Food and liquid intake provides a human body with energy. Yet having a meal before a chess game can cause the body to focus more on digestion than chess. While lack of energy during a six hour game can cause loss of focus and thereby mistakes. Getting thirsty can have a similar adverse effect. When and what should one eat and drink before and during a chess game with classical time controls?
I have played tennis and chess both quite seriously for several years and my preparation towards a game when it comes to food, is basically the same. I try to eat around 4 hours before the game, some pasta with a light sauce or no sauce at all, avoid red meats 24 hours prior to the game and rink only water the day of the game. If I am to start in the morning then a light breakfast. In general I would rather play with a non-full stomach but here's the "secret", I do eat a chocolate bar at the beginning of my chess games and have a bottle of water at hand, this helps me greatly to keep my blood pressure normal, and able to focus on the game without feeling hungry even after 4 or 5 hours of play.
I guess my advice is to know what works for you, I have seen that avoiding heavy meals is generally good for performance but if you (like me) need an extra kick, pick up an energy bar of some sort (I would avoid coffee because of the possibility of needing to go to the toilet at a bad time).
All the best and hope it helps! :)