system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why exactly muscles go away when the person stop working out? Can't the body just get the energy it needs from fat and carbohydrates and leave the grow muscles in place?
gpt
Muscles require energy to just be there, which is why increasing your muscle mass increases your passive  calories burnt per day. 

For efficiency sake, our body doesn't want to consume energy it doesn't need.