ketone body

Ketogenesis occurs in hepatic mitochondria when high rates of fatty acid oxidation generate excess acetyl-CoA that exceeds the oxidative capacity of the TCA cycle, particularly when oxaloacetate is diverted to gluconeogenesis (as in prolonged fasting or insulin deficiency). Acetyl-CoA molecules condense via HMG-CoA synthase to form HMG-CoA, which is cleaved to acetoacetate—the first ketone body. Acetoacetate can be reduced to β-hydroxybutyrate (via β-hydroxybutyrate dehydrogenase) or spontaneously decarboxylate to acetone (responsible for the characteristic fruity breath odor in DKA).

Ketone bodies are released into circulation and taken up by peripheral tissues (brain, muscle, heart) that possess the enzyme succinyl-CoA:3-ketoacid CoA transferase (SCOT/thiophorase) to reconvert them to acetyl-CoA for entry into the TCA cycle. Notably, the liver lacks this enzyme, so it cannot utilize the ketones it produces—this is a key exam point.

Clinically, ketone body production increases in: - Prolonged fasting/starvation (adaptive, glucose-sparing mechanism for brain) - Diabetic ketoacidosis (DKA) in Type 1 diabetes—absolute insulin deficiency leads to unchecked lipolysis and ketogenesis, causing anion gap metabolic acidosis - Alcoholic ketoacidosis—chronic alcohol use alters NADH/NAD+ ratio, favoring β-hydroxybutyrate production - Low-carbohydrate/ketogenic diets

In DKA, elevated ketones cause an anion gap metabolic acidosis (measured by serum bicarbonate, blood pH, and anion gap). Urine dipstick tests primarily detect acetoacetate (nitroprusside reaction), which can underestimate total ketosis if β-hydroxybutyrate predominates (common early in DKA due to altered redox state). Serum β-hydroxybutyrate is the preferred lab test for monitoring DKA.

After approximately 3 days of fasting, the brain shifts from using glucose almost exclusively to deriving up to 60-70% of its energy from ketone bodies, sparing muscle protein from being broken down for gluconeogenesis.