I remember we discussed one ultra-marathoner who went low-carb and improved his performance. In fact, he won that race by a large margin. Dunno how much carbs he ate, but it was much lower carb than he used to eat. I remember he explained the biggest problem isn't really energy, it's the gut. Many of them get to a point where they can't eat or drink anymore, cuz they just puke it out. Another problem is at the other end, they literally had to go several times during a race. With low-carb, that guy didn't have any of those problems anymore. When the race is 12 hours, these things make a big difference.

Since this gut problem makes such a big difference, I'm not sure we can draw any conclusion about energy or anything else just based on this anecdote. But then again, even if that's all it takes to win a race, who cares what else it can do.

It's the often repeated, but unscientific assumption that "fat burns in the flame of carbohydrate" still popular today.

It occurs to me that glucose is required by cells, not necessarily for energy, but more specifically as function. For example, to begin the Krebs cycle, one molecule of glucose is needed, but after that the cycle keeps going without the help of glucose. Or at least that's my naive understanding off the top of my head. Anyway, the point is in this respect, glucose could be seen more as the spark plug in an engine rather than as the fuel that generates the power to move the car. The spark plug ignites the fuel, but then once fuel starts to burn, it's hot enough to keep burning without the help of the spark plug, so there's no need to send electricity to the plug anymore. In a way, trying to run a body on glucose would be like trying to run a car on a thousand spark plugs, but with little actual fuel. When you think about the total amount of various substrates the body can store, it makes sense that glucose wouldn't be used as primary fuel, if at all as fuel. And then from this, it makes less sense to conclude that dietary carbs are needed to burn fat better. Granted, there's no physiological plausibility here (i.e. what we are actually adapted to eat), it's just logic derived from quantities.