Q: I have been breastfeeding but have now started to wean my six month old on to formula. He has developed a rash since starting formula. Is this due to lactose intolerance?
A: No, lactose intolerance does not cause rashes. Lactose is the milk sugar present in breast milk and the milk of nearly all mammals, including cows and goats. Human milk is high in lactose and most babies and young children digest lactose well (the ability to digest lactose decreases with age). In normal digestion, lactose is broken down in the small intestine by an enzyme called lactase. A deficiency of this enzyme causes the undigested lactose to pass straight into the large intestine. Once in the large intestine, lactose draws water into the bowel and ferments with the help of natural bacteria, producing acids and gases. Common symptoms of lactose intolerance include excessive flatulence (wind), bloating and abdominal discomfort, watery diarrhoea, and in infants very acid stools, which can cause scalded buttocks.
Lactose intolerance and milk allergy are quite different conditions. Milk allergy is a reaction of the immune system to the protein in milk. Lactose intolerance does not involve the immune system. Although both lactose intolerance and milk allergy can cause some similar symptoms, and may exist together, lactose intolerance doesn't cause eczema or respiratory symptoms. If your baby has developed a rash it might be an immune response to the cow's milk or other foreign protein (e.g. soy), which would indicate an allergy rather than an intolerance. I recommend that you see your GP or paediatrician to discuss your baby's rash.