I've found that there's a sweet spot as to fasting length: too soon (say a day) and I feel really hungry, so calorie count probably doesn't add up as I eat more than I 'saved'. Wait too long and the something similar happens (starving), my ideal spot seems to be ~3 days and breaking the fast with a 'regular'/not-eating-out meal.
I don't supplement for the fasting, from what I've read ~3 days isn't long enough to warrant that. Just lots of water and some salt (iodized) if I'm doing 4 days. Usually headaches may show up at the end of the 3rd day but that just tells me I'm not hydrating properly (also piss color helps get feedback on this).
What I've seen in my fasting and weight logs, is that doing multiple day fasts helps me keep weight off. The problem I've had in the past was not transitioning to a 'maintenance' schedule.
I'm doing 3 days of water+coffee (this is my third week). Last time I tried this in 2021 I lost 25 pounds (195->175) in 2 months. My plan now is to transition to 2 days a week once I've lost the weight.
I fast for 2-3 weeks quarterly. Water only.
When I am sick for any reason I start fasting immediately.
What has this done for me? My weight is finally where it should be from decades of abuse. Autoimmune conditions and diabetes are under control.
I posted a while back that my sleep was about 2-4 hours nightly. Over the last 5 or so months that has increased to 6 hours a night.
Edit: I drink at least 1 gallon of water per day.
But alas, it wasn't viewed in the light by everyone and other "stacks" became more popular.