Is that true? I thought that the difference between deep thinking energy expenditure and rest expenditure of the brain was not a huge % of the rest energy expenditure. I couldn’t find any source to the 6000 calorie figure, and this article seems to support that the chess player’s calorie deficit was likely due to skipping meals and stress https://www.livescience.com/burn-calories-brain.html

The brain is an organ like any other and gets fatigued more easily with age.