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Unlike "regular" dogs, sled dogs like Siberian Huskies are specifically designed to be incredibly athletic and to be able to run for so long without negative consequence. They have larger hearts to pump more blood more quickly, and through selective breeding, they've adapted to their functions (running great distances while pulling a heavy sled, for instance) by developing a sort of metabolic on/off switch. It was about half a year ago that I heard a short presentation on the subject, so my memory could be a bit foggy, but here it goes: you know how humans produce energy? Basically, they burn glucose, which is a simple sugar. When they run out of that, they start to burn fat. Fat requires much more oxygen to burn than does glucose, so it's a lot more taxing on the system. As a result, fatigue sets in. Sled dogs (who typically consume very fatty diets) burn glucose at rest and in the beginning of long exercise, but after awhile, they're able to switch to burning fat instead. They're able to do so much more effectively. They're also able to pull as much as twenty times as much energy from their bloodstream as humans. It allows them to exercise hard for insane amounts of time without succumbing to fatigue. However, as far as I last knew, the exact mechanism that allows them to use their metabolic on/off switch was not totally understood. Scientists are working hard to figure it out, since they believe that it may benefit humans.

As for what you describe as gaining fat for his thick coat, I wouldn't really call that fat, although I wouldn't know for sure. His just has some loose skin (as do most dogs, to a degree) and a very thick double coat.

If you want to learn more about his metabolism, I'm sure you'll pull up plenty of hits if you Google phrases like "Siberian Husky metabolism," "sled dog metabolism," "sled dog fat," so on and so forth.