At first it is only a dark purple absence of stars in a pinpricked sky. Soon dawn sets the glacier ablaze like a beacon. Kilimanjaro. Africa’s tallest mountain rises up out of the sun-drenched savanna to a place high above the clouds. There, at nearly 20,000 feet, winds top 50 miles an hour and scour what is likely the only indigenous ice on the continent. It’s the first time our group of amateur climbers has seen it this close, and I can’t decide whether I’m excited or terrified.

Upwards of 35,000 tourists attempt to summit the mountain each year. Usually they spend time adjusting to altitude and then embark on a five- or six-day climb, wearing the most advanced mountaineering apparel — waterproof down jackets, insulated trekking pants. Our goal is to reach the peak in 30 hours, with no acclimation to the altitude, on almost no food, on little sleep, and without any cold-weather gear. I’m wearing boots, swim trunks, a wool cap, and a backpack containing emergency gear and water. My chest is bare to the frigid air.

One of our Tanzanian guides watches me warily from beneath his full thermal getup until, finally, he can’t hold his silence anymore. “Please put something on,” he says.

He’s not the only one who thinks I’m crazy. Yesterday a U.S. Army scientist calculated that, given our pace, three-quarters of our group of 29 would come down with debilitating altitude sickness. What the researchers and our guide don’t realize is that the deprivations caused by cold, thin air are the point. I’ve been conditioning my body to environmental stresses for six months, dunking myself in ice water and learning a breathing technique that has given me an almost spooky control over my autonomic body functions.

I suck in 30 breaths of cool air and focus on the blazing orange rock in front of me. There’s no point in checking the temperature. It’s well below freezing, and I’m already burning up.

I don’t like to suffer. Nor do I particularly want to be cold, wet, or hungry. If I had a spirit animal, it would probably be a jellyfish floating in an ocean of perpetual comfort.

That’s not just me; it’s most of us. The body craves homeostasis, the effortless state in which the environment meets our every physical need and the body can rest. So we jack up the heat on cold winter days, ratchet down the air-conditioning in the summer, and don sunglasses when it’s a little too bright outside.

But it hasn’t always been that way. Humans have had the same anatomical makeup for nearly 200,000 years. Which means your office mate who sits on a rolling chair beneath fluorescent lights all day has pretty much the same basic body as the prehistoric caveman who made spear points out of flint to hunt antelope. To get from then to now, we faced countless challenges as we fled predators, froze in snowstorms, sought shelter from rain, hunted and gathered our food, and continued to breathe despite suffocating heat. Variation and stress were the norm; comfort, the exception. To survive, we had to be strong.

Our modern-day struggles pale in comparison to the daily threats of death or deprivation that our forebears faced. But succeeding over the natural world hasn’t made our bodies stronger. Compare your pasty-skinned office mate to one of our prehistoric ancestors, and bets are good that the modern-day man is fatter, lazier, and in worse health. And it’s not just him. The last century saw an explosion of ”diseases of excess” in the developed world, or what happens when you have too much food and your lifestyle is sedentary. Obesity, diabetes, chronic pain, arthritis, and hypertension are all at record highs. We’ve even seen a resurgence of gout. Millions suffer from autoimmune diseases — arthritis, lupus, Crohn’s — in which the body literally attacks itself. It is almost as if our bodies have so little to struggle against that our stored energy instead wreaks havoc on our insides.

There is a consensus among many scientists and athletes that humans were not built for constant homeostasis. In the past, comfort was almost indistinguishable from safety. Now it’s something we take for granted. Human biology needs stress — and not the sort that damages muscle, gets us eaten by a bear, or degrades our physiques, but the environmental and physical fluctuations that invigorate our nervous system.

Our muscles, organs, nerves, fat tissue, and hormones respond and adapt to changes or threats from the outside world. And almost no environmental extreme induces as many changes in human physiology as the cold. Take a plunge into cold water and not only will you trigger a number of processes to warm up the body, but those adjustments will help regulate blood sugar, exercise the circulatory system, and heighten mental awareness. The problem is nobody wants to do that. The bulk of us don’t see environmental stress in the same light as we do, say, jogging.

But we should. Because stepping outside on a frigid day in only a T-shirt creates a cascade of physiological responses that deliver benefits similar to a workout.

