What happens if you don't eat for a whole day, or even two days? Your body starts to panic. It doesn't know when it'll get food again. First, your body uses up all the sugar in your bloodstream. Then, it starts to burn glycogen from your muscles and liver. After 12 hours, your body goes into starvation mode. Your muscles start to break down and your body starts to use its fat reserves for energy. After 16 hours, your body starts to go into ketosis. This means your body is burning fat for energy. After 24 hours, your body starts to break down protein in your muscles. This is not good for your body. When you don't eat, your body starts to panic and it goes into starvation mode.