A chess player can burn up to 6,000 calories a day while playing in a tournament

Well that's interesting.

The human brain consumes approximately 20% of the energy the body produces. I have occasionally wondered whether a brain thinking hard consumes measurably more energy than a brain daydreaming. Conceptually it sort of makes sense that it would but there numerous attributes which would likely confound that easy assumption.

But perhaps I am on to something but through a different mechanism than I was assuming.

From The grandmaster diet: How to lose weight while barely moving by Aishwarya Kumar.

The 1984 World Chess Championship was called off after five months and 48 games because defending champion Anatoly Karpov had lost 22 pounds. "He looked like death," grandmaster and commentator Maurice Ashley recalls.

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

[snip]

Grandmasters in competition are subjected to a constant torrent of mental stress. That stress, in turn, causes their heart rates to increase, which, in turn, forces their bodies to produce more energy to, in turn, produce more oxygen. It is, according to Marcus Raichle, a neurologist at Washington University in St. Louis, and Philip Cryer, a metabolism expert at the school, a vicious, destructive cycle.

Meanwhile, players also eat less during tournaments, simply because they don't have the time or the appetite. "The simple explanation is when they're thinking about chess, they're not thinking about food," says Ewan C. McNay, assistant professor of psychology in the behavioral neuroscience program at the University of Albany.

Stress also leads to altered -- and disturbed -- sleep patterns, which in turn cause more fatigue -- and can lead to more weight loss. A brain operating on less sleep, even by just one hour, Kasimdzhanov notes, requires more energy to stay awake during the chess game. Some grandmasters report dreaming about chess, agonizing over what they could have done differently for hours in their sleep, and waking up exhausted.

The effect size is surprisingly large, but it seems plausible.