It takes some skill to time the right amount of energy for your presentation. Presenting after a very heavy lunch will be difficult. Presenting on an empty stomach is the other extreme. I usually eat a granola bar around 30 minutes before I have to go on stage, to make sure that I am all fired up to go with a fresh shot of energy. Granola bars have a good sugar kick, but also provide some substance that is missing in many processed food snacks and candy.