We’re all familiar with the storybook image of a cat lapping up a bowl of milk. Cats probably first developed a taste for cow’s milk from hanging out in dairy barns, where milk was a coveted treat. What was not to like about this rich and vaguely sweet substance? But milk is not a natural part of an adult cat’s diet, and in some cases it’s not good for them.

Cats may like milk, but milk doesn’t always like them. This is because like some people, many cats are lactose intolerant. This means they lack sufficient quantities of a special enzyme called lactase to digest lactose, the sugar found in milk. Undigested sugars cause havoc in the intestines, including gas, cramping, and diarrhea.

Lactose intolerance is a fact of life for most adult mammals. Baby animals have lactase in their intestines to digest their mother’s milk. But once they are weaned, lactase levels drop. This isn’t a problem in nature, because adult animals would have no reason to drink milk. In our domesticated world, cats drink milk because we offer it to them.

Cow’s milk is never OK for kittens. While young kittens can handle the lactose just fine, cow’s milk lacks many of the essential nutrients, protein, and antibodies a growing kitten needs. Kittens should drink their own mother’s milk if at all possible. If not, they should be fed a milk replacer formulated especially for kittens, available at most pet stores.