Ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone) are water-soluble molecules produced by the liver from fatty acids during periods of low glucose availability (fasting, carbohydrate restriction, prolonged exercise). They serve as an alternative fuel for the brain, heart, and skeletal muscle.
During fasting or carbohydrate restriction, insulin levels drop and glucagon rises, activating hormone-sensitive lipase to release fatty acids from adipose tissue. These fatty acids undergo β-oxidation in hepatic mitochondria, producing acetyl-CoA. When acetyl-CoA exceeds the capacity of the TCA cycle (due to low glucose/oxaloacetate), excess acetyl-CoA is converted via HMGCS2 and HMG-CoA lyase into acetoacetate, which is then reduced to β-hydroxybutyrate. Ketone bodies circulate to peripheral tissues where they are converted back to acetyl-CoA for ATP production. The brain can derive up to 70% of its energy from ketones during prolonged fasting. Ketones also act as signaling molecules: activating GPR109A (anti-inflammatory), inhibiting NLRP3 inflammasome, increasing BDNF, and inducing mitochondrial biogenesis.
Ketone production represents metabolic flexibility and is a marker of successful fat adaptation. Ketogenic diets and fasting increase ketone levels, providing neuroprotection, reducing inflammation, improving insulin sensitivity, and supporting mitochondrial function. Elevated ketones (β-hydroxybutyrate 0.5-3 mM) are associated with improved cognitive function, reduced seizures (epilepsy), neuroprotection in Alzheimer's and Parkinson's, and improved metabolic markers. Short-chain fatty acids (acetate, propionate, butyrate) signal for β-oxidation and ketogenesis.
- Three ketone bodies: acetoacetate, β-hydroxybutyrate (primary), acetone (volatile)
- Produced in liver from fatty acid β-oxidation when glucose is low
- Brain can derive up to 70% of energy from ketones during fasting
- β-hydroxybutyrate 0.5-3 mM indicates nutritional ketosis
- Activate GPR109A receptor for anti-inflammatory effects
- Inhibit NLRP3 inflammasome reducing inflammation
- Increase BDNF expression supporting neuroplasticity
- SCFAs from gut signal colonocytes to use β-oxidation promoting ketone use
- β-hydroxybutyrate — primary circulating ketone body, most abundant
- acetoacetate — first ketone body produced, converted to β-hydroxybutyrate
- fatty acid oxidation — ketone bodies are produced from β-oxidation of fatty acids
- fasting — fasting stimulates ketone production by lowering insulin and increasing lipolysis
- ketogenic diet — carbohydrate restriction drives ketone production
- insulin — low insulin is required for ketogenesis
- glucagon — glucagon stimulates lipolysis and ketone production
- liver — liver is the primary site of ketone body synthesis
- brain — ketones provide up to 70% of brain energy during fasting
- BDNF — ketones increase BDNF expression supporting neuroplasticity
- NLRP3 inflammasome — β-hydroxybutyrate inhibits NLRP3 inflammasome reducing inflammation
- GPR109A — ketones activate this receptor for anti-inflammatory signaling
- mitochondrial biogenesis — ketones promote mitochondrial biogenesis via PGC-1α
- butyrate — butyrate (SCFA) signals colonocytes to use β-oxidation and ketones
- acetate — acetate from gut fermentation can contribute to ketone production
- epilepsy — ketogenic diet reduces seizures through ketone-mediated mechanisms
- Alzheimer's Disease — ketones provide alternative brain fuel when glucose metabolism impaired
- metabolic flexibility — ability to produce and use ketones indicates metabolic flexibility
- adipose tissue — fatty acids from adipose tissue are converted to ketones in liver
- ATP — ketones are converted to acetyl-CoA for ATP production in TCA cycle