system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why, when on a low calorie diet, does your body consume muscle as well as fat? Why not just fat?
gpt
Extra muscle is a luxury.  Your body wants to be strong, but the more muscle it has, the more food it requires.  If it thinks there isn't enough food available, it reduces the amount of muscle you have so you will use less in the long run.