The three ketone bodies are acetone, acetoacetic acid, and beta-hydroxybutyric acid. Ketone bodies are derived from the incomplete oxidation of fatty acids in the liver and are used for energy (ATP generation) in the heart and brain.
Ketone bodies are transported from the liver to other tissues, where acetoacetate and beta-hydroxybutyrate can be reconverted to acetyl-CoA to produce energy, via the Krebs cycle.
The brain gets its energy from ketone bodies when insufficient glucose is available, such as during fasting and starvation. In the event of low blood glucose, most other tissues have additional energy sources besides ketone bodies (such as fatty acids), but the brain does not. After the diet has been changed to lower blood glucose for 3 days, the brain gets 30% of its energy from ketone bodies. After about 40 days, this goes up to 70% (during the initial stages the brain does not burn ketones, since they are an important substrate for lipid synthesis in the brain). By using alternative fuels, the demand for glucose is reduced and muscle proteins are spared further degradation.