I've done a 12 day fast. The first 4-5 days were extremely difficult (caffeine withdrawal didn't help) , but by day 6, I had a feeling of euphoria. My original goal was a 10 day fast, but I didn't want to give up the clear mind and energy I was experiencing, so I extended it to 12 days.

I think this is a pretty common experience. You don't start feeling the "good feels" until you get past the 4-5 day mark.

Anyway, this was about 7-8 years ago and I haven't tried again since, but I often think about it.

For what it's worth, I do IF as a lifestyle (18:6). Basically (for me) it's a very easy way to help restrict calories. Hopefully there are some health benefits, too.

Here's an anecdote:

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.

Careful there with using bodyweight as your gauge. How much of that was fat versus muscle loss?

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.

Yes, whenever I've done 48 hour fasts, I've had the same experience. This is due to the depletion of glycogen in your liver and muscles. At that point your body doesn't have the fat-burning machinery quite going yet, so you are in a temporary dip in energy. You'd have to do another 24 hours or so, I believe, to fully enter into that state. At that point it's business as usual except that you do zero digestion. It's where all the good feelings are. At this point your body balances itself for the time being and produces exactly what it needs to in order to function. You actually get a boost of hormones and metabolism.