Choose the major fuel that is being adapted by the brain after 1 week of fasting.
Ketone bodies
Blood glucose
Fatty acids
Glycogen
The brain then takes up and oxidizes the ketone bodies to derive energy. Consequently, the brain decreases its use of glucose, although glucose is still a major fuel for the brain.Ketone bodies are used by the brain after 3 to 5 days of fasting. This is done to spare protein degradation to provide carbons for gluconeogenesis. Approximately 60% of the brain's energy needs will still be dependent on glucose, but the other 40% can be realized from ketone body oxidation. The brain will not oxidize fatty acids because they have trouble crossing the blood-brain barrier, and the brain has very low levels of glycogen.
Carbohydrates