When your body runs low on glucose, it turns to ketone bodies for fuel. These molecules, produced in the liver from fatty acids, become crucial energy sources for your brain and muscles during fasting or low-carb diets.

Ketone body metabolism is a key part of lipid breakdown. Understanding how your body makes and uses ketones helps explain how you can survive without food for days and why some diets work the way they do.

Ketone bodies: Definition and production

Molecular characteristics and synthesis conditions

• Ketone bodies comprise water-soluble molecules produced by the liver during low glucose availability (acetoacetate, β-hydroxybutyrate, and acetone)
• Production triggers when blood glucose levels drop (fasting, prolonged exercise, low carbohydrate intake)
• Synthesis occurs in liver mitochondria using acetyl-CoA derived from fatty acid oxidation as the primary substrate
• Rate of production inversely proportional to insulin levels and directly proportional to glucagon and other counter-regulatory hormones

Physiological role and adaptation

• Ketone bodies serve as an alternative fuel source for extrahepatic tissues, particularly the brain, during glucose scarcity
• Transition from glucose to ketone body utilization as primary fuel source typically occurs after 3-4 days of fasting or severe carbohydrate restriction
• Brain can adapt to use ketone bodies for up to 70% of its energy needs during prolonged fasting
• Heart and skeletal muscle preferentially use ketone bodies over fatty acids when both are available

Ketogenesis: Process and regulation

Biochemical pathway

• Ketogenesis begins with condensation of two acetyl-CoA molecules to form acetoacetyl-CoA, catalyzed by thiolase
• HMG-CoA synthase adds another acetyl-CoA to acetoacetyl-CoA, forming β-hydroxy-β-methylglutaryl-CoA (HMG-CoA)
• HMG-CoA lyase cleaves HMG-CoA to produce acetoacetate and acetyl-CoA
• Acetoacetate can be reduced to β-hydroxybutyrate by β-hydroxybutyrate dehydrogenase or spontaneously decarboxylate to form acetone
• Rate-limiting enzyme HMG-CoA synthase regulated by succinylation and desuccinylation

Regulatory mechanisms

• Hormonal regulation involves insulin suppression of ketogenesis by inhibiting lipolysis and promoting glucose utilization
• Glucagon stimulates ketogenesis by promoting lipolysis and increasing fatty acid oxidation
• Carnitine palmitoyltransferase I (CPT I) system regulates fatty acid entry into mitochondria, indirectly controlling ketone body production
• Ketone body levels can be measured in blood and urine as diagnostic markers for various metabolic states (fasting, ketogenic diet adherence, diabetic ketoacidosis)

Ketone bodies: Utilization as energy

Metabolic pathway

• Ketone bodies transported in bloodstream to extrahepatic tissues for oxidation and energy production
• β-hydroxybutyrate and acetoacetate serve as primary ketone bodies used for energy production
• Oxidation process involves:
  • Conversion of β-hydroxybutyrate back to acetoacetate by β-hydroxybutyrate dehydrogenase
  • Activation of acetoacetate to acetoacetyl-CoA by succinyl-CoA:acetoacetate CoA transferase
  • Thiolase-catalyzed cleavage of acetoacetyl-CoA to two acetyl-CoA molecules
• Resulting acetyl-CoA enters citric acid cycle for complete oxidation and energy production

Efficiency and adaptations

• Ketone body utilization proves more efficient than glucose in terms of ATP production per oxygen molecule consumed
• Brain adapts to use ketone bodies for up to 70% of its energy needs during prolonged fasting (3-4 days)
• Heart and skeletal muscle preferentially use ketone bodies over fatty acids when both are available
• Transition from glucose to ketone body utilization as primary fuel source typically occurs after 3-4 days of fasting or severe carbohydrate restriction

Ketone body metabolism: Clinical significance

Physiological and pathological states

• Physiological ketosis occurs during fasting, prolonged exercise, and adherence to ketogenic diets (generally considered safe)
• Pathological ketoacidosis can occur in uncontrolled diabetes mellitus, leading to life-threatening decrease in blood pH
• Impaired ketone body metabolism associates with certain inborn errors of metabolism (HMG-CoA lyase deficiency)

Therapeutic applications and research

• Ketogenic diet, characterized by high fat and low carbohydrate intake, has therapeutic applications in:
  • Epilepsy management, particularly in drug-resistant cases
  • Potential neuroprotective effects in neurodegenerative disorders (Alzheimer's disease, Parkinson's disease)
• Ketone bodies implicated in regulation of gene expression and cellular signaling pathways, suggesting broader physiological roles beyond energy metabolism
• Recent research explores potential benefits of exogenous ketone supplementation in various clinical contexts (cognitive function enhancement, athletic performance improvement)
• Ketone body levels measured in blood and urine serve as diagnostic markers for various metabolic states (fasting, ketogenic diet adherence, diabetic ketoacidosis)