This is What Happens to Your Body At Altitude
The higher you go, the weirder stuff gets. Here’s what to expect while up in the clouds and what to watch out for.
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I can’t say I wasn’t warned.
My fellow teammates and even my competitors gave me their best advice as I headed out to race the Breck Epic, a six-day mountain bike stage race that was mostly above 10,000 feet. “Don’t go into the red, or you’ll never recover,” they said with the gravity of those who had seen things racers don’t want to see. So what did I do? Shot off the line like a cannonball, soared into the red and spent the rest of Stage One dropping anchor as everyone in the race passed me by like I was a potted plant.
I learned much from that experience and, following the wisdom of high-altitude experts like Peter Hackett, M.D., director at the Institute for Altitude Medicine in Telluride, Colorado, I learned even more during subsequent two trips to the infamous races in Leadville, Colorado. Most notably, there’s no hiding from or escaping the effects caused by higher elevations, but if you understand what’s happening to your body at altitude, you can take measures to prevent the worst of the potential woes and maximize your performance and enjoyment of your next ride into thin air. Here’s what’s going on.
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Your Brain Cries for Oxygen
As elevation rises, the air pressure becomes lower, meaning oxygen molecules become more spread out, leaving you with less oxygen in every breath you take. It generally becomes noticeable as you hit the 5,000-foot mark, and pretty much affects anyone by the time you cross into high altitude at 8,000 feet. By the time you reach 12,000 feet, there’s a wind-sucking 40 percent fewer oxygen molecules per breath.
Your brain doesn’t like not having enough oxygen, so it sends out the signal to dilate your blood vessels to get more blood flow and oxygen to the brain. “That’s why some people develop a throbbing headache within 30 minutes of arriving at high altitude,” says Hackett. Ibuprofen is one way to help relieve your pounding head, but to really help your body adjust and get the oxygen it needs before your big event, some experts recommend beet root juice: it's rich in nitrates that your body converts to nitric oxide, a gas that causes your blood vessels to relax and widen, allowing more blood flow and making it easier to function in low oxygen environments.
Your Heart Rate Rises
Unsurprisingly, your heart rate increases at altitude as your body compensates for the thinner air. At about 6,500 feet, your heart rate will increase 10 percent over your usual rate at sea level. Heart rate will normalize as you acclimate, but it takes at least two to three weeks to become fully acclimated. If you don’t have time to fully acclimate, it’s useful to go out for a few of your training rides with a heart rate monitor and recalibrate your heart rate zones for high altitude—then stick to them during race day. (This has worked swimmingly well for me at Leadville, where I learned that burning too many matches will fry you for the day.)
Your Power Declines
The same hill you could charge up in the big ring at sea level can leave you spinning in the little ring, still out of breath at high altitude. That’s because your power output drops about three percent per thousand feet above 5,000 feet elevation. “By the time you hit Leadville and other high mountain races at 10,000 feet, you’re operating at fifteen percent less power,” says Hackett. Again, acclimatization helps. But there’s a reason that people who live high often choose to train low—everyone’s power suffers to some degree once you hit high altitude.
You Start to Pee... a Lot
It’s not your imagination. You really are running to the restroom (or roadside, or trailside) more often at high altitude. It’s even got a scientific name—altitude diuresis—and it’s actually a wonderful thing. “Your body wants to be a little drier at high altitude. As you pee out some excess fluids, your blood becomes thicker and your hemoglobin becomes more concentrated,” he says.
The cool thing is that this happens within 24 hours of touchdown at altitude, so within a day you can already carry more oxygen in your blood. “Everyone talks about the dehydration that takes place at altitude like it’s a bad thing,” says Hackett. “But it’s a natural adaptation designed to help you get more oxygen to your muscles and organs.” To that end, resist the urge to drown yourself in fluids every moment of every day in the mountains. “You cannot—nor should not try to—completely counteract attitude diuresis,” he says. “An extra liter a day is a reasonable amount to stay hydrated without overdoing it.”
RELATED: Six Things Your Pee Can Tell You
Your Appetite Declines
Some people get nauseous, or in severe cases even vomit, when they go to high altitude from sea level too quickly. But even if you don’t feel altitude sick, it’s natural for your appetite to decline. Research shows that levels of leptin, a hormone known to suppress appetite, increases at altitude. For the best energy and performance, skew your high altitude diet to be higher in carbohydrates, which provide about 15 percent more energy for the same amount of oxygen as you get from fats.
You Toss and Turn
High altitude can mess up your sleep quality on a few fronts. Most notably, low oxygen directly disrupts sleep centers in your brain, causing frequent waking. You also can experience what is known as sleep periodic breathing, in which (as the name implies) you literally stop breathing for a few moments and then start up again, as the respiratory center of your brain, which senses carbon dioxide, battles with the respiratory trigger in the carotid artery, which senses low oxygen. Both can be quite disruptive to a good night’s rest. “Taking a mild sleeping aid like Benadryl can help while you acclimate,” says Hackett.
Your Body Starts Blood Doping
Getting more oxygen where you need it is priority number one, so along with thickening your blood through a little dehydration, your body will also start producing more EPO—the hormone that regulates the volume and number of red blood cells—within 24 to 48 hours of your arrival at altitude.
Your Genes Go Full Tilt
It can take the better part of a month to fully acclimate to high altitude and, as you may have already noticed, everyone acclimates a bit differently, with some people adjusting relatively seamlessly and others still feeling wheezy and woozy a couple of weeks in. Like so much in life, you can credit (or blame) your personal adaptation process on your genes, says Hackett. “There are over 400 genes that are turned on in every cell in your body in response to altitude,” he says. “There are also genes that get turned off. You’re looking at a tremendous range of gene expression among individuals.”
Generally speaking, however, we all fall along a bell-shaped curve where 60 percent will do about average and take a few days to start feeling better; 20 percent acclimate faster and 20 percent acclimate more slowly. “No matter who you are, big jumps like sea level to Leadville are too much for one day and don't work for anyone. Give yourself at least three to five days at altitude before you want to compete or perform well; seven to ten days if you can afford it.”
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