Why is milk not a substitute for water for my cat?
Cats can become ill if given milk. Today’s domestic cats are descended from those who lived in the wild, feasting on mice, birds and reptiles. They consumed the entire prey, which also had high water content. They ate almost no vegetable matter and were not able to taste sweetness. That included milk. The sugars in milk are a carbohydrate called lactose.
Cow’s milk offers no nutritional value for cats, and it can cause digestive problems. In fact, most cats develop intolerance to lactose shortly after they are weaned. This means that they are unable to digest the sugars that occur naturally in milk. This causes problems that include diarrhea and other unpleasant digestive problems.
Replacing food or water with milk will cause malnutrition. Feral cats are proof because wild cats do not usually have the opportunity to drink cow’s milk.
As long as your cat is eating a high quality food and has access to clean fresh water, she is getting all the nutrition that she needs. Milk alone is not a sufficient diet for any cat, and should never be given in place of food OR in place of water. Replacing a cat’s food or water with milk can cause your cat to become malnourished.
Many cats do seem to enjoy milk, and this causes a dilemma for many cat owners who love to give their cat treats that they enjoy. While most cats are lactose intolerant, some are not. For these cats, milk as an occasional treat is fine. The only way to know how your cat will react to milk is to feed her some. If she does not develop diarrhea then it is safe to assume that she is not lactose intolerant, and you can continue to give her the treat she loves. Again, milk should never be given in place of food, but as a treat. If she is lactose-intolerant, but still craves a treat now and then, there are cat milk substitutes (such as CatSip).