Glucoregulation and hormonal changes during prolonged exercise in boys and girls

A group of 17 children, 8.5–11 years old, performed a 60-min cycle exercise at 60% of maximal oxygen uptake (VO_2max) 2 h after a standardized breakfast. They were 10 young boys (pubertal stage =1) and 7 young girls (pubertal stage 2) of similarVO_2max (respective values were 48.5 ml min^–1 kg^–1, SEM 1.8; 42.1 ml min^–1 kg^–1, SEM 2.4). Blood samples of 5 ml were withdrawn by heparinized catheter, the subjects being in a supine position, 30 min before the test, then after 0, 15, 30 and 60 min of exercise and following 30 min recovery. Haematocrit was immediately measured. Thereafter plasma was analysed for glucose, non-esterified fatty acid, glycerol, catecholamine (noradrenaline, adrenaline), insulin and glucagon concentrations. This study showed two main results. First, the onset of exercise induced a significant glucose decrease (of about 11,4%) in all the children. Secondly, both the glycaemic and the hormonal responses were obviously different according to the sex. In boys only, the initial glucose drop was significantly correlated to the pre-exercise insulin values. Whatever the time, the glycaemic levels and the catecholamine responses were lower in girls than in boys, whereas the insulin values remained higher. However, none of these two hormonal parameters seemed to be really responsible for the lower glucose values in girls. On the one hand, the great individual variability of noradrenaline and adrenaline and differences in their relative intensity at the end of the exercise between boys and girls might contribute to the lower catecholamine levels in girls. On the other hand, the lack of a significant relationship in girls between the glucose decrease after exercise and the pre-exercise insulin values might be explained by a relative insulin insensitivity concomitant with the earlier growth spurt in girls, as demonstrated in subjects at rest by other authors. Finally the mechanisms of all these gender differences remain to be clarified and might be accounted for by a different maturation level in boys and girls.