Running at altitude: The basics
You’ve probably heard that running at altitude is rough. There’s a good reason for this: because of the reduction in pressure, there’s less O2 per unit volume of air, meaning, each breath you take bring less oxygen to your muscles. At 8,000 ft, there’s only 75% the amount the O2 compared to sea level - 25% less is a big discount! At 10,000ft, most runner will spend 30% more energy running the same pace as sea level. This could turn a typical easy training run into a tempo workout! So what’s one to do? The short answer is: slow down. There’s a very handy online calculator that will tell you how much you need to slow down given your altitude. Let’s say you’re running a half marathon and your sea level pace is 11:30/mile. At 8,000ft, you’ll want to add 40 seconds per mile, bringing you down to a 12:10 pace to see an equivalent workout to sea level.
So what happens when your body acclimates, and how long does it take?
First we should say, you’ll never be 100% acclimated/you’ll never be quite as fit as you are at sea level. But over the course of a number of weeks, you will see major differences. The majority of your acclimation will occur within 3 weeks of arriving at altitude, but many ultrarunners say it takes over 6 months to feel “normal” again. In my personal experience, it took 6 weeks for my heart rate to calm down on runs and to feel closer to “normal” running at altitude.
As for the acclimation process itself, here’s what’s going on: when we acclimate, we see an increase in red blood cell volume and total hemoglobin mass, which delivers oxygen to the body. This process is a result of increased erythropoietin (EPO) production by the kidneys upon exposure to altitude, which initiates red blood cell production in the bone marrow. More red blood cells lead to an increase in hemoglobin mass, which is how oxygen is carried around the body. The more oxygen you can deliver to working muscles, the faster you can run. (Source) I did two blood tests, one before moving to altitude and one after: I saw the increase in RBC count and hemoglobin, as well as increased hematocrit and decrease in iron levels (I had high iron before the move, so this was actually a good thing for me: however, women need to be especially careful with this and monitor their iron and ferritin levels as they can get dangerously low at altitude!).
Racing at altitude? Here are some tips:
The first three days you’re at altitude tend to be the worst. Your body goes into overdrive making physiological changes to adapt to altitude, and you adapt more quickly in this time period. Therefore, you’ll feel worse on days 1-3. After the first two weeks you should start to feel major improvements.
Some suggest arriving at altitude weeks before your event to acclimate as much as you can. Given this isn’t reasonable for most, the second suggestion is to arrive the morning of your race, before your body kicks into high gear making more red blood cells.
If neither of the above work for you, your best bet is focus on hydration (including electrolytes!) and eating well. Your body burns more calories at altitude, so you’ll need to account for that. You may feel less thirsty and hungry - but you should try your best to push through and drink as much as you can.
Watch your iron levels! It’s wise to get bloodwork before your race to ensure your iron and ferritin is adequate. If not, it’s possible to supplement.
Account for extra recovery time: it takes you longer to recover at altitude, so plan extras days of recovery.
My story: moving from sea level to Denver:
Probably the worst timing ever, but I moved from sea level to the Denver area 4 days after a 50 miler (do not recommend, since recovery takes longer at altitude, too). It is hard to say what parts of my experience were recovery-issues vs pure altitude acclimation, but it took me 6 weeks to start feeling “normal” again (by that I mean, my HR was closer to normal on runs). I did a quick survey in some ultrarunning communities, and found that others had a similar experience. Many people say the first 6 weeks is the worst, some say it took them 6-12 months to feel like they had most of their fitness back. Some other notable changes I experienced/am still experiencing!:
Need for more sleep and recovery time. Runs and workouts zap me more than than they did at sea level. I find myself needing 8-9 hours of sleep each night, needing a full 100% recovery day (if not two!), and I take longer naps (I used to be great at 20-minute power naps, now they’re 40-60 minutes!).
Need for more fuel and hydration. My appetite has increased (as it should - we burn more calories at altitude), so I carry extra calories on my runs. Additionally, I’m going through water much faster - I used to be able to get away with a 500ml bottle for 10-12 mile runs, but here I need to carry two bottles, and find myself more thirsty after runs. (Altitude is sneaky - you don’t feel yourself sweat as much which can cause you to under-hydrate! I’m still adjusting to this).
I’m struggling more in VO2 workouts. Somewhat counter-intuitive, but I’ve found it hard to push my HR into VO2 zone in interval workouts. In part this could be because I’m trying them mostly outside uphill, and my muscles aren’t up to the challenge with the limited O2, limiting my ability to get time in HR zones. I’ll do more experimenting with this on flats or elliptical.
Pacing. I’ve had to slow down slightly (even after acclimating) to keep my HR in line on my runs.
Eating iron-rich foods. I’ve upped my iron intake to account for the loss experienced through acclimation. I’ll continue to monitor my levels a few times a year, especially before big events.
If you’re racing at altitude, or considering a move to altitude, I hope this helps you prepare!