When the human body begins to create ketones for energy by drawing on its existing fat supply, it enters a state known as ketosis. Ketones are a byproduct of this process, and that is done to ensure that excessive muscle mass is not lost. During the first five days a person goes without eating, it is feasible for them to lose between one and two kilograms (2.2 to 4.4 pounds) of body weight per day.