I've always known that doing low-intensity cardio on an empty stomach is not that big of a deal when it comes to muscle loss, but that doing a HIIT routine on an empty stomach is a big NONO.
Its been recommended over and over that some form of carb/protein preworkout is optimal, but I was recently told that for those with a high BF% (>16%) it isn't necessarlily optimal to have food precardio. The reasoning behind this was that in a fasted morning state, adipose tissue is the main source of energy (thats why low-intensity on empty stomach is no big deal) but doing a high-intensity type routine would turn to glycogen stores as the main energy source, and not the target adipose tissue.
Basically, I was told that with a high BF%, and doing high intensity in the morning, my body will burn the excess fat (glycogen wasn't mentioned at all...i dunno why) so having food precardio is just a waste of the time spent doing the cardio since you are burning the recently ingested nutrients.
Should I just stick to my normal low-intensity morning cardio on empty stomach or should I do HIIT on empty stomach?