Keeps you form passing out getting light headed, etc. I'll that trade off.
Depends on the individual.
For example I pinned ghrp + cjc peptides (anecdotally lowers blood sugar) first thing this morning in a fasted state, then went to the park and did ten second sprints with ten seconds rest for ten sets. No signs of light headedness whatsoever. And later in the day I had a weights workout 4 hours after a meal with the peptides pinned beforehand with no problems.
I've also noticed that no matter how much or little food I eat in the day, I don't feel much difference in energy levels. My conclusion is genetically my body is good at maintaining blood sugar or can tolerate low blood sugar without problems.
I can see how other people might have problems though due to their genetics, in which case some carbs might be needed before training.