Lactose is present in almost all dairy products and is normally digested in the small intestine with the help of an enzyme called lactase which splits the disaccharide into two monosaccharides (simple sugars).
Persons who suffer from lactose intolerance lack this enzyme. This means that the lactose will not be broken down there and as a consequence reaches the large intestine. Here there are bacteria that will process the lactase and this leads to the production of various gases. These, amongst other things, will lead to the typical symptoms of lactose intolerance.
The enzyme lactase breaks the lactose down into two monosaccharide components called galactose and glucose. These monosaccharides will then be absorbed into the body via the intestinal mucosa (the wall of your intestines). In principle all mammals have the ability to digest lactose. However, in all mammals the ability to digest lactose decreases or disappears as they get older since there is no reason, from a biological perspective, for a fully grown mammal to drink milk that is intended for babies.
(medical term: primary lactase deficiency): This is understood to be the most common form of lactose intolerance. Its cause is genetic and it occurs more frequently in warmer and sunny regions (see “Ethnic distribution of lactose intolerance”). It only appears during the process of growing-up (from around the age of 51) and intensifies with age. For this reason around 70% of all Europeans at the age of 60 are no longer able to digest lactose2.
(medical term: secondary lactase deficiency): In this case there is no genetic defect, but it is caused by a disorder in the intestinal mucosa. The enzyme lactase is produced in the epithelium of the small intestine. This layer of the intestine is the first that will be damaged due to other illnesses or other external influences. Sources can, for example, be the taking of antibiotics over a longer period of time. This type of lactose intolerance is reversible, which means that it can be healed.
This is an autosomal recessive genetic defect where not even the smallest amount of lactase is produced. Other disaccharides are, however, synthesised normally. This type of lactose intolerance is very rare and it already affects newborn babies. This type of deficiency is found predominantly in Finland3.
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(1) Ledochowski M., Bair H., Fuchs D., "Lactoseintoleranz", J Ernährungsmed 2003; 5(1): 7–14.
(2) Britta-Marei Lanzenberger, "Laktoseintoleranz", 1. Auflage, systemed Verlag, p.8
(3) Ledochowski M. (Herausgeber), "Klinische Ernährungsmedizin", Springer-Verlag, 2009