Ketone bodies are three water-soluble compounds that are produced as by-products when fatty acids are broken down for energy in the liver and kidney. They are used as a source of energy in the heart and brain. In the brain, they are a vital source of energy during fasting. Although termed "bodies", they are dissolved substances, not particles.
The three ketone bodies are acetone, acetoacetic acid, and beta-hydroxybutyric acid, although beta-hydroxybutyric acid is not technically a ketone but a carboxylic acid.
Uses in the heart and brain
Ketone bodies can be used for energy. 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 citric acid cycle.
The heart gets little energy from ketone bodies except under special circumstances; it uses mainly fatty acids.
The brain gets its energy from ketone bodies when glucose is less available (e.g., when fasting). 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). In time the brain reduces its glucose requirements from 120g to 40g per day.
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