Like plenty of people here already said, I don't usually eat anything while playing because it makes the controller/keyboard sticky/gross. When I was young and stupid I used to eat Doritos or some other chips and drink whatever soda was available.
Now, I take snack breaks if I'm playing for a long time. (in games like WoW the flight paths are great for this) I do consume vast quantities of Mountain Dew White Out, Vanilla Pepsi, Water, or Arizona Tea while on the couch or in front of the computer.