When the body runs out of carbohydrates to burn for energy, it burns fat instead and goes through a process called ketosis. When someone is fasting, it's not uncommon for their body to enter a state of ketosis.
The process has been studied by scientists and found to be useful in treating some kinds of diabetes and epilepsy. Sometimes people eliminate certain kinds of carbohydrates from their diet in order to put their body into ketosis, as a way of burning fat. Ketosis adds the medical -osis suffix, meaning "a state of disease," to ketone, the chemical name for the molecules produced by the liver during ketosis.