BS Biology, former ISSA trainer: The simple answer is - fat mobilizes globally, prioritized by access to circulation. The last 3.5% of body fat is brown adipose, which you can't lose, but if you could, you'd die from hypothermia.
Cool, I've got a related question. Do people have varying amounts of brown adipose and can they develop more of it through training? I've heard that exposure to cold for long periods of time causes your body to produce more brown adipose fat.
Good question. It's very likely safe to assume that we have an adaptive variance for these kinds of things, but it would still be a very small range. If you've heard it, it was probably supported by a study that indicates that correlation. For the most part, it's something you'll almost never even see. Iirc, the minimum healthy, functional bmi for men is 5%, 12% for women, as I was taught years ago. Anything below those ranges and things start to get weird, or it would take great effort and water/diet restrictions to maintain. The point being, anyone who says they're 0%, or even like 3%, has no idea what they're talking about. Thanks for having this discussion with me!
I think that some bodybuilders get to close to that minimum at competition, but they're also really close to death. And a few have died due to the side effects of the drugs they take to get down that low (esp. diuretics). This might be different now though; HGH has been doing weird things to pro-level BBers. Used to be that they'd use shit like 2, 4-dinitrophenol (DNP), which does really weird shit to your metabolism and can very, very easily kill you if you dose too high. Especially since it takes about two weeks for that dose to catch up to you.
I was at 7% (measured on a fancy scale, not the bathtub method) as a male high school long distance runner and I was basically a fastish skeleton. I don't think that would be a healthy BMI for me twenty years later, even if I could maintain it!
I was advised by a doctor to turn the water to cold periodically in the shower to increase the amount of brown fat the body produces. I take it with a grain of salt, but cold tolerance does seem to be a thing and that could be a mechanism for it.
You kind of answered your own question. There are a lot of conditions and feedbacks needed for stasis. As any are pushed to or beyond their limits, a cascade occurs, having catastrophic effects. Body temp regulation is one of the most dire, as we can't survive for long below a certain temp. Regarding that, burn victims can actually die from hypothermia if not treated immediately following 3rd degree burns, due to the amount of fat and skin cells lost to burns. I hope some of this made sense. I'm digging deep to remember, but it's been a while. Cheers