What percentage of cats are lactose intolerant?

If a cat has lactase enzyme deficiency (meaning lactose intolerant) it can cause diarrhea. Lactase breaks down the milk sugar before absorption into the blood stream. A deficiency leads to the cat's inability to break down the milk sugar - lactose.

The same problem affects 50 million American people! That is why there is no information of the internet about cats - it is all about people.

The answer I believe is that all cats are to a certain extent lactose intolerant but the Cat Owner's Home Veterinary Handbook alludes to the fact that some cats are not lactose intolerant, "if a cat has a lactase enzyme deficiency"....Clearly some cats don't have a lactase enzyme deficiency.

There is nothing on the internet or in the best books I have that tell me the percentage of cats that are lactose intolerant.

I would make an assumption that all cats are lactose intolerant to varying degrees and don't feed cow's milk to a cat. There are special cat milk products at pet shops.