system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly happens in my body when I don't eat for a day (or more)?
gpt
Low blood sugar levels trigger the release of glucose, stored as glycogen in muscles and liver. Fat is broken down to undergo oxidation for further energy requirements. If the fasting goes on for long enough, there is a significant breakdown of muscle to supply amino acids for energy. As the starvation continues, the muscle breakdown slows and fat breakdown accelerates. Large amounts of what are known as ketone bodies are now supplying a large component of your energy. 

As a side note, diabetics who don't control their sugar with insulin will observe the same cascade as above since they aren't able to take in the glucose they are eating. That is why some diabetics will present to the hospital with what is known as diabetic ketoacidosis, where their body has been brought to the point where it has to run off these ketone bodies which eventually have dramatic effects on that person's physiology.