Oxygen is arguably the single most important thing to a cyclist. And at altitude, there’s less of it. At about a mile above sea level, the amount of oxygen available to you in the air you breath is about 17% less than sea level. At 8000 feet, it’s about 25% less.
Interesting distinction: There’s not actually less oxygen in the air relative to the other atoms. It’s still air with the same percentages of each. 20% of air is oxygen in Denver or Boston or at the top of the Himalayas. There’s less pressure, which means that there are less molecules squeezed into a given volume of air. So in the volume of air that makes up your lungs, there’s less atoms in general.
So your body needs to do something different to get the same amount of oxygen. The first and obvious one is that you breathe heavier. If you are dropped at 5000 feet when your body is used to zero feet, you’re going to have to breathe heavier sooner if you are exerting yourself. Eventually, your body will adjust to normal breathing rates by adding red blood cells and changing up heart and kidney functions in ways that I don’t understand enough to explain. But it just does.
This means that the possible altitude gain in most single riding sessions probably isn’t enough to make much difference to you as long as you are adjusted to the base altitude in the area.
Which is disconcerting because I’ve been living at altitude for several weeks, and I wanted to blame altitude for my poor performance. I haven’t been exercising though, so maybe my body hasn’t fully adjusted yet.
Or maybe I’m just out of shape. It’s probably 90% fitness, 10% altitude.
Either way, the cure is the same: Keep at it. And shut up legs.