Gargh! This is a real issue. When ever I've been in the slammer to more than two days I order takeaway so I have at least one palatable meal a day. That has created issues on occasions as some hospitals don't like patients bringing their own food, and some try to expressly forbid it.
I don't eat much meat and never want it in hospital. I'm also a bit touchy when it comes to gluten so try to avoid it when possible. That narrows the menu considerably.
I find out from the afternoon staff which restaurants deliver (nurses get meals brought in for themselves all the time) and order a good quality vego dish that I can split over two meals. I also take snack packs of nuts, precooked rice cups, tins of tuna and protein shakes. I keep one of those Tupperware shake makers in my emergency bag. This should not be necessary, but apparently it is.