<prompt> After a fast of a few days, ketone bodies become an important fuelLiverBrainSkeletal muscleRed blood cells  <response>: The brain begins to use ketone bodies when levels start to rise after 3 to 5 days of fasting. Normally, the brain will use only glucose as a fuel most fatty acids cannot cross the blood-brain barrier to be metabolized by the brain, but when ketone bodies are elevated in the blood, they can enter the brain and be used for energy. 