Thank you for finding that. It seems like intermittent cold exposure may be the primary key then. I wonder if breathing exercises help modulate the bodies response to the cold. More oxygen available may mean better brown fat thermogenesis for example.

More oxygen is not the underlying science here. Oxygen saturation is almost always at, or close to, 100%. See the Bohr Effect [0][1].

TL;DR Increased CO2 aids in Oxygen delivery efficiency.

[0] https://en.wikipedia.org/wiki/Bohr_effect [1] https://respiratorycram.com/bohr-effect/

Thanks for posting this. From the wiki page:

> an increase in CO2 results in a decrease in blood pH,[2] resulting in hemoglobin proteins releasing their load of oxygen. Conversely, a decrease in carbon dioxide provokes an increase in pH, which results in hemoglobin picking up more oxygen.

So lowering carbon dioxide concentration (through an activity like hyperventilation) will cause hemoglobin to pick up more oxygen molecules. And increasing carbon dioxide levels will release those molecules. Fascinating.