In the summer of 2021, I moved from Winston-Salem, North Carolina—where I had spent years training at a modest 1,000 feet above sea level—to Colorado Springs, Colorado, which sits at 6,500 feet.
I knew the altitude would be a challenge, but I underestimated just how much it would affect my running. Workouts that once felt routine suddenly left me gasping for air. My paces slowed, recovery took longer, and it felt like my fitness had vanished overnight.
I quickly realized I needed to understand what was happening inside my body and how to adapt properly. Over the following weeks and months, I dove into research on altitude physiology, acclimatization timelines, and strategies to help my body adjust.
Running at altitude impacts your body in immediate and long-term ways—from oxygen delivery and heart rate to hydration and fatigue. And while some runners adapt more quickly than others, no one is immune to the initial hit.
In this guide, I’ll break down how altitude affects your physiology, what changes you can expect both right away and over time, and how to successfully acclimate to avoid altitude sickness.
You’ll learn about the three stages of altitude adaptation—preparation, ascent, and descent—as well as serious conditions like AMS, HAPE, and HACE. Armed with this knowledge, you can train smarter, race stronger, and make the most of your time at elevation.
How Altitude Affects Your Physiology
Most of us have a general understanding of why it is tough to travel and exercise at very high altitudes. The standard answer is that there is less oxygen saturation in the air as altitude increases. This isn’t technically correct.
The amount of oxygen in the air stays constant at 21% regardless of whether you are at sea level or the top of Everest. What changes is the air pressure, which means you are able to breathe in less air as you increase in altitude.
Here is a great TED Talk explaining the process.
So, how does altitude affect your physiology?
Your body responds to altitude through an acclimation process that elicits both immediate and gradual changes. Let’s look at each one in detail:
Immediate changes
The first change your body makes as it becomes hypoxic is to begin hyperventilating. The lower oxygen levels in our body cause our body to respond by making us breathe faster to bring in more oxygen.
Your body will also increase your heart rate and blood pressure.
The increased heart rate will speed up the rate at which blood moves through your body, allowing it to get oxygen to your tissues. Increased blood pressure also helps speed blood through the body.
Your body will increase its metabolism by producing more epinephrine and adrenaline.
These two hormones will also help with the increased blood pressure and heart rate, which we discussed above.
Lastly, your body will also experience fluid loss.
While this can be a negative consequence, leading to dehydration, it will also decrease plasma volume. We will discuss why this is a positive in the next section.
Long-term changes
If you are familiar with professional runners or other endurance athletes, you may be aware that they do stints at high-altitude camps during their training. The reason for this is so they can reap the benefits of the long-term changes that we are about to discuss.
We mentioned fluid loss above. This leads to a decrease in plasma volume. Why is this a benefit?
A lower blood plasma volume leads to a higher concentration of red blood cells, or hematocrit, and a higher concentration of hemoglobin. The more these increase, the more oxygen can get into your tissues.
In addition to a higher concentration of red blood cells, your body will also begin to produce more red blood cells. This actually increases the production of a substance you might have heard of in a negative connotation: EPO.
Our bodies naturally produce this performance-enhancing drug, and we can increase production at high altitudes. Don’t worry; your metrics will be well within the acceptable range should you have to take a drug test.
Your body will also increase the number of capillaries. Capillaries are tiny blood vessels through which oxygen travels to get blood to your tissues. More capillaries mean more blood, which means more oxygen.
An interesting aside, distance running and endurance sports are one of the few things that can cause your body to increase the production of capillaries.
How Long Does It Take To Acclimate To Altitude?
Most people can travel up to 8,000 feet without needing to acclimate. If you are traveling to higher altitudes, the Center for Wilderness Safety states that it typically takes 1-3 days to acclimate to that altitude.1Center For Wilderness Safety | Wilderness First Aid • WFR. (2024, March 28). CWS | Center for Wilderness Safety. https://wildsafe.org/ After this, your body will begin to feel normal at that altitude.
While this might be the case for walking or hiking, I’ve found things to be slightly different for running.
When I lived at sea level and traveled to Colorado Springs (6,000 feet above sea level), I noticed a decrease in my running performance. Runs at my easy pace felt pretty normal, just a slightly elevated heart rate and breathing rate.
As I increased my pace, I noticed that I had to work much harder and was not able to run at the same pace as I had at sea level.
When I moved to Colorado, it was probably 6-8 weeks before I felt like I was back at the sea level benchmarks that I was used to.
Altitude acclimatization can vary from person to person. The best approach is to take it easy for a few days and gradually see how your body adjusts to the elevation.
What Are The 3 Stages Of Altitude Acclimatization?
Altitude acclimatization is the process of gradually introducing your body to higher altitudes, allowing it to adapt. This is typically only necessary if the individual travels to an altitude of around 10,000 feet (3,000 meters) or higher.
To effectively acclimatize to higher altitudes, you can follow an altitude acclimatization 3-step process: preparation, ascent, and descent.
#1: Preparation
The preparation stage of altitude acclimatization involves gradually introducing your body to higher altitudes to prepare it.
For instance, if you were getting ready to run the Leadville 100, which starts at 10,000 feet of elevation and has a high point of 12,500 feet, you might fly into Denver, CO, and stay a few days.
