Fuel oxidation during exercise in middle-aged men: role of training and glucose disposal

The purpose of this study was to test the hypothesis that carbohydrate (CHO) utilization in middle-aged trained men is increased during hard-intensity exercise and decreased during moderate-intensity exercise in comparison with age-matched sedentary men. We also investigated whether a relationship between CHO utilization and glucose disposal exists. Seven trained cyclists (Tr) and seven age-matched sedentary men (Sed) underwent an intravenous glucose tolerance test after an overnight fast (minimal model method) to determine their glucose disposal; they also performed two 1-h trials on a cycle ergometer below and above their individual ventilatory threshold (VT). Substrate oxidation was evaluated by indirect calorimetry. Hormonal responses were investigated during exercise. Insulin sensitivity (SI) and glucose effectiveness (Sg) were significantly higher in the Tr group than in the Sed group (P < 0.001, P < 0.03). CHO oxidation was significantly higher in the Tr group than in the Sed group when exercise was performed above VT, whereas CHO oxidation was higher in the Sed group when exercise was performed below VT (P < 0.05). Epinephrine (Epi) response during hard-intensity exercise was higher in the Tr group than in the Sed group (P < 0.01). SI was negatively correlated to CHO oxidation in the Tr group (r = -0.743, P < 0.05). Endurance training results in increased CHO utilization during hard-intensity exercise and reduced CHO oxidation during moderate-intensity exercise in middle-aged men. During hard-intensity exercise, the increased CHO utilization in middle-aged trained men is associated with a greater response in Epi and is inversely related with SI.