They have adapted
> over time to the cold by a natural increase in capillary
> bloodflow to their extremities.
>
> I don't think it is directly related to diet.

I have the experience of outdoor camping  trip in winter for 3 months along
the rockymountain range .I made only one fire and ate raw dried fruits and
nuts ,honey and salt mostly almost no animal fat.
and yes i adapted to cold ( subzero at all time and minus 25 celsius at the
worst ).I was so adapted to cold that entering a heated home or store the
few times i did while getting supply ,i was not tolerating the heat .
But i have also expereience a big change in tolerating cold water after i
started to eat more animal fats ( before because of almost drown from cold
as a child i was unable to swim long in water without getting hypothermia .)
jean-claude