Hormonal responses associated with the nadir in blood glucose during graded cycling exercise

The response of glucoregulatory hormones associated with the nadir in serum glucose (i.e., glucose threshold) was examined during graded exercise to exhaustion in 22 physically active male participants. A comparison was also made between the glucose, lactate, and ventilatory thresholds. The mean (±SD) age, height, and body mass of the participants was 24.5 (±4.0) years, 182.0 (±9.1) cm, and 84.8 (±17.4) kg, respectively. Blood samples were collected at rest and during the final minute of each power output of a graded exercise test. The nadir in blood glucose concentration occurred at the same time as the lowest concentration of plasma glucagon, and both were significantly elevated at maximal exercise. Insulin response showed a significant decrease from rest but was maintained prior to the glucose nadir followed by a significant decrease. Cortisol was significantly elevated at maximal exercise intensity only. There was no difference in the power output and oxygen consumption (VO2) at the glucose and lactate thresholds but both occurred at one power output prior to the ventilatory threshold. In conclusion, a nadir in serum glucose occurred in a predictable fashion in relation to other measures of anaerobic threshold. The nadir in blood glucose occurred prior to an increase in plasma glucagon concentration and a decrease in circulating insulin concentration during the re-establishment of euglycemia during graded exercise.