Plasma C5 Glucose–to–2H2O Ratio Does Not Provide an Accurate Assessment of Gluconeogenesis During Hyperinsulinemic-Euglycemic Clamps in Either Nondiabetic or Diabetic Humans

OBJECTIVE— Measurement of plasma C2 glucose enrichment is cumbersome. Therefore, the plasma C5 glucose–to–2H2O rather than the plasma C5-to-C2 glucose ratio commonly has been used to measure gluconeogenesis and glycogenolysis during hyperinsulinemic-euglycemic clamps. The validity of this approach is unknown. RESEARCH DESIGN AND METHODS— Ten nondiabetic and 10 diabetic subjects ingested 2H2O the evening before study. The following morning, insulin was infused at a rate of 0.6 mU · kg−1 · min−1 and glucose was clamped at ∼5.3 mmol/l for 5 h. Plasma C5 glucose, C2 glucose, and 2H2O enrichments were measured hourly from 2 h onward. RESULTS— Plasma C2 glucose and plasma 2H2O enrichment were equal in both groups before the clamp, resulting in equivalent estimates of gluconeogenesis and glycogenolysis. In contrast, plasma C2 glucose and plasma C5 glucose enrichments fell throughout the clamp, whereas plasma 2H2O enrichment remained unchanged. Since the C5 glucose concentration and, hence, the C5 glucose–to–2H2O ratio is influenced by both gluconeogenesis and glucose clearance, whereas the C5-to-C2 glucose ratio is only influenced by gluconeogenesis, the C5 glucose–to–2H2O ratio overestimated ( P < 0.01) gluconeogenesis during the clamp. This resulted in biologically implausible negative (i.e., calculated rates of gluconeogenesis exceeding total endogenous glucose production) rates of glycogenolysis in both the nondiabetic and diabetic subjects. CONCLUSIONS— Plasma C5 glucose–to–2H2O ratio does not provide an accurate assessment of gluconeogenesis in nondiabetic or diabetic subjects during a traditional (i.e., 2–3 h) hyperinsulinemic-euglycemic clamp. The conclusions of studies that have used this approach need to be reevaluated.