A kitty lapping from a saucer of milk is an image familiar to everyone. But not everyone knows that milk is bad for cats. Because of cats' unique biology and the way it changes from kittenhood to adulthood, dairy milk can give an adult cat upset stomach and diarrhea.
One reason cats get sick from drinking dairy milk is that they simply cannot tolerate it. Kittens can tolerate nursing milk but not dairy; they live on their mother's milk or a facsimile until they are old enough to eat solid food. Once they grow into adulthood, however, they can no longer tolerate it. If they consume it, digestive upset with diarrhea can result. Dairy milk contains more lactase than their mother's milk, making it less digestible. Kittens nursing from their mother should not be given any other type of milk; they do not need it. Hand-raised kittens who are not enough for solid food should get only milk specially formulated for kittens.
Adult cats do not have enough lactase in their stomachs to digest dairy milk. Lactase is the enzyme that breaks down the lactose in milk and dairy products. Kittens produce lactase in order to digest their mother's milk. As they grow up, however, they produce less and less. Drinking milk or eating cheese can cause an adult cat to throw up and have diarrhea. That won't stop Tom from coming at a run to check out the dairy you just pulled from the fridge.
Dairy milk is not the only form of milk that makes cats sick. Coconut milk is an alternative to dairy milk for lactose-intolerant humans, but not for cats. It contains oils that can cause nausea and diarrhea just as diary does.
If your cat really likes milk, you can give him a few forms as an occasional treat. Check your local pet store for cat milk, which is specially formulated dairy or cow milk for cats. Two brands of this are CatSip and Whiskas Cat Milk. They are treated with lactase and contain taurine, an amino acid necessary to cats.
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