Supposedly chess layers burn excessive calories playing. From one popular culture article[0]

  ...The six-game championship in 2004 left Rustam Kasimdzhanov 17 pounds lighter. And in 2018, one company tracking the heart rate of grandmaster Mikhail Antipov concluded that he had burned 560 calories in just two hours. Sitting still. Playing chess. (An average-sized person would need two hours on a treadmill to break 500.)
  Last week, ESPN looked more closely into the toll the stationary sport takes on the body. ESPN spoke with Robert Sapolsky, a Stanford University researcher who studies primates. Sapolsky explained that some chess players respond to the game like any elite athlete, burning upwards of 6,000 calories a day during tournaments, due to tripled breathing rates, elevated blood pressure, and muscle contractions. That means during tournaments they can lose two pounds each day.

I have done several fasts, and I am quite dubious of the supposed mental improvements. My brain has spent 99%+ of its existence no more than 12 hours away from a steady supply of glucose. I personally feel physically and mentally sluggish. While probably erring much on the side of caution, I avoid driving during a 5+ day fast because I feel my reaction speed is diminished.

Furthermore, the brain is the single most energetically expensive organ in the body. If there were a shortcut metabolic pathway, it seems like it would have been exploited long ago.

[0] https://www.menshealth.com/fitness/a29144951/chess-players-c...