If you don't mind sharing, what kind of starting body fat % (or as a poor proxy, BMI), did you have at the beginning of your multiday fasts? I don't care about the literal numbers, but: bottom of healthy range? Mid-healthy range? Above healthy range? Do you exercise regularly (and did you keep it up during the fasts)?

I guess this is always my question of anyone who has a positive experience with fasts. Personally, I have been at the mid-upper range of healthy bf%/weight for years and exercise frequently (running 30+ minutes 7 days a week for a few years, cycling 60+ minutes 2-3 a week this year) and find much shorter fasts (say, 20h) excruciating and completely draining mentally and physically. And I'm not at 7% body fat like that other guy. Lowest I got was maybe 14% while I was running daily; while I'm up to 18%ish now, fasting hasn't gotten any easier.

(Even if I skipped exercising, just the mental effects made my mood significantly worse by the end of the day, which makes it harder to work effectively in a collaborative environment.)

I'd say that I'm in a similar position to you, though not as fit. I do a short HIIT session every day and am marginally chubby.

What's your non-fasting diet like? Fasting is the quickest way to get into ketosis. If you're used to eating a carb-heavy diet, the switch into ketosis can be brutal. So if you either ease off carbs leading up to the fast, or just all the time, it'll be easier.

The other thing is electrolytes. Your body doesn't store them, so you (IMO) have to supplement. If you start to feel like crap, have 1tsp of salt in about 1L of water and see if it fixes it. I also supplement with magnesium and occasionally potassium.

I was going to say something similar.

I've fasted for up to 5 days before, and I felt just fine - especially on days 4 and 5, I actually feel like I have more mental clarity and drive than usual.