Causes & Symptoms


People with lactose intolerance cannot tolerate lactose because their body produces either none of the digestive enzyme lactase or not enough of it. That leads to lactose in food not being split and, therefore, it cannot be digested.

Lactose intolerance should not be confused with cow's milk protein allergy, in which lactose can actually be tolerated, but not the milk protein.


If the lactose is not split in the small intestine, as it should be, then it continues on into the large intestine, where it is turned into hydrogen, short-chain fatty acids and carbon dioxide. Lactose intolerance is caused by the insufficient digestion of the milk sugar and results in digestive disorders from constipation through to very watery diarrhoea. Large amounts of undigested lactose are decomposed by the bacteria in the large intestine and this creates gases.

Sufferers complain of diarrhoea, stomach cramps, a feeling of fullness, wind, cold sweats, bloated stomach, nausea after eating, dizziness, sleep disturbances, skin blemishes and depression. However, symptoms do not all appear at once and also vary for each sufferer.

The symptoms are very unspecific and could point to a whole number of possible diseases. Therefore diagnosis is sometimes made only relatively late.


Primary congenital alactasia (lack of lactase)

This form is concerned with a congenital, very rare enzyme defect which occurs as early as in infancy and is characterised by a complete lack of lactase. This genetic defect is inherited and can have serious consequences. Therefore sufferers must keep to a lactose-free diet, avoiding even the tiniest amount. Symptoms such as bad diarrhoea, dehydration and malnutrition can occur even in the first few weeks of life, unless there is a switch to a completely lactose-free diet.

Primary lactose intolerance

This is the most common form of lactose intolerance. It occurs after weaning or during the course of growing up. In this form, less and less lactase is produced by the body over time.

Secondary lactose intolerance

This form is due to a disorder in the intestinal mucosa. The intestinal mucosa are damaged by other diseases, such as coeliac disease or Crohn's disease (chronic inflammatory bowel disease) and lactase can no longer be produced in the epithelium in the small intestine. As a result, lactose cannot be utilised. This form of lactose intolerance is reversible and can also be cured by keeping to the requisite diet.


In order to find out whether someone is lactose intolerant, a hydrogen breath test or a blood sugar test, for example, can be carried out by a medical specialist.
In the case of an oral lactose tolerance test, 50 g of lactose is dissolved in water and swallowed. Subsequently a lack of lactase can be diagnosed by two different procedures.

Hydrogen breath test: When there is a lack of lactase, the hydrogen content in the breath rises. This is created by bacteria in the large intestine when digesting the lactose. It then moves into the blood stream and is breathed out through the lungs. The subsequent measure of the hydrogen content in the breath (at 2 to 4 half-hourly intervals) shows whether the lactose has been split in the intestine or there is a lactose intolerance.

Blood sugar test: When the blood sugar rises either not at all or only a little after eating a diet containing lactose, a lack of lactase can be assumed, because the milk sugar is not being split and absorbed in the digestive tract. Therefore there is a lactose intolerance.