For those interested, Jeff Volek's team published a study in 2016, "Metabolic characteristics of keto-adapted ultra-endurance runners" [1] that compared fax oxidation. One of the "fat adapted" participants Zach Kibbter, an ultra-marathon record holder, posted a personal writeup that included a table of his fat usage at various VO2 Maxes (a ridiculous 98% fat usage at 75% VO2max and 76% even at 84% of VO2max!)[2]

[1] https://www.sciencedirect.com/science/article/pii/S002604951...

[2] https://web.archive.org/web/20201111190720/https://zachbitte...