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In 2015, scientists from the Center for Space Medicine and Extreme Environments in Berlin followed athletes competing in the Yukon Arctic Ultra. They wanted to know: How does the human body cope in such a brutal context? When the researchers analyzed the hormones in the bloodstreams of the athletes, one hormone, irisin, was wildly elevated. Irisin is best known for its role in metabolism—it helps the body burn fat as fuel. But irisin also has powerful effects on the brain. Irisin stimulates the brain’s reward system, and the hormone may be a natural antidepressant. Lower levels are associated with an increased risk of depression, and elevated levels can boost motivation and enhance learning. Injecting the protein directly into the brains of mice—not something scientists are ready to try with humans—reduces behaviors associated with depression, including learned helplessness and immobility in the face of threats. Higher blood levels of irisin are also associated with superior cognitive functioning, and may even prevent neurodegenerative diseases such as Alzheimer’s.
The Yukon Arctic Ultra athletes entered the event with extraordinarily high blood levels of this hormone, far beyond levels seen in most humans. Over the course of the event, their irisin levels climbed higher. Even as their bodies fell victim to hypothermia and exhaustion, the athletes were bathing their brains in a chemical that preserves brain health and prevents depression. Why were their blood levels of irisin so elevated? The answer lies in both the nature of the event and what the athletes had to do to get there. Irisin has been dubbed the “exercise hormone,” and it is the best-known example of a myokine, a protein that is manufactured in your muscles and released into your bloodstream during physical activity. (Myo means muscle, and kine means “set into motion by.”) One of the greatest recent scientific breakthroughs in human biology is the realization that skeletal muscles act as an endocrine organ. Your muscles, like your adrenal and pituitary glands, secrete proteins that affect every system of your body. One of these proteins is irisin. Following a single treadmill workout, blood levels of irisin increase by 35 percent. The Yukon Arctic Ultra required up to fifteen hours a day of exercise. Muscle shivering—a form of muscle contraction—also triggers the release of irisin into the bloodstream. For the Yukon Arctic Ultra competitors, the combination of extreme environment and extreme exertion led to exceptionally high levels of this myokine.
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Kelly McGonigal (The Joy of Movement: How exercise helps us find happiness, hope, connection, and courage)