system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why Does Your Breath Smell Bad When You Don't Eat For A Long Time?
gpt
What you are smelling is acetone.  Your brain is the one organ in your body that cannot use fat as an energy source.  When you are eating regularly, your body can maintain the sugar glucose at a high enough level in your blood to provide plenty of energy for your brain.  During short times of fasting, such as when you are sleeping, your body breaks down stored sugar in the form of glycogen to provide glucose for the brain.

When you undergo prolonged fasting, however, your body runs out of glycogen, and your blood sugar drops.  In order to continue providing energy for the brain, your liver converts fat into molecules known as ketone bodies, which your brain can then use as an alternative energy source.  Acetone is produced by the breakdown of ketone bodies for energy, and that's where the bad smell comes from.