A large meal is a big no-no when playing in a tournament. Instead, I usually eat a lot fruit, not candy bars like that, because they give you sugar rush and then your blood sugar level plummits and you are on "low" again. Fruit has sugars and fibers and they keep your blood sugar level up much longer. If you know you're going to have a couple of hours of waiting time between matches, grab a quick light lunch but don't overdo it. A light lunch maybe a salad, or a good bread roll with ham/cheese/anything plus a banana for instance. I usually carry a few bananas with me or dried prunes or something else which give me good energy.
Drinking soda or energy drinks have the same effect that those candy bars, you'll get "high" and "low" quite fast and that's something which you ruin your consistency at the table. The problem is that "low" comes
before you feel hungry, that's why you might be playing very badly without a clue why. Or if you feel a lack of motivation and concentration, it's usually blood sugar related, you are simply low on sugars.
I was recently in a straight pool tournament, and after losing my first match I won 4 matches on loser side, 3 of the last ones in a row and had to play my 4th consecutive match, which I eventually lost playing badly because of lack of nutrition (I ran out of bananas in the previous match

). I think I played like 5-6 hours straight without a decent break. It's in those situations when a good nutrition becomes handy, you just have to eat small healthy snacks to keep your body and mind operating. And also have a glass of water with you during a match, your mind and body cannot work without water.
Well, I don't drink beer or alcohol during tournaments, but it's very obvious that if you drink beer all day long, and reach the latter stage on the tournament and feel like you shouldn't drink anymore, you're going to face another "low", this time it's alcohol low, which makes your game plummit. I don't want to recommend this, but if you need to be a little bit drunk during a tournament, don't stop drinking but don't overdo it. You'll have a better chance of keeping up your good stroke if you don't make radical changes in your liquid and food intake.
Just my $.02