This is a bit of a mess: journals.physiology.org/doi/…

A study put elite athletes who burned almost no carbs on a treadmill. They recorded the highest fat-burning rate ever measured in a human. The FASTER study. 2016. Published in Metabolism. Twenty of the best ultra-endurance athletes on earth. Ultramarathoners. Ironman triathletes. The kind of people whose careers depend on knowing exactly what fuel works. They were split into two groups. Ten ate the standard high-carb athlete diet. The diet every sports nutritionist still pushes. Eat the carbs. Load the carbs. You cannot perform without the carbs. Ten had been low-carb and keto-adapted for an average of twenty months. Same elite level. Same competitions. No carbs. Both groups ran three hours on a treadmill. Researchers measured exactly what fuel each body was burning, breath by breath. Here is what they found. The keto group burned fat at 2.3 times the rate of the carb group. Peak fat oxidation hit 1.5 grams per minute. The textbooks said the human body maxes out near 0.7 grams per minute. The keto athletes doubled the supposed limit. The highest fat-burning rate ever measured in a human, full stop. Then the part that should have ended the carb-loading dogma forever. The fear was that without carbs they would run out of muscle glycogen mid-race. They did not. Their glycogen use during the run and their refill afterward matched the carb athletes exactly. They were running on their own fat at elite intensity. With glycogen behaving identically. The body was never carb-dependent. It was carb-trained. You can train it differently. Almost a decade later, every endurance handbook still tells athletes to load up on carbs. The data has been sitting there the whole time.