Hereditary fructose intolerance, HFI, is a genetic condition, caused by a congenital lack of the enzyme Fructose 1-Phosphate Aldolase. It appears with a frequency of 1:20.000-1:130.000 (depending on research source) and leads to liver damage, kidney damage and hypoglykemia. In this case, fructose is transported without a problem via the transport system (GLUT-5) through the intestine into the body, but it cannot be broken down properly by the liver.
Fructose malabsorption, FM (= dietary fructose intolerance) is, however, an acquired condition caused presumeably by a defective transport system (GLUT-5) in the small intestine. This means that the fructose cannot be absorbed into the body at a sufficient rate. But if it has been successfully transported then it can be broken down without a problem. Every third European suffers from fructose malabsorption!
The term "fructose malabsorption" is more commonly used than "dietary fructose intolerance". On the Food Intolerance Network website we will use the term "fructose malabsorption", and sometimes "fructose intolerance", but in that case we never talk about hereditary fructose intolerance (HFI) unless this is referred to specifically!
Currently it is believed that if less than 25g (0.9 ounces) of fructose cause symptoms, you are suffering from a fructose intolerance. Normally, a person can eat up to 50g (1.8 ounces) fructose without problems.
Sugars are absorbed via the intestines with the help of GLUT and SGLT transporters. The GLUT-5 transporter is responsible for fructose. It takes the fructose from the intestinal lumen into the enterocyte. However, this transporter also reacts to other substances. It is belived that some sugar alcohols like sorbite block it, and glucose stimulates its activity. That is why people with fructose malabsorption should stay away from sorbitol, mannitol, isomalt etc.
But glucose has also a different effect. Is much of it available in the intestine, the GLUT-2 transporter translocate to the apical cell wall. Since they also absorb fructose, they support the GLUT-5 transporters, who only transport fructose. That's why you may add glucose when you have to eat a larger amounts of fructose (after the restriction diet).
Why some people develop fructose malabsorption is not clear as of yet! But it has been observed that over the past few years carbohydrate absorption malfunctions have been on the rise.
(1)Rainer Klinke, Hans-Christian Pape, Stefan Silbernagl (Hrsg.): "Lehrbuch der Physiologie", 5. Auflage. Thieme, Stuttgart 2005
(2)Born Peter, World J Gastroenterol 2007 November 21;13(43): 5687-5691 "Carbohydrate malabsorption in patients with non-specific abdominal complaints"
(3)Manir Ali, Peter Rellos, Timothy M Cox, ItMed Genet 1998;35:353-365, "Hereditary fructose intolerance"