A 28-year-old woman with Type 1 diabetes presents with severe metabolic acidosis (pH 7.15, HCO₃⁻ 12 mEq/L) and elevated blood lactate (8 mmol/L). Her twin sister, who is non-diabetic, develops lactic acidosis after intense anaerobic exercise with similarly elevated lactate (7.5 mmol/L) but recovers within 2 hours. Which feature best distinguishes the lactate metabolism in diabetic ketoacidosis from exercise-induced lactate elevation?