Ketosis is a natural state the body finds itself in when it is using fat as its main fuel. Body fat, or ingested sources of fat. This occurs when following a very low carb, ketogenic diet, and often during intermittent fasting as well. A healthy body should be able to switch between ketosis and carb burning easily. Ketosis is characterized by the controlled and regulated production of ketone bodies in the blood via various metabolic processes. During zero or limited carbohydrate intake, reduced insulin levels lead to a reduction in lipogenesis and fat accumulation. After several days (depending on the person) of fasting, or eating low carb, glucose reserves deplete enough that the body then switches to using fat for another source of fuel.