You actually should exercise while fasting. When fasting, t he body gets rid of fat, and any muscles that are not essential. Which makes a lot of sense - maintaining muscle costs both protein and energy, even if they aren’t used much.

I did not try to run quickly while fasting, but I did keep doing my 5km 3 times per week, and the gym. Anaerobic was hard and quickly tiring, but aerobic exercise wasn’t really any harder.

A friend did point out that my movement was “more economical” in general - I hardly lifted my feet while running or walking, and a few other things. But I didn’t feel more tired or more strained, except for unaerobics.

1) there are tons of substances the body can't make out of nothing/lipids. 2) there are tons of substances the body can't make efficiently with no food, which leads to metabolism/health deficits. and no it's not just vitamins and electrolytes LOL it's much more complex and diverse than that.

3) even if we assume in an ideal world, that the person who fast is able to supplement ALL the useful things and yet not take any external calories/glucose. This person would still reach a huge bioenergetic deficit/stress. Yes he will survive as you say, and might even be asymptomatic but nonetheless I would be very surprised if the biomarkers where doing Okay. Default bloodworks analysises are shit but if you command a proper analysis of your antioxidants ratio and of your bioenergetics biomarkers and apoptotic biomarkers you will see (unless I'm wrong which is very unlikely given my expertise) that the body is having a significantly accelerated aging/damage scheme. Which might not show symptoms before multiple decades later and even might not show symptoms at all in his life but statistically, there is a risk. Now he can mitigate some of those, by taking e.g. ALCAR + NAC daily for a year.

BTW the guy you linked died at 50 years old, which is not a performance.