system
Given the following post, respond as if you're commenting on Reddit:
human
What physically happens to your body when you get "out of shape"?

For example, I used to be a pretty good runner and kept up with running consistently. Then I stopped for several months and it was a lot harder to keep the same pace I used to. Why is this?
gpt
Not sure about ALL the reasons, but for one, the more you exercise, the more mitochondria your muscle cells make (It is here that your cells use up the oxygen you take in and convert it to ATP [energy]). Your aerobic capacity is highly dependent on your mitochondria's ability to turn sugar (among many other things) into ATP. When you stop aerobically exercising (don't run for a few months), your cells see that there is no point in keeping all those extra mitochondria if you are not using them. So the cell breaks them down and uses the parts for things more relevant to your body's current situation :)