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Imagine a man who could endure near-freezing water for 45 minutes at a stretch. Imagine if that same man could run a barefoot marathon in the Arctic or swim 50 meters under the ice of a frozen lake. Imagine that man said the secret to his abilities not only allows him to climb Himalayan mountains wearing shorts, but also eases everything from chronic pain to Crohn’s disease and even Parkinson’s. What would you call that man? A savant? Guru? Prophet of God, maybe?

That’s the character Scott Carney describes in his new book, , about legendary survivalist and icy-water swimmer Wim Hof. The 57-year-old Dutchman, often referred to as the Iceman, has devised a series of breathing techniques and conditioning exercises—mostly various types of hyperventilation and other ways to purge the body of CO2—that he credits as being the key to his extraordinary abilities. Hof, for his part, sees the whole thing in a much more spiritual light—getting back to a purer, more primitive version of ourselves.

The book is a fun read because, at first glance, Hof does seem superhuman. He claims that by slowly conditioning oneself to low-oxygen states (through breathing exercises) or extreme cold (through full-body muscle-clenching exercises), one can channel their spiritual energy and tap into all kinds of hidden powers. Carney is at his best when he tries to explain Hof’s abilities through science. For instance, he suggests that Hof has tapped into a specific type of fat cell called brown adipose tissue that is found in human babies but mostly disappears in adulthood; through his body training, it’s possible that Hof has encouraged this vestigial fat to play an increased role in trapping heat. But the tone of What Doesn’t Kill Us occasionally implies that we should worship the guy. And honestly, it’s hard not to.

Hof is one of those extraordinary characters who pops up occasionally throughout human history seeming to be nothing short of miraculous. For thousands of years, humanity has occasionally glimpsed man’s capacity to do the seemingly impossible or the miraculous using only force of will: walking on burning coals, healing the sick, enduring lethal temperatures for hours. And for all that time, we have been left to our own devices in guessing how such things are possible.

But today, modern science has revealed a number of fascinating mechanisms for how the brain influences the rest of the body, forming a string of enticing bread crumbs leading toward a more satisfying understanding of some of the limits of the human body—and how people like Hof cheat them.

Take one fascinating lead: the effect certain expectations have on bodily functions. The mind has a propensity to make predictions, and then ensure those predictions come to pass through internal “pharmacies” that, when lumped together, are also called placebo effects.

In my book, , I talked to scientists around the world who investigate placebos, internal pharmacies, hypnosis, and the power of belief on the body and mind. One of my favorite quotes came from Alia Crum, a psychologist at Stanford. “I don’t think the power of mind is limitless,” she said. “But I do think we don’t yet know where those limits are.”

In his book, Carney points to Wof’s ability to heal things like Parkinson’s, asthma, chronic pain, and digestive problems, giving us the impression that the mind can do anything it wants. As it happens, all of these diseases are also highly susceptible to the influence of placebo. Contrary to popular belief, not all placebo effects are the same, and not all conditions respond to them equally. That’s because a big part of placebo effects are chemical, employing things like dopamine, endogenous opioids, serotonin, and an untold number of other chemicals your brain idly keeps on hand in case it needs to adjust what’s happening in the body.

That’s what’s at the center of almost every “miracle” I’ve encountered: chemicals that have incredible effects but still follow the rules of biochemistry, even if we don’t yet fully understand what those rules and mechanisms are. Hof claims that one of the secrets to superhuman strength and healing is specialized breathing techniques. Fair enough. But I can introduce him to a healer in Beijing who says it’s about balancing spiritual heat with cold or a witch doctor in Mexico who says it’s about channeling spirits. What do they all share? The chemistry of expectation and belief—which, writ wide, is the world of placebo. A better definition for placebo might be to call it a measurement of the effect of one’s belief on their body.

