[myth is the idea that fat burning decreases as intensity increases.]
The relationship between anaerobic high intensity and muscle glycogen isn’t a myth. Nor is the difference between the metabolism of fast and slow twitch muscle fibers.
Fat burning, stated as a percentage of total energy production, decreases. But total energy production increases, so it's a smaller percentage of a larger number. Total fat calories expended doesn't decrease. Work harder and you'll a lot more glycogen, but you won't burn less fat.
At a moderate level of exercise, you might burn 150 calories in 30 minutes, 80% of them from fat. At a more intense level, you might burn 300 calories, only 50% of them from fat. But 80% of 150 is 120, and 50% of 300 is 150 - and the total number of fat calories burned increased, despite the lower percentage.
The body doesn't stop burning fat, during high intensity exercise. It ramps up the burning of sugar, but the fat burning continues.
[The body doesn’t stop burning fat, during high intensity exercise. It ramps up the burning of sugar, but the fat burning continues.]
Yep. Indeed it seems that the ability to burn carbs as fuel is a beneficial adaptation on top of the aerobic base metabolism.
It would be interesting to see a comparative study of the related physiology in another primate species sans agricultural adaptive influences.
It could be that humans have adapted to carbohydrate rather recently, with varying degrees of success — with the carb role in the citric acid cycle being part of that adaptation.
OR - it could be the carb role in the citric acid (TCA) cycle can be seen in, say, the Bonobo Chimpanzee — which as far as I know, doesn’t do much gardening.