What is lactose intolerance?

Letzte Aktualisierung am 24. August 2026 von Dr. Michael Zechmann-Khreis

Lactose intolerance refers to the body’s inability to properly digest milk sugar (lactose) and the resulting gastrointestinal symptoms. Milk sugar is a disaccharide, meaning it consists of two simple sugars. Lactose is found in almost all dairy products and is normally broken down into its two monosaccharides in the small intestine by an enzyme called lactase. This enzyme is produced in the wall of the small intestine and breaks down lactose into its two components: galactose ( a type of sugar found in mucus) and glucose ( grape sugar). These simple sugars can then be absorbed into the body through the intestinal mucosa with the help of transport proteins. In people with lactose intolerance, this enzyme is missing (or present only in insufficient quantities), which means that not all of the lactose that enters the system is broken down. The undigested lactose then passes into the large intestine. There, bacteria break down the lactose, producing gases and short-chain fatty acids, which, among other things, lead to the typical symptoms of lactose intolerance.

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Types of lactose intolerance

Primary lactose intolerance

There are three different forms of primary lactose intolerance.

Endemic or ethnic lactose intolerance

This is the most common form of lactose intolerance worldwide. About^70%⁵ of the world’s population has this form. It is genetically determined and is not a disease, but rather the normal state. It is more common in warmer, sunnier regions. In principle, all mammals—including humans—have the ability to digest lactose. However, in all mammals, this ability decreases or disappears with age. This is because, biologically speaking, adults do not drink milk. The north-south divide is striking: While only 2–5% of people in the far north have this trait, the figure is 15–20% in the Nordic region and as high as 60% in the southern Mediterranean region. In East Asia and Africa, it is even found in up to 100% of the population.

In some European humans, a genetic variant has developed during evolution that can continue to digest milk as adults. 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.

Please note: The genetic test for lactose intolerance is not conclusive as to whether you have symptoms or not. Please read the article on the diagnosis of lactose intolerance.

congenital lactase deficiency

This is an autosomal recessive genetic defect in which not even the smallest amount of lactase is produced. This form is extremely rare and affects newborns. This defect is particularly evident in Finland^.1

Developmental lactose intolerance

This is 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.

Secondary lactose intolerance

Health-related lactose intolerance

This form has no genetic basis. It develops as a result of a disease or impairment of the small intestine’s mucosa. It can also occur as a side effect of certain medications. This form of lactose intolerance resolves once the mucosa has regenerated or the cause of the intolerance has been eliminated.

If, for example, the intestine is so damaged due to coeliac disease that no or hardly any lactase can 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”.

Diet-related lactose intolerance

It’s also possible to experience symptoms of lactose intolerance because your diet isn’t ideal. This happens when you consume too much lactose on an empty stomach—for example, through milk-based beverages and processed foods containing milk powder. In other words, you flood the intestines with more lactose than the lactase actually present can break down. If, on top of that, transit time is short—meaning the chyme is moved quickly through the digestive tract—lactose reaches the large intestine and is processed by bacteria.

The microbiome also plays a role. Depending on its composition, more or fewer gases are produced, resulting in more or fewer symptoms.

Milk allergy or lactose intolerance?

Milk allergies ( casein allergy, cow’s milk allergy, lactoglobulin allergy, mold ^{cheese allergy4}) 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.

Summary

Lactose intolerance is the inability to properly digest milk sugar (lactose), which leads to gastrointestinal symptoms. Lactose, a disaccharide, is normally broken down into galactose and glucose in the small intestine by the enzyme lactase. In lactose intolerance, this enzyme is absent or present in insufficient quantities, so that undigested lactose enters the large intestine, where bacteria break it down, producing gases and short-chain fatty acids that cause symptoms.

The amount of lactose consumed plays a major role in the onset of symptoms, as does the composition of the diet in terms of fats, proteins, fiber, and minerals. The length of time lactose remains in the digestive tract also plays a role. This means you can’t generally say that you can or cannot tolerate a certain amount of lactose, as it always depends on how you’re feeling on any given day. However, you can usually get a good sense of your own tolerance. Please also read our article on the treatment of lactose intolerance.

Sources and others

1. Catanzaro, R., Sciuto, M. & Marotta, F. Lactose intolerance: An update on its pathogenesis, diagnosis, and treatment. Nutr. Res. 89, 23-34 (2021).
2. Ledochowski M. (Editor), “Klinische Ernährungsmedizin”, Springer-Verlag, 2009
3. Vogelreuter A., “Food intolerances”, Wissenschaftliche Verlagsgesellschaft Stuttgart, 2012Further links
4. DocCheck, retrieved on 1.9.2024
5. R. A. Forsgård, “Lactose digestion in humans: intestinal lactase appears to be constitutive, whereas the colonic microbiome is adaptable,” Am. J. Clin. Nutr. , Vol. 110, No. 2, pp. 273–279, Aug. 2019, doi: 10.1093/ajcn/nqz104.