Energy expenditure and energy intake are two major contributors that dictate your weight loss or weight gain.
When we talk about energy expenditure, the first thing that comes to your mind is probably physical activity. If you are well versed in the subject and you think for a few moments, non-exercise activity thermogenesis (NEAT) comes to your mind. In the end, every little movement counts.
Have you ever wondered how much energy you can actually burn by mental work? I always have.
When I started monitoring my hunger levels more closely, I found out that during the times when I coach, write and create content, I get much more hungry than when I have a physically active day. (I also tend to have very active legs, moving them back and forth while sitting, which can add up to 100-800kcal /a day, an equivalent of 60 minutes of walking/running). 
This is where this article about a grand-master chess player comes to the spotlight. It says that chess players can burn during a tournament up to 6000 kcal! An equivalent of a triathlete training for Ironman!
“Concentration alone isn’t enough to hyper-charge your metabolism. You also have to care… if you and I have thirty minutes to learn a list of words……I’ll probably be a sweaty mess after the thirty minutes are up.I hate to lose. So yeah, I’ll be into it. Even though I’ll just be sitting in a chair I’ll “work” my butt off to learn those words.”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.
Later, I found that these numbers about calories were very rough estimations made by the authors of the article (shame on me for believing it).
To redeem myself, let me correct what I wrote previously and let it be a learning lesson for both of us.
“The definitive study on chess players was carried out by the physiologist Leroy DuBeck and his graduate student Charlotte Leedy. They wired up chess players in order to measure their breathing rates, blood pressure, muscle contractions, and so on, and monitored the players before, during, and after major tournaments. They found tripling of breathing rates, muscle contractions, systolic blood pressures that soared to over 200—exactly the sort of thing seen in athletes during physical competition. See the original report, Leedy’s thesis, “The effects of tournament chess playing on selected physiological responses in players of varying aspirations and abilities” (Temple University, 1975) or their brief report (Leedy, C., and DuBeck, L. 1971. Physiological changes during tournament chess. Chess Life and Review, 708).– Robert Sapolsky
So can thinking hard burn so many calories?
Studies don’t support that idea.
Sure, you need to use more energy when learning new things, or when you think hard. As Peter Sapolsky mentions, “They found tripling of breathing rates, muscle contractions, systolic blood pressures” but breathing harder does not burn that much more energy. As some studies point out, differences between resting and thinking hard seem to be negligible at best. 
I remember that throughout high school I spent 4-8 hours a day playing computer games, living on what is now popular as “bulletproof coffee”. After coming home from school, I would eat 200g of fried cheese, fries, and a ton of ketchup. Then I would follow it up with three of those coffees, each coming at about 300kcal. That is besides breakfast, snacks, lunch, and dinner. Just thinking about it makes me sweat! But I was not getting fat.
As mentioned above, I would spend hours sitting and playing games and I would care about it. I played to win, I brainstormed, strategized, refined and tested stuff. I was invested.
So if it was not the thinking hard part, what was it? I’ve always been a big fidgeter. And playing computer games, or desk games can make you fidget quite a bit, it seems.
Energy expenditure hypothesis
Here is my hypothesis when it comes to energy expenditure.
We know several things about energy expenditure as it relates to our energy availability:
- Our body adapts to states of low and high-calorie availability by expending less/more calories;
- Our body adapts to activities we do regularly such as walking, running, or lifting by getting more efficient and using less energy to do these movements.
- Our body also reacts to energy availability by regulating our NEAT, temperature, choices like willingness to move, hunger, and even heart rate.
Based on these facts I made a hypothesis. To use the most amount of energy you want to train like an athlete. Combine endurance, explosiveness such as sudden direction changes, strength, and be quick on your feet literary and figuratively speaking.
Combining different intensities, movements, and thinking/keeping attention on what’s going on like during combat sports or team sports, you provide your body with a variety of signals.
Training for strength, power, explosiveness, and endurance prevents your body to adapt.
Has anybody mentioned chess boxing?
- Ravussin, E et al. “Determinants of 24-hour energy expenditure in man. Methods and results using a respiratory chamber.” The Journal of clinical investigation vol. 78,6 (1986): 1568-78. doi:10.1172/JCI112749
- Raichle ME, Gusnard DA. Appraising the brain’s energy budget. Proc Natl Acad Sci U S A. 2002;99(16):10237-10239. doi:10.1073/pnas.172399499