first, hospitalization is a pretty big deal. second, try to avoid using items in the first place and avoid the bad statuses to begin with if possible. 3rd, as a general rule, go with whatever food prill wants you to feed your monster, she's generally right and giving them something that they don't like every now and then is okay.
in regards to some of the statuses. if you work them too much, they get thin, if you let them sleep too much, they get fat. a mix of play and enough rest will keep their stress down. Playing also allows the monster to rest for a week when you do it so you can kill two birds a bit with one stone.
That said, even though monsters are supposed to live for only about 5 years, there are many instances through no fault of your own they might live less, some of that can be genetic too.