Choose the major fuel that is being adapted by the brain after 1 week of fasting.

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.