Yep - it's called lactose intolerance. It's a sort of minor allergy, I suppose.

They can manage pretty well with less, but you'll still want to engage them. Cats have a natural prey drive, so they like to hunt, but if there's no vermin or birds or anything for them to hunt, they'll go for toys. Just don't play with them with your hands directly, if you can help it, because they'll attack you any time they want attention if they associate your hands or feet with playthings.