Lactose intolerance is the inability to digest lactose, which is the natural sugar in milk. It is due to deficiency of a gut enzyme called lactase. The undigested lactose is fermented by intestinal flora, producing wind and substances that cause abdominal pain and make the stools (poo) more liquid and acidic.
It should not be confused with allergy to the proteins in cow’s milk, which is a different thing.
Lactose intolerance can be of 2 types:
- Secondary lactase intolerance: which occurs when there is a disease that damages the surface of the bowel and makes the lactase disappear. This can happen after gastroenteritis or in coeliac disease.
- Primary lactose intolerance: this is more common in older children (from 4 years) and adults, and is due to a progressive loss of intestinal lactase after the first few years of life. There are regions of the world where the majority of people are lactose intolerant by this mechanism (Asia and Africa). It is less common in Europe, but even so, nearly 20% of the population is intolerant. However, not all those affected have symptoms, but rather it depends on the degree or threshold of tolerance and the amount of lactose consumed. For example, a person with primary lactose intolerance does not have symptoms if one day they eat cheese and a yoghurt, but they do if they also have a glass of milk or ice-cream.