Clinical meaning
Understanding the detailed biochemistry of ketogenesis and osmotic diuresis is essential for the NP to anticipate clinical findings, guide management decisions, and recognize treatment complications.
KETOGENESIS IN DKA: In the absence of insulin, hormone-sensitive lipase in adipose tissue is uninhibited, releasing massive quantities of free fatty acids (FFAs) into the bloodstream. FFAs are transported to hepatic mitochondria where they undergo beta-oxidation, producing acetyl-CoA. Normally, acetyl-CoA enters the citric acid cycle (TCA cycle), but in DKA, the TCA cycle is overwhelmed because oxaloacetate is diverted to gluconeogenesis (insulin deficiency de-represses PEPCK). The excess acetyl-CoA is shunted into the ketogenic pathway: 1. Two acetyl-CoA molecules condense to form acetoacetyl-CoA 2. HMG-CoA synthase (rate-limiting enzyme of ketogenesis, upregulated by glucagon and downregulated by insulin) converts acetoacetyl-CoA to HMG-CoA 3. HMG-CoA lyase cleaves HMG-CoA to acetoacetate (the first ketone body) 4. Acetoacetate is reversibly reduced to beta-hydroxybutyrate (the predominant ketone body, ratio 3:1 to 10:1 in DKA) or spontaneously decarboxylated to acetone (volatile — exhaled through the lungs, producing fruity breath)
Beta-hydroxybutyrate and acetoacetate are strong organic acids (pKa ~4.7) that dissociate completely at physiological pH, releasing H+ ions that consume bicarbonate buffer, producing high-anion-gap metabolic acidosis.
Why HHS does NOT develop significant ketosis: In type 2 diabetes, portal vein insulin concentrations (even if low systemically) are sufficient to suppress hepatic carnitine palmitoyltransferase I (CPT-I), the enzyme required to transport long-chain FFAs into mitochondria for beta-oxidation. Without FFA entry into mitochondria, ketogenesis cannot proceed. Additionally, the residual insulin in HHS suppresses hormone-sensitive lipase enough to limit FFA release (though not enough to prevent hyperglycemia from hepatic glucose output).