What if your cat loves dairy, but even a treat-sized dollop of milk or cream leads to an upset stomach with unpleasant symptoms like gas, bloating, diarrhea, or vomiting? Actually, that’s pretty normal. Most adult cats have trouble digesting dairy products because they lack the ability to break down lactose, the sugar found in milk. Lactose makes up between 2% and 8% of milk.

The sugar lactose is a kind of carbohydrate that is made up of two other sugars, galactose and glucose. In order to digest milk products properly, the body first has to break the lactose down into those two components. “Lactose intolerance” refers to an inability to separate lactose into its digestible parts.

Lactose intolerance is not an allergy. In an allergic reaction, the body’s immune system misidentifies a benign substance as an intruder and attacks it, causing various symptoms. Lactose intolerance is a digestive issue and doesn’t involve the immune system.‍

When lactose passes through the intestinal tract without being broken down, it pulls extra water into the intestines. Then, to make things even worse, the gut’s resident bacteria get access to the undigested lactose and ferment it.

As a result, feeding milk to a lactose-intolerant cat may cause gas, vomiting, or diarrhea, usually within 8 to 12 hours. In addition, since the lactose can’t be processed and eliminated correctly, this sugar can linger in the digestive tract, continuing to cause gastrointestinal trouble long after the dairy is consumed.

Like other baby mammals, kittens need their mother’s milk to give them a healthy start in life. Among many other benefits, the mother’s colostrum and milk support the development of the gut microbiome, which is crucial for so many aspects of an animal’s health.

Kittens can process their mother’s milk because their digestive system produces a lactase enzyme, which breaks lactose down into those two component sugars. But as soon as the growing kittens begin weaning and start eating solid food, their production of lactase slows way down, so that they no longer have enough to properly digest lactose.‍

Since most mammals have no dietary need for milk once they’ve become adults, this reduction in lactase production—and therefore some degree of lactose intolerance—is actually the norm.

Cats may be attracted to milk, cream, or cheese because these foods taste fatty, but even as a treat, dairy products are generally not recommended for cats.

For one thing, cow and goat milk both have a significantly higher lactose content than a mother cat’s milk, so they’re even more difficult for an adult cat to digest. And in cats with inflammatory bowel disease (IBD) or other intestinal disorders, dairy can make the existing digestive symptoms much worse.

Some dairy products—such as cottage cheese and yogurt—may be tolerated better than straight milk because the bacterial processes they undergo also break down some of the lactose.

As long as your cat has no digestive issues, feeding very small amounts of these human foods may not cause trouble, but if your cat loves an occasional treat, the safest choice is a little lactose-free cow’s milk. (Avoid soy milk, almond, and other nut milks; though these milk substitutes contain no lactose, they have been known to cause stomach problems in cats.)

Better yet, skip the dairy altogether. There is no need for dairy products in your cat’s diet, and what they need for proper hydration is plenty of fresh water, not milk.

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