I did fasting 2 days a week for about 3 months. This is what happened.
First it radically improved my energy. Even just putting my first meal time at 1pm saw a dramatic improvement as my body tapped into my glycogen. It was pretty amazing.
I gradually increased my fasting until I could go a full 36 hours at a time without a meal (I allowed myself some bone broth in the evening before bed so I could sleep.
By this time the adrenaline cycle releasing energy had totally stopped. My body simply adapted to skipping meals and it knew full well that I would eat again in the morning. All extra energy went away.
I was simply fatigued and sitting around hungry on fasting days after that.
One thing you want to be on a fasting day: BUSY. Sitting around fatigued on a fasting day is no bueno.
My weight would fluctuate and then go up. My body did the classic, "Yo-Yo" in yoyo dieting. I'm female and going through pre-menopause and have a high estrogen. Jason Fung has no earthly idea about women and estrogen because he works with older individuals and I've read both of his books and watched his videos, he simply doesn't cover it and doesn't know much about it. Currently, the official recommendation is if you are in childbearing age and female - don't fast over 12-16 hours. Estrogen will kick your butt.
But what do I have to lose? I don't care what they recommended, I simply want to feel better. I was also intrigued - what would happen?
So I started fasting at 162lbs. I ended my 3 months at 168 roughly. The belly fat in my middle went down initially and then built back fortress style as if to say, "Oh NO YOU DON'T!" Again, YoYo.
Being a runner and having dieted for years now, years and years -- and always being really aware of my food intake, I didn't over-do my non-fast days. There were no Buger King Binges or craziness.
I actually ended up eating less over all, but my body had adjusted to do more with less.
Thus now I gain weight easier, my metabolism is uh' more finicky. This is not a good thing.
Also when I would go into Ketosis due to a prolonged fasting period it really screwed up my immune system and my skin developed the classic Keto Rash, and would get sores that would not heal.
At the end of the day, had it worked I would have stuck with it. It simply was doing nothing other than me sitting around hungry, so there was no reason to continue in particular since I was getting a sore on my hand that would not heal.
Hope this helps -- all the best.