The ketogenic diet is a diet high in fat and low in protein and carbohydrates, with up to 90% of calories obtained from fat. Since the diet is low in carbohydrates, the body gets little glucose, normally the main energy source. The fat in the diet is converted by the liver in ketone bodies, which causes a build up of ketones in the blood stream, called ketosis. Ketone bodies are transported across the blood-brain barrier by other means than the GLUT1 protein and thus serve as an alternative fuel for the brain when glucose is not available.