beta-hydroxybutyrate

**Synthesis (liver mitochondria):** During low insulin/high glucagon states, adipose lipolysis releases free fatty acids. These undergo beta-oxidation to acetyl-CoA. Because oxaloacetate is diverted to gluconeogenesis, acetyl-CoA is shunted to ketogenesis: 2 acetyl-CoA -> acetoacetyl-CoA (thiolase) -> HMG-CoA (HMG-CoA synthase, the RATE-LIMITING step) -> acetoacetate (HMG-CoA lyase). Acetoacetate is reduced to beta-hydroxybutyrate by beta-hydroxybutyrate dehydrogenase (NADH -> NAD+). Spontaneous decarboxylation of acetoacetate yields acetone (exhaled, fruity breath odor).

**BHB:acetoacetate ratio:** Normally ~1:1. The ratio reflects the mitochondrial NADH/NAD+ redox state. In DKA the ratio rises to ~3:1 up to 10:1. In alcoholic ketoacidosis (high NADH from ethanol metabolism), the ratio can be even higher (~7:1 or more), and the ketones can be greatly underestimated by nitroprusside testing.

**Utilization (extrahepatic tissues):** BHB is oxidized back to acetoacetate (BHB dehydrogenase), then converted to acetoacetyl-CoA by thiophorase (succinyl-CoA:acetoacetate CoA transferase), then cleaved to 2 acetyl-CoA, which enters the TCA cycle. The liver lacks thiophorase, so it produces but cannot use ketones. RBCs cannot use ketones (no mitochondria). The brain adapts to use ketones after ~3 days of fasting, which spares muscle protein breakdown for gluconeogenesis.

**Clinical significance:** - **DKA** (type 1 DM, insulin deficiency): high-anion-gap metabolic acidosis, hyperglycemia, Kussmaul respirations, fruity breath, abdominal pain, dehydration, and total-body K+ depletion (serum K+ may be normal or high due to acidosis and insulin lack). Treatment: IV fluids, insulin, K+ repletion (check K+ before insulin), and treatment of the precipitant (infection, missed insulin, MI). - **Alcoholic ketoacidosis:** chronic alcohol use plus poor intake and vomiting. Typically low or normal glucose and a high-AG acidosis. Treat with IV dextrose, thiamine, and fluids. - **Starvation ketosis / ketogenic diet / SGLT2 inhibitor-associated euglycemic DKA:** euglycemic DKA is a classic pharmacology association. - **Diagnostic pitfall:** The nitroprusside reaction (urine dipstick, Acetest) detects acetoacetate (and acetone weakly), NOT BHB. Early in DKA, predominant BHB can give a falsely low or negative result. With treatment, BHB is converted to acetoacetate, so the test may paradoxically look worse while the patient improves. Direct serum BHB measurement is preferred for diagnosis and monitoring. A level >3 mmol/L is generally consistent with DKA. - **Hypoglycemia without ketones:** suggests a fatty acid oxidation defect (e.g., MCAD deficiency) or hyperinsulinism, because ketogenesis is impaired or suppressed.

**Pearls:** Ketone bodies are acidic (acetoacetate and BHB are organic acids), which is why accumulation lowers bicarbonate and raises the anion gap. HMG-CoA synthase (mitochondrial) is the rate-limiting enzyme of ketogenesis; the cytosolic isoform is for cholesterol synthesis (statins inhibit HMG-CoA reductase). Key mnemonic: the liver makes ketones but can't use them.