When and how are ketone bodies produced and utilized?

Ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone) are produced in liver mitochondria when acetyl-CoA from fatty acid oxidation exceeds TCA cycle capacity, primarily during fasting, starvation, or uncontrolled diabetes. Production (ketogenesis): two acetyl-CoA form acetoacetyl-CoA, then HMG-CoA, cleaved to acetoacetate and acetyl-CoA. Acetoacetate can be reduced to beta-hydroxybutyrate or spontaneously decarboxylated to acetone. Utilization (ketolysis): in extrahepatic tissues (especially brain, heart, muscle), ketone bodies are converted back to acetyl-CoA for TCA cycle oxidation. During prolonged fasting, ketone bodies become the brain's primary fuel, sparing glucose and reducing protein breakdown.