Medical Encyclopaedia › Lactose intolerance (short definition)
In brief: In lactose intolerance, milk sugar (lactose) is split only inadequately in the small intestine because the enzyme lactase is missing or reduced. The undigested sugar reaches the large intestine and causes symptoms there.
Causes
Worldwide, most adults lose part of their lactase activity after weaning – this is the natural, genetically determined form. In Northern Europe, by contrast, lactase persists in many people. Temporary (secondary) lactose intolerance can occur after intestinal infections or in diseases such as coeliac disease.
Symptoms
About half an hour to two hours after eating foods containing lactose, flatulence, abdominal pain, diarrhoea or nausea occur. The severity depends on the amount consumed and the individual residual activity. Lactose intolerance is not an allergy and does not damage the gut.
Diagnosis
The standard test is the hydrogen (H2) breath test: after drinking a lactose solution, it is measured whether more hydrogen, produced by bacterial fermentation, is exhaled. In addition, a genetic test can detect the hereditary form. An elimination trial lasting several weeks followed by a challenge can support the diagnosis.
Diet
Complete avoidance is usually not necessary: many of those affected tolerate smaller amounts, such as a splash of milk in coffee. Hard cheese and many yoghurts contain little lactose. Lactose-free products and lactase preparations taken with meals containing milk can help. Anyone who severely restricts dairy products should ensure an adequate calcium intake.
Related terms in the encyclopaedia
Hydrogen breath test, Enzyme, Small intestine, Bloating, Diarrhoea, Fructose intolerance, Calcium, Irritable bowel syndrome
Sources
- Bundeszentrum für Ernährung (BZfE): Laktoseintoleranz
- IQWiG: gesundheitsinformation.de – Gesundheitsinformationen des Instituts für Qualität und Wirtschaftlichkeit im Gesundheitswesen
Note: This article is for general information and does not replace advice, diagnosis or treatment from a doctor or pharmacist. Last updated: October 2026.