How do our bodies adjust to extreme temperatures?

This man clearly harbors no qualms about the cold. The native Norwegian achieved viral renown some weeks back for skating on and swimming in Norway's Lake Goksjo almost entirely nude, save for a pair of speedos and a wreath of ice worn round his neck. We know what you're thinking: How I can I be like this man? Let us show you the way.

The secret to adjusting to extreme temperatures is a gradual physiological process known as acclimatization. Our bodies, highly amenable machines that they are, can acclimatize to cold environments and warm environments, alike. It's why Bostonians' blood runs thick, and why Arizona natives thrive in 100-degree heat. But how does acclimatization work?

Acclimatization

Before we begin, we must establish an important distinction between immediate and long-term physiological response. Our bodies can of course react to extreme temperatures rather quickly, one obvious example being their ability to sweat. Sweating – whether brought on by the strain of exercise, the heavy heat of a muggy afternoon, or both – is an almost immediate physiological response (and an evolutionarily invaluable one, at that). But the sweat mechanism as such is an inaccurate characterization of acclimatization.

Acclimatization refers to those physiological responses of a deeper origin: the hormonal and metabolic programming that governs not only your tendency to sweat, but how you sweat, when you sweat, and even the amount of salt your sweat carries with it. This temperature-regulation system is controlled in large part by a collaboration between your hypothalamus and pituitary gland, and manages a range of physiological responses. These include the readiness with which you shunt blood to vessels in your skin (which has a cooling effect); the meter and sensitivity of your heartbeat; your body's overall production of thermal energy; and the allotment of bodily resources to protecting your liver, brain, kidneys, and other vital organs, to list a few examples.

These are all thermoregulatory tasks your body performs normally, of course, but when we talk about acclimatization, what we're actually referring to are the adjustments your body undergoes to optimize the function of those tasks with respect to your environment. Allow me to belabor this point: A bodily mechanism like sweating, in and of itself, is not an example of acclimatization; your body adjusting to warmer temperatures so that it sweats earlier, more profusely, and at a lower salt concentration – that's acclimatization.

The deep biological adjustments that are required to acclimatize to an environment take more time to dial-in than the regulatory mechanisms they control. In this way, the process of acclimatizing to temperature is similar to, and often overlaps with, the one experienced by runners who live and train at altitude. A prime example is Kilian Jornet Burgada, one of the most formidable endurance athletes of this or any generation.

Training for Cold and Heat

Jornet spent his adolescence romping through the Spanish Pyrenees at an elevation of 6,500 feet. "When you are born and bred at altitude," explains Stanford physiology and nutrition scientist Stacy in a profile on Jornet, published last year in the New York Times, "you tend to have a higher blood volume and red-cell count for oxygen-carrying capacity." This translates to greater endurance and better performance – a cause and effect that, in Jornet's case, has served him well on the path to endurance-sport supremacy (a fact to which the opening paragraph of the aforementioned profile speaks volumes).

Acclimatizing to heat and cold occurs in a similar fashion, with deep physiological adjustments becoming hormonally and metabolically ingrained over long stretches of time. That's not to say you have to be born and raised in the Spanish Pyrenees (or, in the case of the man in the above video, Norway) to handle living in the cold. Generally speaking, the longer you spend in an environment, the more adept your body becomes at performing under its particular conditions (plunging, semi-nude, into the frozen slush of a Norwegian lake, for example); but numerous studies, conducted from the early 1960s onward, suggest that 10-to-14 days of exposure to relatively higher or lower temperatures is enough to begin reaping the benefits of acclimatization.

For instance, a recent study conducted by researchers led by Wouter D. van Marken Lichtenbelt at the NUTRIM School for Nutrition, Toxicology and Metabolism corroborated earlier findings that ten days of cold-exposure was enough to increase the body's ability to generate warmth without shivering. As with previous studies, the researchers observed that, upon cold acclimation, test subjects judged the controlled environment as warmer, felt more comfortable in the cold, and reported less shivering.

The secret to the spike in heat production? According to the researchers, an increase in activity in brown adipose tissue, in parallel with an increase in nonshivering thermogenesis. Brown adipose tissue's main physiological purpose is to generate heat independently from the teeth-chattering muscle contractions that we often experience in the cold. Once thought to serve this role primarily in human infants, recent studies like this one have demonstrated that brown adipose tissue plays an important role in acclimatization to cold temperatures in adults, as well. As van Marken Lichtenbelt and his colleagues explain:

Upon prolonged cold exposure, shivering will gradually decrease, but energy expenditure remains elevated, indicating increased [nonshivering thermogenesis]. This metabolic adaptation over time is called adaptive thermogenesis. In rodents, the increase in [nonshivering thermogenesis] can be fully attributed to [brown adipose tissue]... [Human studies have shown] that prolonged cold exposure (12°C, 8 h/d, 31 days) in healthy men also resulted in a gradual decrease of shivering, while heat production remained elevated.

The same way that sweating as a cooling mechanism is fundamentally different from the prolonged metabolic-overhaul required for true heat acclimatization, the immediate benefits of shivering to generate heat are very distinct from the role of brown adipose tissue in the long-term metabolic reworking required for adaptive thermogenesis. It is perhaps unsurprising, then, that the initial benefits of heat-acclimatization have been shown to reveal themselves on a similar 10-to-14-day time scale as those of cold-acclimatization. The U.S. Army's Guide to Heat Acclimatization, which is used by elite soldiers undergoing advanced military training in hot weather, summarizes the current research in plain terms:

Generally, about two weeks of daily heat exposure is needed to induce heat acclimatization. Heat acclimatization requires a minimum daily heat exposure of about two hours (can be broken into two 1-hour exposures) combined with physical exercise that requires cardiovascular endurance, (for example, marching or jogging) rather than strength training (pushups and resistance training). Gradually increase the exercise intensity or duration each day.

Your mileage may vary, of course. As NASA researchers Hanna Kaciuba-Uscilko and a John E. Greenleaf put it in their 1989 report on cold-acclimatization in humans, "physiological response to cold varies with the age and physical fitness of an individual and depends on the intensity of the cold as well as on the duration of exposure." Similarly, the length of time that effects of heat- and cold-acclimatization persist after leaving a given environment is liable to vary from person-to-person; a San Diego native who spends just two weeks acclimatizing to the frigid conditions of a Norwegian winter will take less time to adjust, upon returning to balmy climes of his native habitat, than a Norwegian native on her first visit to the Sonoran Desert.

Similarly, if you want to wade in the slushy shallows of a frozen Norwegian lake, you might want to give your body more than two weeks' notice to adjust.