Metabolic and hormonal response to short term fasting after endurance training in the rat

The metabolic and hormonal response to short term fasting was studied after endurance exercise training. Rats were kept running on a motor driven rodent treadmill 5 days/wk for periods up to 1 h/day for 6 wk. Trained and untrained rats were then fasted for 24 h and 48 h. Liver and muscle glycogen, blood glucose, lactate, beta OH butyrate, glycerol, plasma insulin, testosterone and corticosterone were measured in fed and fasted trained and untrained rats. 48 h fasted trained rats show a lower level of blood lactate (1.08 +/- 0.05 vs 1.33 +/- 0.08 mmol/l-1 of blood glycerol (1 +/- 0.11 vs 0.84 +/- 0.08 mmol/l-1), and of muscle glycogen. There is a significant increase in plasma corticosterone in 48 h fasted trained rats from fed values. Plasma testosterone decreases during fasting, the values are higher in trained rats. Plasma insulin decreases during fasting without any difference between the two groups. These results show higher lipolysis, and decreased glycogenolysis in trained animals during 48 h fasting. The difference between the groups in steroid hormone response could reduce neoglucogenesis and muscle proteolysis in trained animals.