Fructose is a monosaccharide, that is, a simple sugar that does not break down into other simpler carbohydrates. It is used as a sweetener for diabetics, and in nature it is found in vegetables, fruits and honey. On the other hand, sucrose, better known as common sugar, is composed of two molecules, glucose and fructose, and once in the intestine, enzymes in the duodenum break it down into its two sugars.
Sorbitol, or glucitol, is a sugar alcohol found in red algae and, together with fructose, glucose and sucrose, in fruits such as pears, apples, cherries and peaches. It is used as a sweetener, thickener and humectant and is able to sweeten while providing few calories.
What is fructose intolerance?
Fructose intolerance is a malabsorption caused by a deficiency of the transporter of this sugar from the lumen of the intestine to the interior of the cells. This transporter is GLUT5, which also serves to transport sorbitol. When we mix foods with sorbitol, fructose absorption is more difficult because both compete for the same transporter.
On the other hand, the transporter called GLUT2 is shared with glucose, galactose and fructose, and glucose facilitates the absorption of fructose by this transporter. This explains why fructose intolerant individuals can tolerate sucrose, a common sugar composed of glucose and fructose, without difficulty.
Why does fructose intolerance occur?
Fructose intolerance can occur in three situations:
- A primary intolerance can occur, which is a genetic problem that results in a deficit of the transporting enzyme and develops throughout life.
- A secondary intolerance can develop, due to another intestinal disease (gastroenteritis, irradiation enteritis, celiac disease, Crohn’s disease, ulcerative colitis, etc.) that alters the edge of the intestinal mucosa and prevents the transport of fructose transiently or permanently.
- Fructosemia or hereditary fructose intolerance is a rare disease (1 in 30,000 births) and is due to a metabolic deficit of the enzyme aldolase located in the liver. Symptoms appear in the first months of life when common sugar or fructose is introduced into the diet.
How does it manifest itself clinically?
When fructose is not absorbed by the intestine it reaches the colon and the bacteria of the intestinal flora ferment the fructose and sorbitol and produce pain, abdominal distension, intestinal movements and noises, and may cause diarrhea and sometimes vomiting and headache.
How do we diagnose it?
The best test for diagnosis is the exhaled hydrogen test. When unabsorbed fructose or sorbitol sugars in the small intestine pass into the large intestine, the existing bacteria use them as food and produce hydrogen as a waste product. The hydrogen is absorbed into the blood and expelled by respiration. The test consists of administering a fructose-sorbitol solution to the fasting patient and collecting exhaled samples for hydrogen measurement. This has to be accompanied by some preconditions of preparation, which are:
- Have intact intestinal flora, avoiding taking antibiotics, Asprina® or laxatives (including Plantaben® or fibers) or probiotics or prokinetics (Motlilium®, Cidine®, Blastón®, etc) for four weeks before the test. Likewise, the test will not be performed if a colonoscopy or intestinal cleansing has been performed four weeks prior to the test.
- A diet free of sugars and starches should be followed 24 hours before the test. Such as bread, rice, pasta, cereals, cereals, potatoes, peas, corn, fruits, fruit juice, milk, yogurt, biscuits, candy, soft drinks and chewing gum. Diets with residues slow down the intestinal transit. The day before the test you can take o For breakfast, chamomile or tea without sugar with scrambled eggs without dairy products. o For lunch and dinner, white rice, fish or meat without seasoning, except for aromatic herbs.
- Do not smoke 8 hours before the test.
- Do not perform physical exercise for 6 hours before or during the test, because it influences the elimination of hydrogen.
- Go to the test without having brushed your teeth. You can only perform a rinse with Chlorhexidine.
The test lasts 4 hours and you should not eat or drink during the test.
When the test is started, a basal sample is taken at the beginning and if the value is below 10 ppm, it indicates that these preconditions have been met. Subsequently the patient will blow every 30 minutes for 3 to 4 hours. It is considered positive (the patient is intolerant) if the elevation of the hydrogen level is 20 ppm above the patient’s basal level before the test.
What can I do if I have fructose intolerance?
Fructose intolerance means a dietary restriction of fructose and sorbitol. However, a very strict diet can be difficult to follow and can be associated with nutritional deficiency, therefore the diet will be adjusted depending on the degree of intolerance.
The association of fructose with sorbitol should be avoided, since it hinders the absorption of fructose. Sorbitol is contained in chocolates, pastries, commercial juices, medicines in syrups, chewing gums and jelly beans. And also the sweetener E-420. On the other hand, the association of glucose with fructose facilitates the absorption of this one as we have commented previously.
The most intolerant fruits are the Rosacea family (peach, apricot, apple, pear, plum, cherry,) raisins, jams and quince.