>TL;DR: Mainly the heat is from an increased metabolism in brown adipose tissue

Isn't that basically the conclusion that Wim Hof guy came to after being studied by doctors while he submerged himself in icy water? He has some breathing technique that supposedly activates his brown fat tissues and keeps him warm. Dude claims to have hiked Kilimanjaro wearing nothing but a pair of shorts. Not sure how real any of that actually is, but it's a pretty neat though. I wish I could activate my brown fat tissues on purpose sometimes.