6000 calories seems absurd. But having played chess tournaments I have lost a bit weight during them despite eating generous amounts of bacon, eggs, hash browns etc at the hotel breakfast, then more crappy fast food in the evening, with mostly sleeping and relaxing in between. So I feel like there must be something to it. Although 6000 kcal in one game is ridiculous, even factoring in nervously pacing the playing hall when it's not my move.