Hiya, most cats are lactose intolerant so it is the lactose in the milk that they cannot digest. I'm not 100% sure but something to do with not producing the thing that breaks down lactose once they are weaned (please someone correct me if I'm wrong)

You can buy cat milk that has reduced lactose in it. Personally I buy a lactose free milk (from tescos) which is a lot cheaper, it's in the bit with all the ling life milk etc

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