A 20 min HIIT won’t significantly raise cortisol whether fasted or untasted. The fasted component doesn’t impact cortisol – that’s the intensity and duration.
Catabolism (breaking down of muscles) is lack of fuel plus exertion so keeping it short and fuelling straight after avoids this.
Plus fasted trading has other positive benefits like increased metabolism, insulin response etc so benefits are huge compared to risk of above when done properly.
So, keep sessions should short and not more than 3 a week and fuel straight after.
