"In the aerobic exercise domain up to ~100% of maximal oxygen uptake (VO2max), CHO is the dominant fuel, as CHO-based oxidative metabolism can be activated quickly, provide all of the fuel at high aerobic power outputs (> 85-90% VO2max) and is a more efficient fuel (kcal/L O2 used) when compared to fat."
https://www.gssiweb.org/sports-science-exchange/article/regu...
Try eating 100g of grass per hour during a marathon and you will see. That's the metabolic edge horses have over humans.
1 food calorie as listed on a food label is enough to heat 1kg of water by 1c
https://www.futilitycloset.com/2008/11/16/the-mensa-diet/
(If the nutritional calories in the drink had been only the same number of thermodynamic calories, the drink would have been energetically negative for the body because of its low temperature.)