How long can you go without eating before it really affects you?

I know exactly how you feel - I had the cold over the weekend and am still bothered with it. Your body gets used to a routine of food, so when the body doesn't get the energy it needs, you can sometimes get dizzy or hunger pains. I normally eat jelly when I'm ill or toast, but if I can't eat anything at all, then I take lucozade for sugar and energy (or at least take a normal drink like coke, which you can take the fizz out of). Hope this helps!!