For a normal healthy person it is a quick-fix mechanism that we have.The trouble is of course, if you have ME/CFS it doesn't work, because most of that quick-fix energy does not do anything useful at all.
I idly wonder what very elevated glucose levels would do to PEM, but don't have diabetes.
Indeed, the recent serum analysis which found all sorts of elevated/depressed metabolites found glucose was completely normal.
Which is probably a good thing - because if on PEM, the body attempted to ramp up the glucose, bad, bad things may happen, and it might look a lot like diabetes. (though such a large and obvious biomarker might have gotten work done earlier)