ESPN has an interesting article on the physical toll Chess Grand Masters go through.
“Grandmasters sustain elevated blood pressure for hours in the range found in competitive marathon runners,” Sapolsky says.
It all combines to produce an average weight loss of 2 pounds a day, or about 10-12 pounds over the course of a 10-day tournament in which each grandmaster might play five or six times. The effect can be off-putting to the players themselves, even if it’s expected. Caruana, whose base weight is 135 pounds, drops to 120 to 125 pounds. “Sometimes I’ve weighed myself after tournaments and I’ve seen the scale drop below 120,” he says, “and that’s when I get mildly scared.”