Ketosis is a metabolic state in which your body uses fat, rather than sugar, for energy. Your body shifts into ketosis when your blood sugar is low, and the glycogen in your muscles has been depleted. Typically, this happens when you eat a low carb diet or fast from food altogether for a prolonged period. Ketosis is usually heralded as a fast and effective way to lose body fat. Research suggests that ketosis may positively affect health in other ways, as well.