The Feline Diet - Why Adult Cats Should Not Drink Milk

It's true that cats love milk. Most cats will drink it if you let them. And who hasn't shared a little milk with their feline friend now and then? So, what's the problem?
Cow's milk is not a natural food for any cat. Even kittens, who naturally drink nothing but milk for the first few weeks of their lives, specifically need milk that is higher in protein, certain fats, and other nutrients, than the stuff we get from dairy cows. In other words, the only milk they need is the milk that their mothers provide.
Once kittens are weaned (at 8-12 weeks of age), they often lose their ability to digest milk. Many adult cats are lactose intolerant, and can develop an upset tummy and diarrhea from drinking cow's milk. 
There are other problems with cow's milk, as well. Much of the milk we can buy at the store comes from cows that have been injected with a growth hormone, called rBGH. This hormone causes changes in the milk itself, including elevated levels of another hormone, IGF-1, which is known to promote cancerous tumors. Milk from rBGH-treated cows is also different in the types and amounts of fatty acids and proteins it contains.
rBGH also causes higher rates of painful mastitis (inflammation of the udder) that must be treated with antibiotics and anti-inflammatory drugs. This increases the chances of chemical residues in the milk that could harm your cat. While there are laws preventing dairies from selling milk with illegal levels of certain drugs, inspections are often inadequate. Other drugs are not even tested for. Chemically contaminated milk can still end up in your refrigerator.
Even without rBGH, cow's milk is full of hormones, including estrogens, which are linked to many health problems. Milking cows are kept pregnant 11 months of the year; and all those pregnancy hormones flow right into the milk. Milk may also contain traces of pesticides and other contaminants of the food the cows are fed. 
Unlike the milk produced by nursing queens (mother cats), which kittens drink in its natural raw state, store-bought milk is pasteurized. Pasteurization is necessary to kill harmful bacteria that are rampant in high-production dairy facilities. But it also destroys much of milk's natural vitamins A, C and some B vitamins; it makes calcium less absorbable; and it deforms (denatures) proteins and enzymes. Such altered proteins may cause allergies. Additionally, casein, the major protein in milk, may itself cause allergies. Dairy products are one of the top allergens in cats (the other most common allergens are chicken, beef, fish, wheat, and corn). 
So, even though she may love it (don't we all love to eat things that aren't very good for us?), cow's milk is definitely not a good choice for your cat.