Ketosis is a metabolic state where most of the body’s energy supply comes from ketone bodies in the blood, in contrast to a state of glycolysis where blood glucose provides most of the energy. It is characterised by serum concentrations of ketone bodies over 0.5 millimolar with low and stable levels of insulin and blood glucose. It is almost always generalized with hyperketonemia, that is, an elevated level of ketone bodies in the blood throughout the body. Ketone bodies are formed by ketogenesis when liver glycogen stores are depleted (or from metabolising medium-chain triglycerides). The main ketone bodies used for energy are acetoacetate and β-hydroxybutyrate, and the levels of ketone bodies are regulated mainly by insulin and glucagon. Most cells in the body can use both glucose and ketone bodies for fuel, and during ketosis free fatty acids and glucose synthesis (gluconeogenesis) fuel the remainder. During the usual overnight fast the body’s metabolism naturally switches into ketosis, and will switch back to glycolysis after a carbohydrate-rich meal. Longer-term ketosis may result from fasting or staying on a low-carbohydrate diet, and deliberately induced ketosis serves as a medical intervention for intractable epilepsy. In glycolysis higher levels of insulin promote storage of body fat and block release of fat from adipose tissues, while in ketosis fat reserves are readily released and consumed. For this reason ketosis is sometimes referred to as the body’s “fat burning” mode.