How Altitude Affects Calorie Burn for Cycling
It’s a lot harder to ride at altitude. We all know this, but the point was reemphasized to me recently at the Tour of the Gila. Coming up from sea level to the 2,000 meter heights of Silver City, NM, I was noticeably winded just walking around. The cycling, almost needless to say, did not go well.
It was a very fun weekend, but not a productive one as far as racing and upgrade points go. That’s okay though. At the end of the day, cycling is about the enjoyment, and I enjoyed being off the back amongst the trees of the Gila National Forest about as much as I enjoy anything in cycling. While off the back, I did have plenty of time to speculate about what effect the altitude was having on my body. Aside from the obvious breathlessness and lack of power output, what else was changing? When I got home, it was time for some research on these effects, starting with how the high peaks and low oxygen affect my caloric usage. I’m working harder, shouldn’t I be burning more?
In short, no. Here’s why:
;tldr: Exercise at altitude burns fewer calories than at sea level. Read on for why.
Altitude and Calorie Burn
Over the course of an endurance bike ride like a road race or time trial, your muscles take in oxygen and glucose and push out carbon dioxide. That’s the big picture view of what’s going on in every single muscle cell. But let’s zoom in just a bit, to the actual machinery that carries out this process.
That’s the mitochondria the gold dots in the incredible photo above. This is a photo of cow epithelial cells, the same kind of cells that line your nasal passages, intestines, and most body cavities. Give the picture a second look. Every individual splash of gold is a mitochondria. There are thousands in there, but the muscle cells you use to ride those miles have many more mitochondria than those cells above, more than 4,000 per cell.
Every individual mitochondrion is constantly working to produce energy from glucose, converting that sugar molecule into ATP, an energetic molecule that muscles use for fuel, and Carbon Dioxide, a waste product (I call it muscle poop, but that phrasing hasn’t caught on for some reason…). This process requires dozens of specific chemical reactions and is an incredible mechanism, but we’re only focused on the overall picture. Here it is in brief:
Glucose and Oxygen go in, Carbon Dioxide and Energy come out.
Without the two inputs, we don’t get the outputs. That’s why it’s essential to keep your energy supplies topped up throughout a long energy ride by eating gels and bars along the way. Usually, it’s the glucose that’s in short supply, but altitude changes that relationship.
At 2,000 meters, there’s 20% less oxygen in the air than at sea level and it’s just not enough for this reaction to work smoothly. For the mitochondria to continually pump glucose through and energy out, oxygen has to be available. At altitude, it’s not.
This is why calorific burn is lower at altitude, there’s not enough oxygen available to perform this chemical reaction. This reaction is almost identical to what happens when you burn something. Hydrocarbons (like oil and sugar) and oxygen go in and carbon dioxide comes out.
You know how a flame goes out when you cover it? That’s exactly what happens to in your muscles when you go to altitude.
There’s limited oxygen, so the reaction slows down. If you took away all the oxygen, the reaction would stop completely (and you would die). Fortunately, 2,000 feet is not that extreme a condition.
How this helps your cycling
This reemphasizes why it’s best to “sleep high, train low” if available. Training at low altitude lets your mitochondria be maximally effective at converting glucose to Watts, and sleeping at high altitude lets your body adapt to those conditions. Day to day, this won’t make a big difference for the average cyclist, but it’s something to keep in mind if you have the option of altitude exposure as you prep for an event.
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