I feel like I’ve seen guidance from the coaches that Z1/2 fasted is fine, but they recommend fueling for strength and Z3 work.
If I have a morning Z2 workout I usually do it fasted with a cup of black coffee, but always eat something before CMF or any weighted carry’s or strength sessions. If my schedule permits I’ll usually do z2 when I wake up, but move the others later in the day since they are not fasted.
I do all my shorter Z1 or 2 workouts in the morning before work fasted, I don’t have time to eat and let food settle and work out and get the kids to school and get to work on time so this works for me. I try to fuel up for any interval work or sprint or strength workouts and I plan longer workouts (over 90 min or so) for after work or on the weekend and eat something before and fuel during a run or hike over 90 min. just my $2.
I also do my Z1/Z2 runs fasted in the morning (after eating only breakfast and lunch the previous day) and it has worked well for me so far. I tried to do strength training fasted but prefer a meal about 2-3 hours before.
I BELIEVE any workout done after about 4-5 hours of not eating anything is considered a fasted workout; one doesn’t have to do the fast overnight.
Shorter z1/2 workouts can certainly be fasted IMO. Longer Z1/2 efforts can be but you want to progress this over time and realize you may well be compromising the rate of your recovery. I don’t think its always worth it.
I would not bother fasting Chamfit and would certainly not fast for any Z3 of higher intensity but particularity and muscular endurance type heavy work. Always fuel for these.