What about water fasting for longer periods? Have you gone 24/48/72 hours without any food? I find that longer fasts tend to work better for me, since I end up eating more calories in the 8 hours window in a 16/8 IF regime. But if don't eat for close to 36 hours, even with the increased food intake, I would still come out ahead.
If you fast long enough, say 48 hours I think, your body enters a mode of "autophagy", which is highly beneficial. So there's that.
I've done extended water fasting several times. 5-days, 7-days, 10-days, 17-days. It's MUCH easier for me to fast for 7-days than do 7 24-hour fasts. I feel much less hunger and fatigue when doing extended. All the negative experience is in the 1st day or 2 and gets really easy by day 3. So the result is much less net discomfort with extended fasting.
I fasted for around 40 hours once, and by the end I felt extremely low energy. I noticed I was avoiding standing, leaning on things if I did, and my normal foot-tapping or idle movements completely stopped.
Does anything like that happen to you? Do you know if it would feel better during autophagy?
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.
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.
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.
I believe autophagy doesn't kick in for the first 5 days, so if you want to preserve all muscle mass, that's the window to stay under. Once you go after that, the changes aren't that dramatic either, so you'll recover whatever muscle loss you incur fairly quickly as long as you're fasting while still having fat reserves. Once you're out of those, fasting is a bad idea, in the sense that you will start dying.
It's not like your body is going to eat its own muscle! It's not that stupid. The muscle loss is due to the normal wear and tear of daily activities. If you train hard during a prolonged fast, it's likely going to lead to more muscle breakdown. And autophagy is not that either. Also, it kicks in much sooner than 5 days.
As for fat reserves, even fairly lean individuals can probably go a week or two without food without breaking down much muscle. 14 days at 2500 calories is merely 10 pounds of fat. Unless you are morbidly skinny, you should have 10-15 pounds of fat ready.
If you fast long enough, say 48 hours I think, your body enters a mode of "autophagy", which is highly beneficial. So there's that.