Re: How do humans adapt physiologically to the cold?

Kristi,

Like so many, this question is surprisingly complex.  I didn’t know much 
about this, beyond basic medical understanding, so I did some reading.  
Turns out there is a tremendous amount of research done to address cold 
adaptation, because there is a lot of practical application.  If we 
understood the cold response better, we could potentially help athletes, 
rescue workers, the elderly (who have a lower cold tolerance), immersion 
victims, and so on.  I must say, though, that I have not learned too much 
beyond my basic medical knowledge.

First off, I assume that you are talking about the situation where the 
whole body is cold.  Some of the information below is actually not correct 
if you are talking about just a part of the body being cold (like sticking 
your hand in ice water).

The way to think about this problem is as follows.  When it is cold around 
you, the environment is stealing heat from your body.  There are two ways 
to respond to this - defend the heat you have, and make more heat.

In the acute sense ("acute" means in the short term, when you find yourself 
being in the cold when you are normally warm), your body has several 
mechanisms to defend the heat it has.  The first is not physiological, but 
behavioral - you go where it’s warm, put on a coat, etc.

The most important physiological defense response is called "peripheral 
vasoconstriction".  Basically, you have blood vessels running all through 
your body, including through skin on the surface of your body (we call 
these "peripheral").  The blood running through these vessels is, of 
course, warm.  But when your skin is surrounded by cold air or cold water, 
the heat in that blood is easily lost to the environment (much more easily 
than from deep inside your body).  Since the blood is constantly 
circulating, this can rapidly cool down your whole body.  To prevent this, 
your body responds by closing down a lot of these peripheral blood vessels 
(using a kind of blood vessel called an arteriole that has muscles in the 
wall of the vessel), thereby limiting the amount of blood going through the 
skin and slowing the rate of heat loss.  This is why your skin turns white 
when you are cold - very little blood is going through it.

A final mechanism to defend body heat is often forgotten, but important - 
you STOP sweating.  Water on the surface of the body is a very effective 
way of stealing heat from the body, and so you don’t sweat when you’re 
cold.

Increased heat production in the acute sense is also achieved by several 
mechanisms.  The first mechanism is again not strictly physiological, but 
rather neurological - it is called shivering.  Yes, shivering is a very 
complicated process that your body executes "on demand" when you are too 
cold.  Muscles make heat when they work (like when you exercise).  This is 
a basic principle of physics - when energy is taken from a storage source 
(such as fat) and broken down to do work, it can be converted to another 
form of energy, but ultimately it is all dissipated as heat.  Thus, when 
your muscles do the work of contracting back and forth repeatedly, they 
generate heat.  It is very interesting that different people have very 
different thresholds for shivering, and in general people who shiver more 
easily have a higher tolerance for cold (but even that is complicated, 
since once you adapt to the cold over the long term, you generally shiver 
less).  

The major physiologic mechanism of generating more heat follows the same 
logic as shivering, but is a little harder to grasp, perhaps.  It is called 
"metabolic heat production".  Again, when stored food is broken down to do 
work some of the energy is dissipated as heat.  In this case, the work we 
are talking about is chemical work, like making new proteins, changing 
sugar into fat, and so on.  Your body does this kind of chemical work 
constantly, which is what generates most of the heat in your body.  When 
you are cold, this chemical work is stepped up a bit, and so more heat is 
generated.  Perhaps the most interesting example of metabolic heat 
production is in a special kind of tissue called "brown fat".  Brown fat is 
a lot like normal fat in that lipids are stored, and then broken down for 
energy when needed.  The special thing about brown fat is that it breaks 
down the lipids in a very wasteful way.  Normally, the energy from fat is 
used by tissues to do work (which again does generate some heat).  But 
brown fat breaks down the lipids in a way that does no work at all, and so 
the energy in the lipids is converted entirely to heat (to be complete, the 
mitochondria in brown fat lack the ability to make ATP as the final product 
of oxidative phosphorylation).  Adult humans have only a small amount of 
brown fat, and so this probably does not contribute much to heat production 
for us.

On a mechanistic note, many of the above processes are mediated by a part 
of the nervous system called the sympathetic nervous system.  This is the 
system that is primarily responsible for responding to stressful 
situations.  

Things get quite a bit harder to pin down when we consider the body’s 
response to cold in the chronic sense ("chronic" means over a long period 
of time, like when you live in a cold environment), what is called "cold 
adaptation" or "cold acclimatization".  Basically, several sources I have 
seen indicated that there really is no single, easily definable pattern of 
human adaptation to cold in the long term.  Researchers have studied 
various human populations that live in cold environments, as well as 
experimental subjects who were put in a cold environment for a long time.  
Some people and populations adapt by increasing their metabolic rate by 
longer term mechanisms like increasing the rate of thyroid hormone 
production (this hormone stimulates metabolism).  Others seem to defend 
their body heat more aggressively by making more fat, which acts to 
insulate the body better.  Still others learn to tolerate slight decreases 
in their core body temperature.  Still others never really adapt at all, in 
a strict sense, since no physiological changes are apparent - thus they 
adapt primarily by behavioral mechanisms, or perhaps by simply increasing 
the efficiency of the acute defense mechanisms.

This list can go on, but I think you get the idea.  In the end, the reason 
this question continues to interest and confuse researchers is that it 
seems like there should be a single, simple answer.  But from everything I 
have read, there just isn’t one.  Bottom line, we are warm blooded animals, 
and our primary mechanisms for surviving in the cold are fire, clothing and 
shelter, and these are far more important than any physiological mechanism.  
If we are exposed too much, our body temperatures drop and we die, and this 
is true whether you live in Alaska or Hawaii.  Now if you were a frog or a 
fish.....

I hope that answers the question that you asked (and not some other 
question!).

Tom Wilson, MD PhD