University of Virginia researchers recently compared the effects of several types of diets on growth hormone (GH-1) release during exercise. The study involved seven subjects, with an average age of 24, who consumed one of three diets:
1) high fat (70% fat, 10% carb and 20% protein)
2) high carb (70% carb, 10% fat and 20% protein), or
3) isocaloric (45% carb, 35% fat and 20% protein).
During the exercise sessions, no differences were noted in OH release among the three groups. Measurements taken during rest, however, did show that general OH output was lower in subjects eating the high-fat diet than in those consuming the high-carb regimen.
The authors of this study interpret these results as suggesting that "exercise is a robust stimulus of OH release, and overrides the effect of diet observed at rest, or the 13-hour interval between the last meal and the exercise session was sufficient to eliminate the effects of the diet." Why does a high-carb diet produce greater OH release than a high-fat diet? Free fatty acids are known to inhibit OH release. And although high carb intake increases blood glucose, thus inhibiting OH release, it also stimulates insulin release. The insulin then lowers blood-glucose levels, providing a direct stimulus to increased OH release.
From a practical point of view, if you want to maximize OH release during exercise, it's best not to eat anything before you train. The presence of either fatty acids or glucose in the blood will depress OH response during exercise.