What I do when I can't eat (which is almost all the time) is that I try to stay hydrated, like faerie said, because I find that helps with the appetite. I also try to find small things to eat that don't require much chewing, just to keep the blood sugar on a non-critical level. Not eating makes it easier for the brain to switch over to mania or hypomania (I think my brain has figured that fact out and is abusing it), and it can also affect your mood badly (even if it makes you feel better), so you should try to eat regularly.