Determination of glucose turnover and glucose oxidation rates in man with stable isotope tracers.

Determination of the turnover rates of glucose gives a more dynamic view of carbohydrate metabolism. Using 2H- or 13C-labelled glucose, stable isotope methods have been established which are free of risk for volunteers or patients and are in accordance with the legal requirements for radiation protection. The aim of the present study was to determine the main parameters of glucose turnover in vivo by using two stable-isotope-labelled glucose molecules, [6,6-2H]glucose and [U-13C]glucose. Under steady state conditions, the following parameters were analysed: glucose turnover rate, glucose oxidation rate, recycling of glucose, hepatic glucose production rate, and glucose clearance. In healthy volunteers the following data were obtained for the glucose turnover rate: 2.42 +/- 0.11 mg/kg x min, glucose oxidation rate 1.34 +/- 0.08 mg/kg x min, glucose clearance 3.04 +/- 0.17 ml/kg x min, and glucose recycling 24.7% (about 0.6 mg/kg x min). Under conditions of the euglycaemic-hyperinsulinaemic clamp (insulin levels about 80 mU/l) the glucose turnover rate increased to 9-10 mg/kg x min, and the hepatic glucose production rate was totally suppressed. Under these conditions identical glucose turnover rates were measured by rate of appearance Ra and euglycaemic-hyperinsulinaemic clamp. These data clearly demonstrate that by using differentially labelled glucose molecules at least five parameters of glucose metabolism may be determined in vivo. High insulin levels (70-80 mU/l) stimulate glucose turnover rate by 300-400%, and the glucose infusion rate agrees well with the rate of appearance (Ra) of glucose, determined with [6,6-2H]glucose. Thus, this glucose tracer provides relevant and presumably accurate data under basal and under hyperinsulinaemic conditions.