Lactose is the number one carb/sugar in human milk. We wouldn’t survive as a species if babies were lactose intolerant. Human milk actually has 50% more lactose than cow’s milk! Our bodies produce a protein called lactose that breaks down lactose in the gut. Lactase is supposed to disappear after baby is weaned- usually by seven years of age. Yes, babies are supposed to be breastfeed until between 2.5 to 7 years of age. Human babies are supposed to drink only human milk and our bodies are designed to digest it efficiently and effectively. When this protein disappears, the body has a hard time digesting lactose. In reality up to 70% of adults are lactose intolerant as adults because this protein is supposed to disappear. We’re not meant to drink milk after childhood. But this shouldn’t happen until after baby is weaned- as a toddler or preschooler. The reason babies can have indigestion and upset from milk is from bovine protein either that mom is eating (those excessive cheese lovers know what I’m talking about) or from introducing artificial baby milk (aka formula that is cow’s milk based).
The proteins in milk can be divided into two categories: caseins and whey proteins (remember Little Miss Moffett on her tuffet eating her curds and whey? Curds are like the globs in cottage cheese and the whey is the watery substance). Human milk contains these in a ratio of 40:60 casein to whey; while in cow’s milk the ratio is 80:20 respectively. The amount of total protein in cow’s milk is more than double that of human milk to help baby cows double in size very quickly after baby. Cow’s milk contains considerably more casein than human milk to achieve that growth. Casein can be difficult to digest, in fact it is used as the primary ingredient of some glues! Artificial baby milks have to be formulated or altered to contain more whey than casein, to try to replicate the ratio of whey to casein to be as similar to that of human milk as it can to be better digested. But it is still a forgoing protein that the body wasn’t designed to digest.
Now there are truly some babies who have difficulties with digestion, however congenital lactose intolerance is very, very rare. It would be seen immediately after birth with very severe symptoms and should be diagnosed by a pediatrician as soon as possible. A small percentage of breastfeeding mothers notice an obvious difference in their baby’s behavior and/or health when mom eats certain foods. As previously stated, cow’s milk products are the most common problem foods and the only foods conclusively linked by research to fussiness/gassiness in babies because of the protein found in cows milk. Food sensitivities in breastfed babies are not nearly as common as many breastfeeding mothers have been led to think.
There are some really well written articles if you feel your baby has a protein intolerance. For more information, check out the following resources:
The Comparative Composition of Human Milk and of Cow’s Milk. http://www.jbc.org/content/16/2/147.full.pdf
A Comparison Between Human Milk and Cow’s Milk. https://www.viva.org.uk/white-lies/comparison-between-human-milk-and-cows-milk