Ketosis What Is Ketosis? Ketosis is a metabolic state in which the body shifts from using carbohydrates as its primary fuel source to using ketones , which are molecules produced when fat is broken down in the liver. This typically occurs when carbohydrate intake is significantly reduced, lowering insulin levels and prompting the body to rely more heavily on stored fat for energy. Ketosis can occur during: Fasting Prolonged endurance exercise Very low-carbohydrate diets The k