A 35-year-old male athlete undergoes a 2-hour endurance run and recovers normally with blood lactate returning to baseline within 90 minutes. A 35-year-old male with cirrhosis and portal hypertension develops persistent lactate elevation (6 mmol/L) 4 hours after mild exertion, which does not normalize despite rest. Which metabolic feature best discriminates the lactate clearance capacity in these two individuals?