system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we lose unused muscle and not unused fat?
gpt
Because muscle is expensive to maintain, you need to eat a lot of food just to keep the muscles even if you aren't using them.

This is not true of fat, it's pretty cheap for your body to maintain when it just sits there.

This is important because for 10s of thousands of years food was very scarce for humans. So our bodies evolved to not waste it. Because people that didn't lose unused muscle would starve to death so the gene for "keep your muscles" was lost.

It's only within the last 100 years or so, and even then only in rich countries, that a "keep your muscles" gene wouldn't result in you starving to death after a single tough winter (and they are all tough).

Additionally, the entire *point* of fat to stick around until you need it. Before the abundance of food we have now getting rid of unused fat made about as much sense as withdrawing all the money in your savings and burning it because you haven't needed for a couple years now.