Winter has arrived here in Europe. For some of us the cold is a major factor and, let’s face it, we humans are poorly equipped to combat the cold – especially if you compare us to other mammals that live at the same latitudes. But did you know we can learn to acclimatize to the cold over short periods of time? That, at least, could help to take the edge off.
During the first few winters of my surfing life, wetsuits obviously weren’t as good as they are now. The cold was a major factor. After surfing, I remember always having numb hands and feet, sometimes coming out of the water violently shivering and always being ravenously hungry. I didn’t know that those ‘symptoms’ were mechanisms that the body had evolved to defend itself against the cold.
The human body is designed to function within a certain temperature range, like a car engine. If the working core becomes too cold, it will not function properly. It has built-in control mechanisms to keep it within a certain temperature range while the outside temperature happily swings outside that range. For example, to keep the core warm in cold conditions, a human body burns more calories, just like a car engine burns more fuel.
When we get cold, we shiver. Shivering is when large groups of muscles produce random contractions exclusively to produce heat. In this way, extra heat energy is available to avoid your core temperature dropping. You’d be surprised how much energy is used by shivering; research has shown that shivering can increase our base metabolic rate by around 400 per cent – equivalent to a brisk walk or a slow bike ride.
Shivering comes in different stages. First, you get a barely perceivable tightening of the muscles, called ‘pre-shivering’. Then, before long, you get those short, rapid movements such as chattering of the teeth. Those movements get larger and larger until you enter a state of uncontrolled violent shaking, where you find it difficult to see and difficult to talk.
Shivering normally only slows down the rate of cooling, but doesn’t stop it altogether.
So if you are in the water and get to that third stage of violent shaking, you need to get out as soon as possible So if you are in the water and get to that third stage of violent shaking, you need to get out as soon as possible, because your body is still losing heat. Therefore, in the end, shivering is not a very efficient process and can’t be relied upon to stop you freezing up.
Also, when we get cold, our hands and feet go numb. This is due to the blood vessels in our hands and feet contracting and restricting the blood supply: a process called cold-induced vasoconstriction. In normal circumstances, where our core temperature is above the outside temperature, the core is prevented from overheating by pumping warm blood to our extremities, where it cools before being pumped back. In cold conditions, the blood is shunted away from the extremities straight back to the inner core, so the vital organs stay warm.
As a result, the extremities, which are not receiving warm blood, cool down a lot more than they normally would. That is why your hands and feet go numb, even though the inside of you might not feel that cold.
Unfortunately, cold-induced vasoconstriction isn’t a very efficient process either. It comes into action far too soon, way before our vital organs are put in any significant danger. Our hands and feet go numb at relatively high temperatures, and the discomfort (and possible danger of frostbite in extreme circumstances) is an almost unnecessary sacrifice.
So, it seems that the two major processes we have evolved to keep us warm – shivering and vasoconstriction – aren’t really doing their job properly. Why not? It is because they evolved when we were still living in warm climates, on the plains of Africa. We have been out of Africa for such a short time that evolution hasn’t had time to give us the efficient cold-defence systems that we see in other mammals. Polar bears, for example, have an impressive array of built-in central-heating mechanisms, in addition to a lot of hair and blubber.
In fact, the major reason why we are able to live in cold climates is because our brains evolved so that we became clever enough to change our environment before Nature had a chance to change our bodies. We discovered fire and learned how to build shelters, make clothes and predict the weather.
However, even though we haven’t evolved physical mechanisms to combat the cold, the good news is that we can temporarily change the behaviour of our bodies to allow short-term adjustment to the cold. This is called acclimatisation as opposed to adaptation.
Acclimatisation can be seen in various modern cultures around the world. For example, the Ama woman divers of Japan and Korea used to spend many hours a day in 7ºC water using only thin cotton outfits. Members of some modern hunter-gatherer societies such as Native Australians and Namibians sleep outside on the desert floor at around 0ºC with little or no clothing. In our own culture, you have cold-water swimmers like Lewis Pugh. In 2007, Pugh did a one-kilometre swim in -1.7 ºC degree water at the North Pole wearing just a pair of speedos.
So how do we do it? Well, there is evidence that we temporarily adapt ourselves to the cold by simply conditioning our bodies to function at a lower temperature. There are two ways in which we do this: hypothermic habituation and cold-induced vasodilation.
Hypothermic habituation is where we let our core temperature drop a small amount, and then learn to operate at that lower temperature. Those desert nomads get a good night’s sleep because they are able to let their core temperature drop a couple of degrees, which allows the shivering threshold to drop to a lower temperature. If you or I went straight out and tried to do this we would probably spend all night shivering. However, given a few weeks of sleeping outside, most of us could probably teach our shivering threshold to creep to a lower temperature, and could enjoy shiverless nights under a desert sky.
The second mechanism is cold-induced vasodilation. Earlier, I talked about cold-induced vasoconstriction, the shunting of blood away from the extremities and towards the inner core, to keep the vital organs at the right temperature. Well, it turns out that the blood vessels don’t actually shut down continuously; they intermittently open and close. If you repeatedly expose your hands and feet to colder temperatures, the vessels tend to stay closed for shorter periods of time and stay open for longer. Result: your hands and feet don’t go so numb so quickly. This has been studied in people like fish-factory workers, whose hands are continuously cold.
Cold-induced vasodilation seems contradictory. Why close the blood vessels to protect the vital organs but then open them again? At first, the body is not used to dealing with the cold, so it overreacts by closing the blood vessels right down so that the vital organs are protected no matter what. But then, once it becomes accustomed to the cold, it can start to risk opening them up a bit more until it finds the right balance between open and closed phases for a particular outside temperature.
In the same way that you can train yourself to shiver less, you also can train your hands and feet not go so numb, by gradually exposing them to the cold. Experiments have shown that about half an hour a day over about a month can be sufficient.
So, even though evolution hasn’t given us very efficient ways of insulating ourselves from the cold, it seems that all of us have the ability to become accustomed to the cold by training our bodies. In the same way as half an hour of jogging a day will increase your heart and lung capacity, half an hour a day in cold water will bring down your shivering threshold and stop your hands and feet getting too numb. Think about that when you are planning your next winter trip to Iceland or Nova Scotia.