Milk is the first food that infants ingest for sustenance. It contains a sugar called lactose which cannot be absorbed by the body as it is. Lactose is broken down in the intestines by the enzyme lactase into glucose and galactose, which is then absorbed.

At birth, the intestines contain plenty of lactase. Its production is determined by a gene which is switched on at birth. The lactase gradually decreases as the child gets older. After the age of one, 400ml of milk is all that is required in a day.

The production of lactase is genetically determined. About 60 per cent Indians are unable to digest large volumes of dairy products as they age because the level of this enzyme drops. In Europe, the gene is slightly different. Most of the population continue to produce large quantities of lactase well into adult life. They can tolerate larger volumes of milk and dairy products.

If the lactase production is insufficient, then the lactose remains undigested and moves into the large intestine. The bacteria present there break down and ferment the lactose, causing the typical symptoms of intolerance such as abdominal distention, bloating, gas formation, stomach cramps and diarrhoea.

Lactose intolerance runs in families and usually sets in with increasing age. Such people continue to be able to tolerate small amounts of milk but they have to be careful not to overload their digestive tract. Curd is often better tolerated than milk. Low lactose milk, ice cream, dairy products and probiotics are widely marketed to help with this condition.

Diarrhoea due to viruses, bacteria or food poisoning can cause intestinal hurry. The small hair (villi) lining the intestine and responsible for the breakdown and absorption of lactose can get washed off. Avoiding all milk and milk products for 48 hours gives the intestines time to recover and regrow the villi. The diarrhoea will subside faster if this is done. Lactose intolerance is diagnosed based on the symptoms. It can be confirmed with the hydrogen breath test and lactose tolerance test.

Cow’s milk allergy can occur in up to 15 per cent babies and manifest as weaning starts. It can also occur in breastfed infants if the mother is drinking cow’s milk. The allergy can also occur with sheep, goat, buffalo and camel milk, so changing the origin of the milk is not a solution. It is due to an abnormal response by the child’s immune system.

Symptoms can start immediately after ingesting milk or a few hours later. There may be tingling around the nose and mouth, swelling of the lips, vomiting, abdominal pain, diarrhoea, rashes, itching and urticaria. Abdominal cramps, diarrhoea with blood, watery eyes and a runny nose may take longer to appear.

Milk allergy requires avoiding milk and milk products. It tends to run in families. Read food labels carefully to make sure no milk protein or powder is added. Minute quantities of milk can cause allergies, unlike milk intolerance where the symptoms are dose-related. The good news is that children can suddenly outgrow the allergy.

The writer is a paediatrician with a family practice at Vellore and the author of Staying Healthy in Modern India. If you have any questions on health issues please write to yourhealthgm@yahoo.co.in