Denver, the Mile High City, sits at an elevation of 5,280 feet. Staying a day or two at this elevation will allow you to acclimate before heading to Leadville, CO, which sits at an elevation of 10,000 feet.
It’s also important to take it easy the first few days at altitude as your body adjusts. Pushing too hard early on could mean you spend the rest of your time trying to recover. For locations even higher in altitude, like a Himalayan peak, it is even more important to spend time in the preparation stage.
Base Camp at Mt. Everest sits at 20,000 feet. Most climbers who climb the peak will fly into Kathmandu, which sits at 4,500 ft. They will then spend several days or weeks hiking to base camp to allow their bodies to acclimate to the high altitude. Once there, they spend several days doing short hikes and acclimating before starting up the mountain.
#2: Ascent
When making the ascent, it’s best to keep in mind that you typically don’t want to sleep higher than 1,600 feet per day. In addition to this, for every 3,300 feet you climb, you will want to spend an extra day at that elevation to acclimate.
On Everest, climbers will typically spend time climbing up to camps 1, 2, and 3, staying a day or two, then coming back down. This process enables them to adhere to these tenets and acclimate to the high altitude as they prepare to make a push for the summit.
#3: Descent
You’ve made it to the summit. The hard work is over, and now it’s just time to come back down. While the descent may seem like the easiest part, your body is still working hard at a high altitude. You want to ensure that you are not overexerting yourself during this period.
If you are in Colorado bagging a 14er (the nickname for a peak over 14,000 feet), take a few minutes at the summit to rest and enjoy the view before starting back down.
I ran two marathons last summer, the Leadville Marathon and the Pikes Peak Marathon. Both included a challenging climb that brought me over 13,000 feet to their summits (13k for Mosquito Pass at Leadville and 14k for Pikes Peak at the Pikes Peak Marathon).
Both of these races had an aid station at the top with food and hydration for runners. I made sure to take my time to get my calories in and let my breathing and heart rate return to normal before starting the descent back down.
If you are traveling to altitudes higher than 10,000 feet, you may want to watch for symptoms of AMS, HAPE, and HACE.
AMS
AMS is acute mountain sickness,2https://www.facebook.com/WebMD. (2017, January 31). Altitude Sickness: What to Know. WebMD; WebMD. https://www.webmd.com/a-to-z-guides/altitude-sickness also commonly referred to as high altitude illness or altitude sickness. In its mildest form, altitude sickness can feel like a short-term hangover. Symptoms of altitude sickness may include a headache, loss of appetite, dizziness, muscle aches, and nausea.
Symptoms of altitude sickness typically begin 24-48 hours after arriving at extreme altitude, or even moderate altitude, and go away as you experience high altitude acclimatization over prolonged exposure.
While you can generally mitigate the onset of altitude sickness by taking your time when going from low altitude to extreme altitude, and taking it easy the first few days you are at altitude, it can affect anyone of any fitness level.
Severe AMS can be life-threatening. If you or anyone you are with is experiencing severe AMS, it is important to seek medical attention immediately.
The acclimatization process can take some time and varies from person to person, especially depending on susceptibility and different risk factors such as experience, cardiovascular fitness, and body composition. Don’t be afraid to take a step back and slow down as you scale Mount Everest (or other high-altitude adventures).
Common treatments for acute mountain sickness include acetazolamide, dexamethasone, and nifedipine. These medications help with the symptoms associated with hypoxemia while trekking at altitude.
HAPE
HAPE stands for high-altitude pulmonary edema. This is a buildup of fluid around the lungs caused by hypoxia at high altitudes. It is very dangerous, if not life-threatening, and is the most common cause of death from altitude sickness.
Ventilatory symptoms of HAPE include shortness of breath at rest, cough, decreased exercise performance, and chest congestion. If someone is suspected of suffering from HAPE, it is best to get them to a lower altitude as quickly as possible.
HACE
HACE is high-altitude cerebral edema. This is a buildup of fluid around the brain and is the most severe form of altitude sickness. It is life-threatening, and the individual should seek medical attention immediately.
Symptoms include loss of consciousness, fever, ataxia, and rapid heartbeat. If someone is suspected of suffering from HACE, they should be taken to a lower altitude and provided with supplemental oxygen.
While altitude sickness can affect anyone, regardless of their fitness level, it can be mitigated by gradually increasing altitude and allowing sufficient time for proper acclimation.
With proper planning, you can properly acclimate to high altitude so that you can enjoy your time in the mountains.
For a list of some of the best places to train at altitude, check out our guide: The 5 Best Altitude Training Destinations in the World.
Very well written. Before finding your website and this story I ran across several discussions of how to acclimate at 8 or 9,000 ft in ski areas. 2 to 3 days seem to be enough. Well, we spent 27 years at 8900 ft. The last few years, and we were by then well into our ’70s, we would head for Phoenix for about 5 months. On the return home we might have very light headaches for a couple of days, and I was not back up to Ranch work for about 2 weeks. By the way, I first crossed Wolf Creek Pass from the Alma side in 1971. Been doing it occasionally until we left in 2016. Something about that feeling I get at the top by the priest’s Monument really puts me in touch with a greater power. And I’m far from a religious guy.
Great information! I’ve come up from Seattle and spent 4 days in Denver with family. I was bike riding about 12 miles a day and feel ok. Today, up to Dillon for a couple of days and then Leadville for a week with friends hiking a few 14’s. Thanks for the article.