Two travelers close up. Girlfriends are in the winter forest. Two women with backpacks and trekking poles in a winter hike. Friends walk along the snow covered path. Girls in the mountains.

“You don’t need the evidence to know that the more you exercise, the better your mood is, the less likely you are to have anxiety and depression,” says Dr. Hadi Halazun. “Everyone thinks the cardiovascular system is the primary beneficiary of exercise, but it’s just one component.”

It’s well-studied that increased physical activity reduces the risk of developing heart disease. Arteries are elastic, and the more you exercise, the healthier they become. “What happens with age and sedentary lifestyles is that they harden and that’s how you develop high blood pressure and heart disease,” explains Dr. Halazun.

New research from Dr. Jack Kreindler, an emergency medic and chief medical officer of The Centre for Health and Human Performance, a London-based multidisciplinary practice that treats all patients from preoperative to Olympic-level athletes, shows that being fit is not just beneficial to the heart, but to cells all over the body. By studying people going through extremes — fitness in extreme environments (i.e. climbing Everest or other high-altitude, low-oxygen locales) — Dr. Kreindler has discovered that “there is an advantage to being fit, and it’s not just being able to run for a bus.”

When you exercise, you put your cells through stress. “Every time you push it to produce energy, it also creates damage, free radical production,” says Dr. Kreindler. “A fit cell is one that can do a lot of work with a little amount of fuel and a minimal amount of damage to itself.” That means you’ll be better equipped, not just for moderate stress environments (running for a bus, for example), but also for the extremes — an environment where there’s less oxygen, or an emergency medical situation. “We know that people who are fit will be able to cope with operations much better. They will be able to survive periods in ICU much better,” Dr. Kreindler explains.

“Metabolic fitness affects almost every cell in your body in a beneficial way.”

Our understanding of fitness on this cellular level has also helped inform larger connections. For example, working big muscle groups in the legs is linked to hormones produced by the right side of the heart that stimulate the brain. “So cognitive improvement happens as a result of using the big muscle groups in your legs,” says Dr. Kreindler. The more you stress the system, the stronger your bones get; the more you move, the better your vertebrae can breathe, squeezing out waste; the more you work on your coordination and balance, the better your neuromuscular firing (the connection between your brain and muscles). “Metabolic fitness affects almost every cell in your body in a beneficial way,” he says.

The American Heart Association recommends 150 minutes of moderate exercise a week, and as Dr. Halazun points out, the collective decrease in everyday movement over the past two years — no longer walking on your commute to work, being stuck at home for days at a time — has highlighted that these “activities that are not necessarily considered exercise (but are movement) are more and more important.”

But Dr. Kreindler’s research shows that being fit is more than getting your 10,000 steps (although you do achieve a certain level of fitness this way). “Incredible things are happening to the body when you push it a little bit and allow it to recover properly,” he says. What’s key is mixing aerobic exercise (low-grade activities, which can range from long walks to running for some) and anaerobic activities — high intensity exercise in short bursts. “Anaerobic [exercise] forces your system to get better at coping with high intensity stresses,” Dr. Kreindler explains.

“Incredible things are happening to the body when you push it a little bit and allow it to recover properly.”

Stretching all parts of your bodily system is important — low and high intensity exercise, activities like yoga that improve flexibility, or climbing, which improves balance and coordination. “When you eat, you’re trying to [eat] as many brightly colored vegetables and fruits,” says Dr. Kreindler. “The more variety you’ve got [in how you move], the more you fill in the gaps that constitute a healthy metabolic system and make it fit.”

Getting to this state — aerobically fit, anaerobically fit, strong, flexible, and able to balance — can all be done without equipment, but not without proper nutrition and sleep. “We were not born into a world where [food] was overloaded on us. We were always in a slight calorie deficit,” says Dr. Kreindler. “You can burn more if your cells are on fire and super fit.” But none of that matters if you’re not sleeping. “That’s the time when you repair and adapt,” he says. One of Dr. Kreindler’s findings from studying extreme altitude situations was that “people who fail to adapt their breathing during sleep are the ones that do the worst with respect to altitude sickness.” Even at sea level, sleep is “the most important fuel of all the fuels.”

Next up in Dr. Kreindler’s research endeavors is studying the differences between men and women. Later this year, he’ll be co-leading an expedition to the South Pole studying how men and women fare in the environment. This expedition follows two previous ones that were just men and just women, respectively. The findings showed that, compared to men, women “did spectacularly well and didn’t lose as much muscle mass.” Their hypothesis for this upcoming expedition is that women will perform much better in hyper-endurance situations. The science is something he and his team are still working to understand. Dr. Kreindler suspects that “it’s something to do with survival in mass migrations where there’s a famine or a drought. Women need to leave and travel much longer. And men perhaps were intended for dealing with acute crises like an angry rhino.”

This science of human performance is what helps us continue to understand how everyday people can survive, reducing risk and getting through emergency situations, what Dr. Kreindler calls the “tough stuff.” Fitness is “a superpower. It can save your life.”