The famine response is how the body of a human or animal responds to malnutrition.
The brain normally gets its energy from glucose, but during starvation it is forced to get its energy from ketone bodies. In starvation, most other tissues have additional energy sources besides ketone bodies (such as fatty acids) but the brain does not. After fasting for 3 days, the brain gets 30% of its energy from ketone bodies. After 4 days, this goes up to 70%. (During the initial stages of starvation the brain does not burn ketones, since they are an important substrate for lipid synthesis in the brain.) The brain retains some need for glucose, because ketone bodies can be broken down for energy only in the mitochondria, and brain cells' long thin axons are too far from mitochondria.
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