Influence of physical training on the fuel-hormone response to prolonged low intensity exercise

The effects of physical training on the fuel-hormone response to prolonged (3 hr), low intensity cycle ergometer exercise (40% maximal aerobic power) which in the untrained state fails to produce a rise in blood lactate, was examined in six healthy male subjects. The training program consisted of one hour cycle ergometer exercise performed 4 times weekly for 6 weeks and resulted in a 19% increase in maximal aerobic power. Prior to training, prolonged low intensity exercise resulted in a 20% decline in plasma glucose, a 2.5-fold rise in plasma free fatty acids (FFA), a 7-fold rise in plasma epinephrine, a 3-fold elevation in plasma norepinephrine, and a 2.5-fold rise in plasma glucagon. Following training, the exercised-induced decline in glucose was 60% less than before training, the elevations in plasma FFA and norepinephrine were respectively, 45% and 90% less than before training and no significant increment in plasma norepinephrine and glucagon was observed. Training also blunted the exercise-induced elevations in circulating ketones and growth hormone and resulted in a lower respiratory exchange ratio during exercise. The data indicate that training markedly diminishes the fuel-hormone perturbations associated with low intensity exercise and in the face of a lessened increment in plasma FFA results in a greater utilization of fat and less dependence on carbohydrate during the exercise.