Over the course of an intense multi-day tournament, a chess grandmaster could burn up to 6,000 calories a day, Robert Sapolsky, Stanford professor of neurology and neurosurgery, told ESPN.
Some players in this elite category find that they lose weight after a competition.
"Sometimes I've weighed myself after tournaments and I've seen the scale drop below 120, and that's when I get mildly scared," Fabiano Caruana, the No. 2 chess player in the world, told ESPN. Typically, he weighs 135 pounds.
That may seem surprising given that chess is a sedentary game, but a mental workout can burn calories too.
Many people associate "burning calories" with exercising, but calories are simply a unit of energy that you can get from eating. Your body needs energy to perform all of your daily activities, including using your brain.
The brain relies heavily on glucose, a form of sugar, to complete functions such as thinking, memory and learning, according to the Harvard Mahoney Neuroscience Institute. Studies suggest that brain activities account for 20% of the calories utilized by the body.
The more mentally taxing a task is, the more energy your brain needs to complete it, Marcus Raichle, M.D., a distinguished professor of medicine at Washington University School of Medicine in St. Louis told TIME in 2018. For example, learning how to play an instrument for eight hours could require 320 calories of brain power, he estimated. (To put that in perspective, a 150-pound person could burn about 340 calories playing 30 minutes of basketball.)
Stress adds another layer to your calorie expenditure. The acute stress that a chess grandmaster might encounter can cause a cascade of physiological changes, such as an increased heart rate and oxygen intake, which would also cause you to use more energy, Dr. Raichle told ESPN in 2019.
To be clear, thinking or intense concentration can't replace physical activity, nor is it an effective weight-loss strategy. The calorie expenditure you get from mental tasks "would be very modest," Dr. Raichle told TIME.
Eventually you'd get distracted or tired as you use up glucose. "You'd run into this depletion effect where you can't sustain the same level of cognitive performance," Dr. Raichle said.
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