This is an absence of the enzyme lactase and is common in people from Mediterranean origin and those from the Indian sub-continent and also occurs in other populations. The best diagnosis is a hydrogen breath test (see investigations, and can be diagnosed with a test at endoscopy when a small piece of the small intestine lining is place in a special bedside lactose testing advice, and manifests as before).

The activity of the enzyme lactase can be objectively looked at by taking a small piece of the lining of the duodenum at endoscopy.

Equally sometimes using a lactose free milk to distinguish between cow’s milk protein allergy and lactose intolerance due to the lactase enzyme deficiency can help, ie if lactose free milk is tolerated then it is not a cow’s milk protein allergy because lactose free milks contain cow’s milk protein.  Equally if lactose free milk is not tolerated then it is more likely to be a cow’s milk protein allergy than a lactose problem.