In A Nutshell
Few species of animal enjoy lactose outside of infancy, and even fewer of those enjoy the lactose of another animal. As common a theme as a cat lapping at a saucer of milk might be, lactose intolerance is the norm for most of any adult mammal. Only humans are found to enjoy the milk of another animal well into their adulthood.
The Whole Bushel
Typically, mammals end up on a diet of milk in their early lives brought to them by their mother’s milk. This is often the first source of nutrition available, and “mother’s milk” also has a lot of other positive properties outside of base nutrition, such as proteins to help build the immune system. To properly handle and digest lactose our bodies have to produce its own enzyme known as lactase. Lactose is made up of two sugars, glucose and galactose, so for mammals to properly digest lactose it needs to be broken into two singular sugars, which is where lactase comes into play.
Lactase is produced in the body whenever lactose is consumed, so as long as lactose is in your diet, lactase will help you keep it all down. However, since most cats are natural carnivores, milk stops playing such a role in their diet once they are old enough to hunt and forage. Once cats move on to field mice, birds, and other local rodents, the build-up of lactase ceases. So as darling an image as a saucer of cream might be for that newly arrived feral cat, it is much safer and healthier to opt for a bowl of water.
While a lactose-intolerant cat typically won’t die from ingesting lactose, there are some troubling symptoms and complications that could arise. Upset stomach, diarrhea, and gastrointestinal cramps are the most common signs of lactose intolerance. Your cat should fully recover after the lactose has made it through digestion, so no worries there.
There is actually little harm in testing the waters to see if your cat is lactose intolerant. Giving your kitty a small amount of milk and watching for signs of arising symptoms is often regarded as safe practice. Some cats might even prefer milk over water, despite their digestive tract hating it, and for them there is a lactose-free alternative made special for cats.
As with most things, lactose intolerance can be taken on a case-by-case basis with each individual cat. Some cats that grew up with a heavy-lactose diet all their life would likely still produce the lactase needed to make milk a comfortable option for hydration. Some cats might find a preference for milk over certain tap waters, depending on the types of chemicals used in the local filtration.
Other potential dairy products should be treated similarly to milk, though some cheeses are much lower in lactose, and are even recommended by some veterinarians for slipping pills into, which allows an easier time medicating your kitty.
Most cat foods will provide your cat with the proper, square nutrition needed to be healthy. There are very few reasons to actively find out if your cat is or is not lactose intolerant, short of wanting to make your cat more comfortable with more options or preferences.
Show Me The Proof
ABC Science: Lactose intolerance
ASPCA: Feeding Your Adult Cat
Cats and Dairy: Get the Facts
NCBI: Diagnosis and management of food allergy and intolerance in dogs and cats