system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does the human body eat muscle before fat when it's in starvation mode?

I would think (but could be totally wrong) that your body would want to eat the fat first in order to preserve the muscles, which would help with survival odds. What happened biologically/evolutionarily that made it this way instead, and why?
gpt
While your premise isn't entirely correct that protein metabolism and fat metabolism happen sequentially, one right after the other, you are on to something here.

An important issue is that glucose is the preferred form of energy especially for your brain, and your body will attempt to keep the level of glucose in your bloodstream stable. Your reserves of glucose are in the liver. Once those are depleted, protein is essentially carbohydrate with added nitrogen. So, in order to maintain your blood sugar, your body will metabolize amino acids into carbohydrate by stripping the nitrogen (and throwing it away in your urine), and converting into glucose.

Your brain's second most favorite food, after glucose, is ketone bodies, which are created from fat. The metabolism of ketone bodies is the goal of the Atkins diet.