Know that it's uncommon for an infant to be lactose intolerant. "Lactose intolerance" means that your body can't produce enough lactase, the enzyme that helps digest lactose (the primary sugar in dairy products like cow's milk). If it can't be digested, lactose hangs around in the intestine, causing gastrointestinal problems like diarrhea, cramping, bloating, or gas.
Very rarely, a baby is born with true lactose intolerance. (This is genetic and would have to be passed on to baby by both parents.) Babies with this rare condition have severe diarrhea beginning at birth and can't digest the lactose in their mama's breast milk or in cow's milk formula. They need a special, lactose-free formula.
It's possible for babies to temporarily have a little trouble producing lactase, like when they are having severe diarrhea or are taking certain medications. And sometimes preemies might not produce enough lactase for a short time.
If you think your baby might have a problem with lactose, talk to his doctor. She’ll want to check out baby’s symptoms and talk with you about the possibilities.
What's important to note is that there is a more common condition involving the protein found in cow's milk. Some babies are sensitive to this protein, beta-lactoglobulin, and have gastrointestinal problems and/or other allergic responses to the cow's milk in their moms' diets. This is an immune response and is different from lactose intolerance. This protein is only found in your milk after you eat or drink cow’s milk products. (It isn't naturally found in human milk.) So usually the best course of action is cutting dairy out of mom’s diet.