"In 2004, winner Rustam Kasimdzhanov walked away from the six-game world [chess] championship having lost 17 pounds"

ESPN looks at the physical demands of high level chess:

In October 2018, Polar, a U.S.-based company that tracks heart rates, monitored chess players during a tournament and found that 21-year-old Russian grandmaster Mikhail Antipov had burned 560 calories in two hours of sitting and playing chess -- or roughly what Roger Federer would burn in an hour of singles tennis.

Robert Sapolsky, who studies stress in primates at Stanford University, says a chess player can burn up to 6,000 calories a day while playing in a tournament, three times what an average person consumes in a day. Based on breathing rates (which triple during competition), blood pressure (which elevates) and muscle contractions before, during and after major tournaments, Sapolsky suggests that grandmasters' stress responses to chess are on par with what elite athletes experience.

"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.

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Stress and anxiety, in fact, are the greatest drivers of the phenomenon.

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not one of these grandmasters has perfected his fitness routine like the current world champion, Magnus Carlsen

*Previously: "Chess Coach to Leave Texas Tech With Her Team’s Best in Tow"