Current advice is to not go much past 36 hours straight fasting. Somewhere between there and 48 typically your body switches into a starvation mode. Your thyroid hormones shift dramatically, you will feel abnormally cold because your body starts conserving instead of burning, and stress hormones start to rise.

Once won't kill you but repeatedly entering that state isn't great for your long term health.

Tried it on myself going well past 36 hours and the S curve of weight loss gets really flat because of the heightened conservation mode you enter. 84 hours is around 1.5x the fat loss I get out of 36 hours even though it is over 2x as long.

You'll get more positive health outcomes by refeeding for a few days and then starting a new 36 hour fast.

0 calorie electrolytes like relyte or LMNT will also reduce stress on your body and hunger sensation while fasting.

36 hours does have dramatic positive benefits compared to 16/8 and 18/6.

My current protocol is 36 hours once a week. I do dinner Monday and then don't eat until breakfast Wednesday. Every other day I eat a typical 3 meal day.

This is all based on years of experimentation on myself with different fasting protocols and comparing my bloodwork results, athletic performance, and waist size.

I ran an experiment with eating every day all day and am very unhappy with my bloodwork and waist though my athletic performance is better than it was doing 84 hour fasts. I'll probably space out my fasts to every other week, every 3 weeks, or once a month once I get back to where I want to be. I see no future where I go beyond 36 hours again. I'll do 36h 2x a week if once a week doesn't get me the results I want.

Hope this helps another #fasting n00b get up to speed and get the results they want with less hassle than I went through dialing in my protocol based on health influencer bullshit combined with trial and error.