Inactivity and excessive and inappropriate caloric intake stimulate development of visceral obesity that is associated with increased risks of type 2 (non-insulin-dependent) diabetes, hypertension, and coronary heart disease (CHD). Both exercise and dietary restriction have been successfully used as a means of body fat loss. In addition, exercise, with or without fat loss, can reduce risks of diabetes, hypertension, and CHD but its effects have not been consistent. In particular, there is little consensus regarding the effects of exercise intensity on either insulin sensitivity, body fat loss, or risks of hypertension and CHD. Our premise is that if exercise is to be used effectively to prevent or reduce different health risks or to be used as a tool in mechanistic analyses of hormone action, a better understanding of dose-dependent effects of exercise intensity is needed. Our pilot study, initiated two years ago, tested the hypothesis that exercise training at high walking intensities could increase basal pulsatile growth hormone (GH) secretion. The preliminary results supported this hypothesis, but in addition revealed that increases in training intensity attenuate increases in insulin sensitivity (assessed with an indirect measure of insulin sensitivity) that usually accompany exercise training. The goals of the present study are to (1) replicate in a larger group of 18 subjects the observed attenuation of insulin sensitivity at high training intensities with the more specific test of insulin sensitivity, Bergman's frequently sampled intravenous glucose tolerance test (FSIVGTT); (2) determine whether the effect of exercise intensity on insulin sensitivity is a consequence of, or independent of, changes in body fat mass and distribution; (3) replicate in a larger group of 18 subjects the observed increases in pulsatile GH secretion between 15 and 30 weeks of exercise training at high training intensities, and (4) collect some preliminary information on the effects of exercise intensity on plasma lipids (HDL-cholesterol , LDL-cholesterol, and triglycerides) which also constitute a risk of developing CHD.
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