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.