Well, the brain is the largest consumer of calories in the human body so it isn't ridiculous that intense mental activity over a prolonged period could burn a l

Well, the brain is the largest consumer of calories in the human body so it isn't ridiculous that intense mental activity over a prolonged period could burn a lot more calories... but yeah, 6k is on the level of daily caloric expenditure you'd see in a farmer working with hand tools, so it is difficult to believe.