Lactose intolerance is due to the deficiency of an enzyme called lactase, normally found in the small intestine. When lactase is insufficient, the lactose (milk sugar) contained in dairy products cannot be fully digested.

When undigested lactose moves to the colon from the small intestine, it interacts with normal intestinal bacteria.​

"Common symptoms of lactose intolerance include bloating, abdominal cramps, nausea and diarrhoea," say​ doctors at the Department of Gastroenterology and Hepatology at Singapore General Hospital, a member of the ​SingHealth​ group.​ These symptoms usually begin about 30 minutes to two hours after consuming milk or dairy products including powdered milk, ice cream and evaporated milk.

Symptoms usually disappear when foods with high levels of lactose are removed from one’s diet.

Lactose intolerance can be caused by genetic factors or it can also develop due to age, bowel injury or gastrointestinal disorders.

If the small intestine produces less lactase after an illness, surgery or injury to the small intestine, the lactose intolerance can last from a few days to a couple of weeks, and may be reversible. In children in particular, such a temporary lactase deficiency can also result from a viral or bacterial infection.

In rare cases where​ babies are born ‘lactose intolerant’ due to a complete absence of lactase activity, these babies require lactose-free formula as they are intolerant to lactose found in their mothers’ breast milk.

Ref: O17​​​