands at nearly 40 grams per day in the United States and France, an increase of around 15% since the 1970s. This trend has not only favored intolerance to fructose. fructose, but also diseases such as dyslipidemia (cholesterol), gout (linked to uric acid metabolism) and, above all, type 2 diabetes (due to too high a level of glucose).
Hereditary fructose intolerance is more annoying. It manifests itself by sweating, tremors, dizziness and nausea, but can cause disturbances of consciousness which can lead to coma or even death of the patient.
Causes of fructose intolerance
The mechanism at work in the event of a fructose overdose is well known. To take in food and then transport the substances the body needs, the cells of the intestinal mucosa produce specific protein molecules, adapted to the food components. Just for the different types of sugar, there are several. The molecules responsible for transporting fructose, however, have limited capacities. In people sensitive to fructose, these are exceeded with less than 25 grams per meal. As a result, unabsorbed fructose molecules accumulate in the small intestine and are then recovered in the colon. There, the bacteria reuse them but at the same time generate problems such as gas or diarrhea.
When inflammatory diseases, such as Crohn’s disease or celiac disease, attack the lining and damage its cells, the small intestine can no longer absorb fructose as well. The same symptoms as in people with fructose intolerance then appear. Fructose malabsorption should not be confused with hereditary fructose intolerance, which is much rarer (approximately 1 in 20,000 people). In this case, fructose absorption is normal, but an enzyme normally produced by the body (called hepatic fructose-1-phosphate aldolase) is missing. However, the latter helps prevent the accumulation of substances with toxic effects for the liver, kidneys and brain.
Diagnosis and therapies for fructose intolerance
It is not always easy to conclude that you have fructose intolerance, especially since the symptoms are similar to those of lactose intolerance or irritable bowel syndrome. To be sure, we can do a breathing test, the principle of which is simple: measure the quantity of hydrogen, which comes from fructose transformed by bacteria in the large intestine, which is found in the blood then in the lungs, from where it expired. The patient thus drinks water containing 25 grams of dissolved fructose, then the doctor measures the quantity of hydrogen exhaled, at least twice over several hours. Result: the more hydrogen the air exhaled by the patient contains, the less the body tolerates fructose.
If this is the case, then fructose consumption should be reduced in order to reduce the symptoms. A consultation with a nutritionist allows, through other tests, to determine the amount of fructose that each person is able to absorb. It also and above all helps to make changes in diet. But rest assured: while certain fruits have a high fructose level – it can reach 70% when the fruits are dried! – certain possibilities of low content (citrus fruits, pineapple, etc.). To be effective, the diet must also eliminate – or limit – certain vegetables containing a high amount of fructose, including onions, carrots, zucchini and red pepper. Also be careful with honey: it contains almost 40% pure fructose!
That said, giving up fruit is not recommended – except for people with hereditary fructose intolerance – because they are essential in a balanced diet. Instead, it is best to combine foods containing fructose with glucose-based ingredients. Because due to a complex biochemical mechanism, the absorption of fructose and that of glucose are linked: in the presence of glucose, the molecules responsible for the assimilation of fructose increase their capacities tenfold. This is why table sugar (sucrose), which is composed of glucose and fructose, is generally well tolerated. The same goes for fruits like blackberries, raspberries, citrus fruits, yellow melon and papaya, which contain fructose and glucose in relatively balanced proportions.
Conversely, watch out for sorbitol! This sugar substitute found in low-fat drinks and chewing gum, and in many foods for diabetics, produces the opposite effect of glucose: it blocks the molecules responsible for transporting fructose, thereby worsening its absorption. Be especially vigilant as several studies have shown the harmful effects of fructose on health: memory and concentration problems, stress, and even depression. One more reason to consume fructose in moderation.
➤ Article published in the magazine GEO HORS-SERIE SCIENCES, “The digestive system, Understanding – Treating – Preventing” from December 2020 – January 2021.
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