Belief and placebos don’t just affect disease. They also boost athletic performance, as Hof demonstrates when he swam under 50 yards of ice. This is where scientists have begun asking some really interesting questions.

https://youtube.com/watch?v=DpeM4FNXRsE

Placebo effects have long been studied in medicine, but Christopher Beedie, a sports psychologist at the Canterbury Christ Church University in England, is among the few scientists who study it in athletics. His work often examines how elite athletes perform under intense fatigue when they think they have some kind of performance enhancement. The interesting question for Beedie isn’t what can the human body do, but rather, what more can the human mind add to that?

“I don’t think there’s anything surprising about people who exist at the end of continua,” says Beedie. “[Hof] is an extension of the classic example of a unique athlete optimized on nearly all variables who’s also probably learned to capitalize on every component of placebo responding he can.”

One of the most studied mechanisms of placebo in medicine is that of pain relief. Scientists have documented an extensive network of self-medicating pathways in the brain involving internal opioid stores that kick into gear when our bodies expect a treatment—from aspirin to acupuncture—and don’t get one. And there’s a lot of overlap between pain and athletic performance. Because what is intense exercise but extended pain resistance? In fact, pain relievers like morphine are strictly regulated in athletics for their performance-enhancing powers.

In addition to painkillers, there may be a whole network of internal chemicals our bodies can dip into for increased performance. In one from 2008, legendary Italian placebo scientist Fabricio Benedetti told weightlifters that they were getting performance-enhancing drugs when they were actually getting placebos and, secretly, lighter weights to lift. Once they believed the drugs were working, as perceived by the lighter weights, the loads were surreptitiously returned to their normal weight. The force the athletes were able to produce with their muscles increased while perceived fatigue stayed the same.

Beedie has done a lot of similar placebo performance experiments—consistently demonstrating their ability to give an impressive edge to cyclists, runners, and many other athletes—to the point where the athletes at his school don't always believe what he says. He claims belief taps into “headroom” that every athlete has in their potential—or the idea that that athletes can push themselves to operate between their perceived maximum execution and the maximum that physics and their bodies will allow. By either removing energy-wasting anxiety or tapping into chemicals like opioids or as-yet-undiscovered internal performance drugs through one’s expectations, the brain can coax the body into that magical zone.

In fact, Beedie is convinced this headroom is the same space filled by performance-enhancing drugs. (Indeed that some banned drugs, , can outperform placebos.) He’s just finished the largest (not yet published) placebo study ever done in athletics—600 subjects in all—and found that the people most likely to respond to placebo were the ones experienced using supplements. Perhaps the previous supplements the athletes had taken primed them to have a placebo response. Perhaps people who naturally respond to a sports placebo are also likely to have taken performance enhancers. Either way, it suggests that artificial boosted performance and boosted performance from expectation produce similar effects.

“This [whole idea of expectation-based bodily responses] is an evolved mechanism that allows us to capitalize on untapped resources at critical points in our existence,” Beedie says. Belief is belief, so it’s possible that drugs—real or placebo—fill the same space that superstitious baseball pitchers fill by wearing mismatched socks or dirty underwear and the same space filled by Hof and his breathing methods. None of this is to say Hof isn’t incredible. His feats of endurance are astounding and perhaps even scientifically significant, like his ability to control his body temperature so well. But he’s not magic, and we should be careful about trusting important health decisions to any belief-based technique—even one that allows a person to swim under ice.

Perhaps the most interesting question is what can people like Hof really tell us about the effect of our mind on our bodies? Scientists already know that Parkinson’s disease, pain, and depression all respond very well to all kinds of beliefs, whether through special breathing, secret pills, or magic crystals. But could that same belief fuel unprecedented feats of athleticism? Beedie says that, especially for elite athletes, there’s a limit to the benefits of both psychological and pharmacological performance enhancers, so why not just use belief in place of drugs?

“We’re trying to educate athletes into the idea that the headroom is there to be filled, and drugs are not necessarily the only way of filling that headroom,” he says. “Confidence is the drug of champions.”

Erik Vance is the author of the new book . Reporting for this project was supported in part by the Pulitzer Center.