Hormone and energy substrate changes during prolonged exercise in the heat

Eleven trained men (aged 34.5 +/- 2 yrs) were studied during a 16.1 km run in the heat (Ta = 30.2 degrees C). Fasting blood samples were taken prior to the run and at 6.4, 12.9, and 16.1 km, and 3 h recovery. Serum or plasma glucose, insulin, glucagon, glycerol, and catecholamines were measured. Mean values were: exercise intensity, 80% of VO2max; final rectal temperature, 39.9 degrees C; and weight loss, 4.0%. Glucose increased 61% by 6.4 km, then decreased significantly by 16.1 km. Glycerol increased by 415% at 6.4 km, and continued to increase throughout the run. Epinephrine increased progressively during the run, but norepinephrine increased at 6.4 km, and did not change further during the exercise. Insulin increased slightly at 6.4 km, then decreased significantly from 6.4-16.1 km. Glucagon increased from 6.4-12.9 km and remained elevated at 3 h recovery. Hormone and substrate measurements obtained only before and after prolonged exercise may not reflect changes that occur during the course of the exercise. The observed insulin-glucagon relationships vary from previous findings in nontrained subjects at lower exercise intensities.