To explain why, you need to look at the human circulatory system, a complex network of arteries and veins that carry blood and oxygen to and from every tissue. In a single day, roughly four to seven liters of blood travel thousands of miles. This blood superhighway is more than just a series of tubes; it’s an active and responsive system. Tiny muscles line the arteries and veins and help push the blood through the body, which is critical for circulation and regulating blood pressure. The second you step out and have a brush with near-arctic winds, these tiny muscles flex.

Exercising that system is important: Cardiovascular diseases contribute to 31 percent of the world’s mortality. A main way to trigger those circulatory muscles is to actually go outside to feel the cold. But living in a perpetually climate-controlled environment — in our homes, cars, and offices, or simply by being bundled up outdoors — means that those muscles are never challenged by the elements. Even a fit body with chiseled abs might be hiding weak circulatory muscles.

Experiencing cold can also spur your body to activate brown adipose tissue (BAT), also known as brown fat. The primary purpose of BAT is to pull ordinary white fat from storage and burn it to keep you warm. So as counterintuitive as it may sound, the more active brown fat you have, the higher the capacity you have to stay lean.

Everyone is born with about 5 percent of his body mass as brown fat. But thanks in part to years of artificial heating, many of us in the developed world have almost no active BAT left by the time we reach adulthood. The good news is that placing yourself in even moderately cold temperatures, such as setting a thermostat to the 50s or low 60s for a few weeks, can activate your brown fat.

It’s a lesson that hasn’t been lost on Ray Cronise, a former NASA scientist. He spent 15 years conducting experiments at the Marshall Space Flight Center that assessed how the body changed in extreme conditions, but his career took a turn when he decided to create a way to shed weight that wasn’t focused on counting calories. Cronise prescribed himself daily hourlong walks in sub-60-degree temperatures, along with regular exercise. In six weeks he dropped almost 30 pounds. During the process, he developed a deeper theory on health.

“We’re overlit, overfed, and overstimulated, and in terms of how long we’ve been on Earth, that’s all new,” he says. We’re living in an “eternal summer” and missing out on what Cronise calls “metabolic winter,” a time when the body adjusts to discomfort and scarcity between times of plenty. As he wrote in a 2014 paper published in Metabolic Syndrome and Related Disorders, “Our 7-million-year evolutionary path was dominated by two seasonal challenges — calorie scarcity and mild cold stress. . . . We solved them both.” The inevitable result of losing seasonal variation, he says, is obesity and chronic disease. As proof he points not only to the population of his home state of Alabama, which ranks second in obesity levels in the U.S., but also to the fact that even our pets are fat. “There’s a connection,” he says.

The key to fixing the problem, according to Cronise, is to bring cold back into our lives. Doing so can add just enough mild stress to reinvigorate our evolutionary programming, improve our circulation, and kick our metabolism into high gear.

I can vouch for the benefits of a little environmental suffering. Back in July 2012, I was at a personal low point while living in Long Beach, California. I had been sitting in a desk chair in front of my computer for almost eight hours straight. Palm trees gently swayed outside my window. Despite my relatively comfortable perch, my legs throbbed from underuse and my back ached. I told myself that since I was now approaching my mid-thirties, it was perfectly normal for my stomach to sag over my belt. I figured a moderate amount of exercise and an occasional dip into the organic aisle of my grocery store should suffice to maintain a level of decorum.

That was when the internet coughed up a picture of a nearly naked man sitting on a glacier somewhere north of the Arctic Circle.

His name was Wim Hof, a Dutch adventurer and biohacker who proved he could raise and lower his body temperature at will and influence his immune system with the power of his mind. He ran a training camp in the snowy wilderness of Poland, where people from around the world converged to study his secrets. He promised that he could teach someone to survive in arctic environments with almost no gear. He said he had invented a breathing method that allowed any one to tap into his own biology to strengthen endurance and to put certain autonomic processes — like constricting blood vessels and producing body heat — under conscious control. What’s more, it took only a few days to learn. It all seemed crazy to me.

I was sure Hof was a charlatan, so I booked a ticket to Poland to test his “method.”

