Ketone Body Metabolism

Ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) are water-soluble fuels made by the liver from acetyl-CoA when fatty acid oxidation is high and carbohydrate is scarce. They are exported to the brain, heart, and skeletal muscle, where they are reconverted to acetyl-CoA for energy, allowing the body to spare glucose during fasting, prolonged exercise, and the neonatal period.

Definition

Ketone body metabolism comprises ketogenesis, the hepatic mitochondrial pathway that condenses acetyl-CoA into acetoacetate and beta-hydroxybutyrate (with HMG-CoA synthase as the controlling enzyme), and ketolysis, the extrahepatic pathway that reactivates these bodies to acetyl-CoA for oxidation in the citric acid cycle.

Scope

The entry covers hepatic ketogenesis from acetyl-CoA, peripheral ketolysis, the hormonal and metabolic conditions that turn ketone production on, and the emerging role of beta-hydroxybutyrate as a signaling molecule. It distinguishes physiological ketosis from pathological ketoacidosis at a conceptual level. It is a biochemical reference and does not provide guidance on managing ketoacidosis or ketogenic diets.

Core questions

• Under what metabolic conditions does the liver produce ketone bodies?
• What is the controlling enzyme of ketogenesis?
• Why can the liver make but not use ketone bodies?
• How do peripheral tissues convert ketone bodies back to usable fuel?

Key concepts

• Acetoacetate, beta-hydroxybutyrate, and acetone
• HMG-CoA synthase (mitochondrial, rate-controlling for ketogenesis)
• Ketogenesis in liver mitochondria
• Ketolysis and the role of SCOT (thiophorase)
• Glucose sparing during fasting
• Physiological ketosis versus diabetic ketoacidosis
• Beta-hydroxybutyrate signaling

Key theories

Ketogenesis as overflow of hepatic fatty acid oxidation

When fasting drives high rates of hepatic beta-oxidation and oxaloacetate is diverted to gluconeogenesis, acetyl-CoA accumulates beyond citric-acid-cycle capacity and is rerouted into ketone bodies, so ketogenesis tracks the same regulatory signals (low malonyl-CoA, high fatty acid flux) that govern oxidation.

Beta-hydroxybutyrate as a signaling metabolite

Beyond its role as fuel, beta-hydroxybutyrate acts as an endogenous inhibitor of class I histone deacetylases and as a ligand for certain receptors, linking the fasted metabolic state to changes in gene expression and signaling.

Mechanisms

During fasting or carbohydrate restriction, low insulin and high glucagon promote adipose lipolysis and hepatic fatty acid oxidation, raising mitochondrial acetyl-CoA. Because oxaloacetate is consumed by gluconeogenesis, acetyl-CoA cannot all enter the citric acid cycle, so two acetyl-CoA condense to acetoacetyl-CoA, which mitochondrial HMG-CoA synthase (the controlling step) converts to HMG-CoA; HMG-CoA lyase then releases acetoacetate, which is reversibly reduced to beta-hydroxybutyrate or spontaneously decarboxylated to acetone. These ketone bodies enter the blood and are taken up by extrahepatic tissues, where beta-hydroxybutyrate is reoxidized to acetoacetate, activated to acetoacetyl-CoA by succinyl-CoA:3-ketoacid CoA transferase (SCOT), and split into two acetyl-CoA for oxidation. The liver cannot use ketone bodies for energy because it lacks SCOT, ensuring it remains a net exporter.

Clinical relevance

Ketone body metabolism explains how the brain is fueled during prolonged fasting, the basis of nutritional ketosis, and, at the pathological extreme, the uncontrolled ketone production of diabetic ketoacidosis when insulin is absent. This entry presents normal physiology and biochemistry for reference and education and is not a basis for diagnosing or managing ketoacidosis or for prescribing dietary regimens.

History

Ketone bodies were long regarded mainly as toxic byproducts seen in diabetes, but mid-twentieth-century physiology, including the work of Krebs and of Williamson, established them as normal and important respiratory fuels, notably for the brain during starvation as shown by the Cahill fasting studies. McGarry and Foster integrated ketogenesis into the broader regulatory scheme of hepatic fatty acid oxidation, and more recent work has revealed beta-hydroxybutyrate's roles as a signaling molecule.

Key figures

• J. Denis McGarry
• Daniel Foster
• Dennis Williamson
• Hans Krebs

Seminal works

• mcgarry-foster-1980
• robinson-williamson-1980
• puchalska-crawford-2017

Frequently asked questions

Why can the liver make ketone bodies but not use them?

The liver lacks the enzyme succinyl-CoA:3-ketoacid CoA transferase (SCOT) needed to reactivate acetoacetate to acetoacetyl-CoA, so it exports ketone bodies to other tissues rather than oxidizing them itself.

Is making ketone bodies normal or a sign of disease?

Modest ketogenesis is a normal physiological response to fasting, prolonged exercise, or low carbohydrate intake; only when ketone production is uncontrolled, as in insulin deficiency, can it progress to harmful ketoacidosis.