The first time I did a longer fast (72 hours) I felt low-energy, as you describe. Then I ate a single large meal and did another long fast; that felt quite good for the duration of that fast... so I did that for a month, more or less, and lost a lot of body fay (I'm 6'4" and I went from 225 to 200).
And then I was being pretty healthy for 8 months before going back to occasional drinking, but I still don't do sweets other than fruit.
I mostly eat a single meal in the middle of the day (though I make a lot of compromises due to being around other humans) and I don't usually find myself hungry in a way that is intrusive on my thoughts. However, I did start rock climbing in a gym and I upped my caloric intake and would eat a small meal afterwards.
I did another series of fasts this spring to bring my weight down to 175, and it's stayed there. And then I did a short series of longer fasts this fall, and didn't feel low-energy during them.
It's very likely that it was most of it. It's just how fasting works. Your body enters ketosis in a day or two and uses fat for fuel. Your body would have to be rather dumb to burn muscle at that point.
I don't have a good way to measure that, but if you have one let me know.
I can say that after all this, I was climbing hard 5.10s and easy 5.11s, and I'm 40, and I haven't been able to climb stuff like that since I was about 22.
The first time I did a longer fast (72 hours) I felt low-energy, as you describe. Then I ate a single large meal and did another long fast; that felt quite good for the duration of that fast... so I did that for a month, more or less, and lost a lot of body fay (I'm 6'4" and I went from 225 to 200).
And then I was being pretty healthy for 8 months before going back to occasional drinking, but I still don't do sweets other than fruit.
I mostly eat a single meal in the middle of the day (though I make a lot of compromises due to being around other humans) and I don't usually find myself hungry in a way that is intrusive on my thoughts. However, I did start rock climbing in a gym and I upped my caloric intake and would eat a small meal afterwards.
I did another series of fasts this spring to bring my weight down to 175, and it's stayed there. And then I did a short series of longer fasts this fall, and didn't feel low-energy during them.