Ketogenesis

Ketogenesis is the hepatic mitochondrial biosynthesis of ketone bodies — beta-hydroxybutyrate (BHB), acetoacetate (AcAc), and acetone — from acetyl-CoA derived from fatty acid beta-oxidation. It provides an alternative energy substrate to glucose during states of carbohydrate scarcity, particularly for the central nervous system, which cannot directly oxidize long-chain fatty acids but readily uses ketones.

Biochemistry

Ketogenesis proceeds in four enzymatic steps within liver mitochondria. Two molecules of acetyl-CoA (the product of fatty acid beta-oxidation) condense via thiolase (ACAT1) to form acetoacetyl-CoA. A third acetyl-CoA is added by HMG-CoA synthase 2 (HMGCS2, the ketogenic isoform, distinct from the cholesterogenic cytosolic isoform) to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). HMG-CoA lyase cleaves HMG-CoA into free acetoacetate and acetyl-CoA. Acetoacetate is then reduced by beta-hydroxybutyrate dehydrogenase (BDH1) to D-beta-hydroxybutyrate, or spontaneously decarboxylates to acetone (which is largely exhaled).

Triggers

Ketogenic flux is driven by three conditions that converge on the liver mitochondrion: (1) elevated fatty acid delivery from adipose lipolysis, which requires low insulin; (2) high beta-oxidation flux, which requires activated carnitine palmitoyltransferase 1 (CPT1), disinhibited by low malonyl-CoA; and (3) limited oxaloacetate, which shunts acetyl-CoA away from TCA entry toward ketogenesis. The physiological states meeting all three are: fasting (>16-24 hours), very-low-carbohydrate diet (ketogenic diet), prolonged exercise, lactation, and pathologically, insulin-deficient diabetes (diabetic ketoacidosis, DKA) and alcoholic ketosis.

Liver-peripheral asymmetry

The liver synthesizes ketones but cannot oxidize them, because hepatocytes lack SCOT (succinyl-CoA:3-oxoacid CoA-transferase, OXCT1), the enzyme that reactivates acetoacetate for TCA entry in peripheral tissues. This asymmetry ensures that hepatic ketone production is directed outward to tissues that can use them — brain, skeletal muscle, heart, and renal cortex — rather than futilely consumed at the production site.

Ketone body utilization

In peripheral tissues, BHB is oxidized back to acetoacetate by BDH1, activated by SCOT to acetoacetyl-CoA, cleaved by thiolase to two acetyl-CoA, and oxidized in the TCA cycle. During prolonged fasting or ketosis, the brain derives 60-70% of its energy from ketones, reducing its glucose requirement from approximately 120 g/day to 30-40 g/day — one of the key adaptations that permits survival during extended starvation.

Nutritional vs. pathological ketosis

Nutritional ketosis produces blood BHB concentrations of approximately 0.5-5 mmol/L and is associated with maintained blood pH and normal insulin signaling. Diabetic ketoacidosis produces BHB concentrations of 5-25 mmol/L combined with acidemia, driven by absolute insulin deficiency that removes the normal brake on lipolysis and hepatic ketogenesis. The distinction matters clinically: nutritional ketosis is a physiological state; DKA is a medical emergency.

Exogenous ketones

Ketone esters (R-BHB-butanediol monoester) and ketone salts (BHB-sodium, BHB-potassium, BHB-calcium) are commercially available supplements that raise blood BHB without requiring endogenous ketogenesis. Evidence for their application beyond specific athletic or neurological use cases (metabolic epilepsy, possibly Alzheimer's research contexts) is limited.