The body has a range that it considers to be maximizing survival to reproduction that it will aim you for, given thousands of factors most of which you are probably barely aware of. Ideally your body is going to make sure that you have enough body fat to can go a few days without food. You will need around 800 kCal/day from body fat to do that, so your body will start to tap out around 25lbs (800 kCal / 31.5 kCal/lb/day) of body fat - that’s if you have negligible fasting insulin and so have 100% access to body fat. If you are a uncontrolled type 2 diabetic with insulin levels that never go below 300 and you can only access 20% of your energy from body fat then you may need more like 125lbs of body fat to meet your bodies expectations.
If you want your body to go below your minimum amount of body fat to be able to sustain a day without food it’ll put up a fight. When you think about it, it really
should put up a fight, because what you’d be trying to get it to do is leave you with so little body fat that you couldn’t survive a day on your energy reserves.
Of course you don’t totally go without when you don’t have enough body fat, there is a backup emergency source of energy for when you have inadequate glucose or fatty acids to keep the lights on … protein.
So any attempt to go below around 25lbs of body fat should involve some wasting of lean tissue for energy. Obviously that is unsustainable … so most people who get their body fat below 10% end up yoyoing down into that range only for competitions, and pound in protein to try to recover the damage afterwards.
Would you be able to explain the formula you used (800 kCal / 31.5 kCal/lb/day) and the 25 lbs of fat rule you mentioned? I’m a male, 5’8”, 133 lbs, 12-15% body fat, and 18-19 lbs of body fat. I want to water fast long term, but I want to make sure it’s not a dangerous idea with my body composition. Thanks so much for your help!