Veterinarians often recommend spaying a female cat if the owner does not want her to have kittens. What does biology know about the effect of spaying on female cats' organism and health (apart from lack of fertility, of course)?
From the standpoint of the skeletal system, spaying is definitely not good for her health. Removal of the ovaries mimics what happens at menopause, when circulating estrogen levels fall. Estrogen is necessary for maintenance of bone mineralization. Without estrogen, bones start to demineralize (the same process that happens in human females after menopause, leading to osteoporosis).
Then again, some animals, if they do not get pregnant, will cycle indefinitely until they get anemic and die. Ferrets are an example. So it's better to have these spayed.