Q Is giving milk to adult cats, especially older cats, unnecessary and harmful to them?
A I'm sure many of our readers are lactose intolerant, so they know what drinking a glass of milk does to them! Most cats (and dogs) develop essentially the same condition as they age because they lose the enzyme, lactase, necessary to help them digest lactose, a sugar present in milk. (Kittens, of course, have ample quantities of this enzyme so they can digest their mom's milk.) As the undigested lactose passes through the intestinal tract, it draws water along with it. In addition, bacteria present in the colon ferment the undigested sugars and produce volatile fatty acids. The result is diarrhea.
Cats can probably tolerate a little milk, but larger quantities will almost assuredly cause diarrhea in most. Since cats who consume a nutritionally complete and balanced commercial diet don't require any of the nutrients in milk, there is normally no reason to give it.