Letzte Aktualisierung am 7. December 2023 von Dr. Michael Zechmann-Khreis
Lactose intolerance refers to the inability of the body to properly digest milk sugar (lactose) .
Lactose is a carbohydrate that consists of two sugars. It is found in almost all dairy products and is normally broken down into its two individual sugars in the small intestine by an enzyme called lactase. This enzyme is formed in the wall of the small intestine and breaks down lactose into its two components galactose (mucose) and glucose (grape sugar). These simple sugars can then be absorbed into the body via the intestinal mucosa with the help of transport proteins. In people with lactose intolerance, this enzyme is missing (or is insufficiently present), which means that the lactose is not broken down and cannot be absorbed into the blood. It then passes into the large intestine. This is where bacteria are located that process the lactose and produce gases and short-chain fatty acids, which then lead to the typical symptoms of lactose intolerance.
A distinction is made between these types of lactose intolerance:
Primary lactose intolerance
There are three different forms of primary lactose intolerance.
congenital lactase deficiency
This is an autosomal recessive genetic defect in which not even the smallest amount of lactase is produced. Other disaccharidases are synthesized completely normally. This form is extremely rare and affects newborns. This defect can be observed above all in Finland3).
Developmental lactose intolerance
A very rare lactose intolerance that is only seen in premature babies. From the 8th week of pregnancy, the embryo already produces lactase. This production increases until the 34th week in order to reach a maximum at birth. Premature babies born before 32 weeks are often unable to produce sufficient lactase, which is why they are lactose intolerant. This form of lactose intolerance is reversible and has no effect on lactose intolerance later in life.
Endemic or ethnic lactose intolerance
This is the most common form of lactose intolerance worldwide. Around 70% of the world’s population is affected by this form. It is genetically determined and occurs more frequently in warmer, sunny regions. All mammals, including humans, have the ability to digest lactose. In all mammals, however, this ability decreases or disappears with increasing age. For example, 70% of Europeans can no longer digest lactose by the age of 602. Most Asians and black Africans also lose the ability to break down lactose after childhood. From a biological point of view, there is no reason for an adult mammal, such as an adult human, to drink milk.
In some European humans, however, a special form has developed during evolution that can continue to digest milk. The reason for the continuation of enzyme production is a mutation of a specific gene. Lactase continues to be produced, even in old age. You can find a more detailed explanation of this form of lactose intolerance in our article on the global distribution of lactose intolerance.
Secondary lactose intolerance
In some of these European people mentioned above, who can digest lactose in old age, this ability may be lost. They lose the ability to digest lactose again with increasing age. This is often due to external triggers, e.g. damage to the mucous membrane of the small intestine. As a result of this damage, lactase can no longer be formed and the person is then lactose intolerant.
The following diseases or external influences can be the cause of secondary lactose intolerance:
- Taking antibiotics over a longer period of time
- Bacterial overgrowth (small intestinal bacterial overgrowth)
- Chronic inflammatory bowel diseases
- Untreated celiac disease
- Other intolerances such as fructose malabsorption
- Parasite infestation
- Chronic alcoholism
- …
Secondary lactose intolerance can disappear as soon as the cause of the disease has been eliminated. This means that if, for example, the intestine is so damaged due to coeliac disease that lactase can no longer be produced, this is secondary lactose intolerance. If the intestine recovers on a gluten-free diet, this ability can return. Secondary lactose intolerance is thus “cured”.
Milk allergy or lactose intolerance?
Milk allergies (casein, lactalbumin, cow’s milk allergy, …) differ from lactose intolerance or lactose intolerance! These are not intolerances to lactose (a sugar), but genuine allergies to certain proteins in milk. In the case of allergies, the smallest amounts are enough to cause symptoms. In the case of lactose intolerance, small amounts are harmless.
Sources and others
- Ledochowski M. (Editor), “Klinische Ernährungsmedizin”, Springer-Verlag, 2009
- Vogelreuter A., “Food intolerances”, Wissenschaftliche Verlagsgesellschaft Stuttgart, 2012
- Catanzaro, R., Sciuto, M., & Marotta, F. (2021). Lactose intolerance: An update on its pathogenesis, diagnosis, and treatment. Nutrition Research, 89, 23-34. https://doi.org/10.1016/j.nutres.2021.02.003