Ketosis is a metabolic state in which most of the body's energy supply comes from ketone bodies in the blood, in contrast to a state of glycolysis in which 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 ß-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.