system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How the body decides whether to burn fat or muscle, and which area of the body it targets first.

Just like the subject says. For further explanation see this hypothetical: A man is eating 1700-1900 calories and burning 2200-2400 calories per day via interval training or other forms of cardio. To  make up this deficit what ratio of fat:muscle will the body typically consume, and which area of the body would it come from first? Does the body favor to burn fat closest to the muscles being used, or does it target a specific area first?
gpt
Mostly the adipocytes, the fat cells, when they're triggered just release fatty acids and the blood moves it around bound to some proteins.  They're not locally targeted really. The liver sends the body signals when it starts to get a low on blood sugar supply and triggers the fat cells to send out free fatty acids. The liver doesn't direct it to a special place, and the blood just carriers it wherever cells need energy. 

Fat -> blood -> muscle. Which muscles you're exercising don't control where the fat comes from.