Prediction of glucose oxidation rate during exercise

Recently, the relationship between percentage maximal heart rate vs. glucose oxidation rate has been proposed as a tool for estimating glucose oxidation rate during exercise in insulin-dependent diabetic patients. The reliability of this relationship and its applicability to long-term exercise is evaluated. Eight healthy volunteers performed a graded cycloergometric exercise (10-min steps at 30, 50, 70, 90 % of ventilatory threshold). Heart rate and glucose oxidation rate (by indirect calorimetry) were measured during the last 5 min of each step. Volunteers underwent then three 1-hour constant intensity rides at 40, 60, 80 % of ventilatory threshold. Heart rate was recorded continuously; glucose oxidation rate was determined over 15-min periods. The percentage maximal heart rate vs. glucose oxidation rate relationship obtained from the graded exercise matched that previously reported. Independently of intensity, glucose oxidation rates observed during the 1-hr rides were linearly related to the estimated ones (R(2)>0.96, p<0.001), being, however, progressively over-estimated in subsequent exercise periods. The proposed correction yields values close to the identity line (y=1.001 x x; R2=0.974, p<0.001), the difference between observed and "corrected" values amounting to 0.23+/-2.17 mg x min (-1) x kg (-1). In conclusion, glucose oxidation rate can be estimated from heart rate, once proper correction factors are applied for long duration exercises.