I've eaten a power bar or a bagel with peanut butter right before I hop on my bike, and I've never had a problem with getting cramps or getting sick. The only time I've had a problem is if I have milk or something greasy before I ride. Also, if I'm doing a long ride or a hard ride, I usually do a gel or a serving of shot blocks every 10 miles or so. I have low blood sugar, and I've figured out that this is the best formula to keep me from bonking. You'll probably have to adjust to figure out what works best for you.
Good luck!
Aimee
A bicycle does get you there and more.... And there is always the thin edge of danger to keep you alert and comfortably apprehensive. Dogs become dogs again and snap at your raincoat; potholes become personal. And getting there is all the fun. ~Bill Emerson, "On Bicycling," Saturday Evening Post, 29 July 1967