Lactose intolerance occurs because the body is not producing enough lactase, which is the enzyme that breaks down lactose in the small intestine.

Most people are born with the ability to break down lactose but a range of factors increases the risk of developing lactose intolerance later in life.

Genetic factors:

Some people have lactose intolerance because they have a genetic difference that reduces the effectiveness of their body in being able to break down lactose. The tendency to produce less lactase enzyme with age is more common in people of Asian, African, South American, Southern European and Australian Aboriginal heritage than in people of Northern European descent.

Medical factors:

• damage to the small intestine, such as due to surgery, or coeliac disease, or Crohn’s disease
• gastroenteritis, which can strip the intestines of lactase for a week or two
• parasitic infection, which can temporarily reduce lactase levels
• lack of iron in the diet, which can interfere with lactose digestion and absorption.

Some people find that their ability to tolerate lactose goes down as they get older.