Cold exposure makes you thermodynamically more efficient at the mitochondrial level, which helps energy production at the most fundamental level.

Guess what cold plunging does to thyroid? It Up regulates it. 

The reason is simple. 

Mammals who must adapt to the cold ( any mammal in the wintertime) (think shivering) must increase their basal metabolic rate, which is controlled by thyroid. This is done by uncoupling their mitochondria to produce more heat (infrared radiation). The more heat generated requires more fat to fuel. So fat stores are utilized in cold (winter) to generate more heat. The colder you can become on a consistent basis, the more efficient your biology becomes. 

This is cold thermogenesis.

It is all about thermodynamics and mitochondria. 

Life is an energy game.

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