The acetyl-CoA formed in fatty acid oxidation enters the citric acid cycle only if fat and carbohydrate degradation are appropriated balanced. The reason is that the entry of acetyl-CoA into the citric acid cycle depends on the availability of oxaloacetate for the formation of citrate.

In fasting or diabetes, oxaloacetate is consumed to form glucose ny the gluconeogenic pathway and hence is unavailable for condensation with acetyl-CoA. Under these conditions, acetyl-coA is diverted to the formation of acetoacetate and D-3-hydroxybutyrate. This procces is known as ketogenesis, wich occurs primarly in liver mitochondria. Acetoacetate, D-3-hydroxybutyrate and acetone are sometimes referred to as ketone bodies.

The brain, under normal circumstances, uses only glucose as its energy source, but during starvation ketone bodies become the brain's major fuel source.