When I had my cats spayed (I know, you said neuter), the vet told me not to feed them for at least 8 hours before the surgery. As far as after the surgery, I had food and water out for them. Neither of my cats wanted food for a good 12 hours though!! I don't believe Izzie threw up, but I know Charlotte's tummy was upset after the surgery. Then again, I don't believe neutering is as traumatic as spaying.