i dont think all rats are lactose intolerant but they can be. same as dogs and cats some can have a little milk no problems but others look out! and you know what a lot of it has to do with the processing of the milk. if you got fresh from the cow most people wouldnt have as much problem with it either.
Poor performance and cataract formation occurred in rats fed lactose or galactose (Day and Pigman, 1957); diarrhea was also observed in weanling rats fed α- or β-lactose (Baker et al., 1967)