I'm diabetic. Type II, so what that mostly means is if I am very hungry, I AM VERY MOTHER****ING HUNGRY AND MUST EAT RIGHT NOW!!!!! GRRRR!!!
So I have learned to eat a light meal pre-show, try to keep the beers to 1 or 2, and of course yes I pee right before I go on. Any diabetic will tell you this -- pee early and pee often.
Other than that the only time I've actually thought my condition would impair a show was one VERY COLD day I went out the night before drinking, ran 2 miles that day to try to flush the excessive Guiness from my system, and then took one of those 5-hour power drinks. I am not sure if it was the stimulants, the hangover, the incredible difference in temperature between the front door and under the stage lights, or a combination of all of thse, but I went on and started to sing my first loud note, and my head felt like it was exploding. I made it through the first song, considered very briefly telling the guys I just could not continue, and then decided I was going on regardless. Sang the whole set with my head feeling like it was being crushed in a vise every single time I stopped singing a note.