At his training center in Przesieka, Hof introduced me to the basics of body hacking. First he taught a breathing routine that alternated between controlled hyperventilation and breath holds with empty lungs. Cycling between these helps expel CO2 and fully saturate the blood with oxygen. With a little practice, the routine allowed me to hold my breath for three minutes at a stretch. The point of the exercise, Hof said, is to reprogram the way the nervous system responds to the stress of not breathing. Will yourself to hold on a little bit past the point at which you’d normally gasp for air and you gain a measure of conscious control over a function that’s normally automatic. Hof explained this breathing process would help you to withstand environmental stressors, too, helping you to stay warm — even get hot — in very low temperatures.

Which brought us to the second half of the method, which is brutally simple: Get used to being cold, and suppress the urge to shiver. Shivering is an autonomic method the body uses to warm up. Hof taught us that simply relaxing and taking calm breaths would help quell our shivering and force our bodies to switch from using muscle movement for heat to burning fat.

Every morning I woke up and made my way down from the second floor of Hof’s dilapidated training center to a makeshift meditation room full of rumpled sleeping bags and well-worn yoga mats for the morning breathing routine. For almost an hour, the five of us on the retreat alternated between rapid breathing cycles with face-contorting breath holds. At the end of the session, we tested how the method changed our ability to do pushups. Even after one hour of training I could bang out 50 reps on a single breath, whereas just a week earlier I could barely manage 20 pushups while breathing the whole time. I think it was that moment when I realized my body was changing, and I went from being a skeptic to giving the method a chance.

As I began to trust Hof’s teachings, I found that my body was capable of mind-boggling things. On the first day, I could stand barefoot in the snow for only five minutes before excruciating pain forced me to retreat inside. But after breathing with Hof on the second day, I managed to will myself through 20. On the third day, 45 minutes in the snow wasn’t a problem. Then Hof took us to an icy waterfall behind his house, where we meditated on the banks until the snow melted around us. We sat in the near-freezing water for minutes at a stretch, and then in a final feat to put Hof’s method to the test, we spent eight hours walking up a nearby ski hill wearing nothing but shorts and hiking boots. The combination of intense trekking and Hof’s breathing technique left me sweating despite subzero winds. And though I hadn’t gone on the trip with the intention of losing weight, at the end of the seven days, I had shed seven pounds of fat.

The Poland trip is why, four years later, I find myself shirtless and marching up Kilimanjaro. I wanted a new frontier to prove, at least to myself, how far human resilience can really go. I’m with Hof and a handful of other intrepid climbers, and during the course of the last day we have busted past every established protocol for safe and slow ascents. Hof’s method, I discover, does not make a person completely immune to the elements. We pause for a few moments in one gusty exposed area, and it’s difficult to generate the heat I need to fight the cold. So I drop my backpack and pull out a thin merino-wool shirt to provide my skin with a little protection. It’s a temporary measure to handle the cold while I focus on battling the altitude.

As we continue up, the rhythm of the march sometimes lulls me out of my conscious breathing. My mind starts to wander, and I take in only as much air as my brain might see fit, and I forget to use Hof’s technique to lead and control my breath. That’s when the high altitude creeps in. The world dims imperceptibly, and every footstep seems to fall just a little heavier. I start the breathing method when I realize I’m fading, and it creates an immediate effect. I take 30 rapid breaths, and the world brightens as easily as if I were taking off sunglasses. My steps are lighter, and I have the energy to continue.

We reach Gilman’s Point, roughly 700 feet below the true summit. It’s about 5 degrees, but I would later calculate that the magnifying effect of the wind on skin brought the real temperature down to –24. That’s enough to cause frostbite in a half hour of exposure. I’ve been shirtless for the bulk of the journey, caving into covering my skin with the merino only as I neared the lip of the volcano. We check our watches, subtract away our departure time, and find that we’ve more than beaten our 30-hour goal — we’ve crushed it. It has been 28 hours and 6 minutes since we left the park entrance. To the best of our knowledge, it is the fastest-ever unacclimated ascent to Gilman’s Point by amateur climbers.

I breathe in the success in Kilimanjaro’s thinnest air. I take 30 breaths and I’m hot. So I take off my shirt to enjoy the cold.

WARNING: As you might imagine, prolonged breath holding and cold exposure has inherent risks. Consult a physician before beginning, and practice while seated or lying down, away from water, and not while driving; increase intensity gradually. The method should challenge you, not end you.

This excerpt was created from Scott Carney’s What Doesn’t Kill Us: How Freezing Water, Extreme Altitude, and Environmental Conditioning WilL Renew Our Lost Evolutionary Strength, available January 4.

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