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