Our expert says:
Well, the Krebs cycle and pyruvate are distinct, so you couldn't stay within the Krebs and use pyruvate. Pyruvate is formed as the product of glycolysis, and goes one of two ways - one is to be converted into acetyl CoA, which then enters the Krebs cycle. The other is the conversion to lactate. Both allow more energy to forum, but the Krebs cycle only happens at lower intensities when the rate at which pyruvate is formed is lower than what can be handled by the Krebs cycle, if that makes sense. Any "overflow" is converted to lactate.
And you never use up all your glucose - if you did this, you'd be dead, because the brain is fully reliant on glucose, so it's never allowed to deplete fully. The thing that determines whether you go to the oxidation of the Krebs cycle or the lactate system is the intensity of exercise, nothing more.
The use of fat is via the Krebs cycle, though that molecule acetyl CoA. This again doesn't happen at very high intensities. So to answer your question as well as possible - if you do HIIT, the primary source of energy is the formation of lactate, with energy being generated through glycolysis. A small amount comes from oxidation of glucose in the Krebs cycle, and a small amount from fat oxidation, and a small amount from creatine phosphate.
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