After a fast of a few days, ketone bodies become an important fuel
Liver
Brain
Skeletal muscle
Red blood cells
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.
Nutrition & Digestion