Alterations Of Growth Hormone Levels During a Swimming Training Period as They Are Affected By Sex

Introduction

Human Growth hormone (hGH) during exercise it may be important for potentiating or maintaining blood levels of free fatty acids and glucose in longer durations of exercise. However, hGH is also affected by sex. Higher hGH levels have been documented in females during rest and mild exercise (1). This sex difference has been attributed to a positive influence by oestradiol, since both males and females exposed on oestrogen theraphy have exhibited exaggerated responses (2). Since hGH plays a complementary role in metabolism during exercise, this study attempted to examine the training adaptations of hGH at rest, and how this process is affected by sex.

Methods

Eight competitive swimmers, four males (mean age = 15.8 + 0.8) and four females (mean age = 15.0 + 0.7) were examined during a two month training swimming season, and three weeks after the termination of the training period. Total distance covered was 45.000 meters per week. Height, weight and percent body fat were recorded during the tested period. Venous blood samples were collected at rest, early in the morning during the period of intensive training (Test 1) and under the same conditions three weeks after the training period was terminated (Test 2). The samples were analyzed for GH which was measured by the method of Radioimmuneassay (two site IRMA). The level of significance was set at p < 0. 05.

Results

For both sexes values for height, weight and percent body fat remained relatively constant during the tested period. On the contrary, significant changes were found for GH responses between Test 1 and Test 2 for both males and females, indicating that resting levels of GH responses appear to be diminished when the subjects were free of training. This was found to be independed of sex. More specifically, male GH responses were found significantly lower during Test 2 measurements, as compared to Test 1 measurements (6.64+0.24 vs 12.6+2.75) respectively and this was also true for females (17.7+6.7 vs 24.9+5.4) for Test 2 and Test 1 measurements respectively. Furthermore, it was found that females had higher resting serum GH concentrations, compared to their age and fitness matched males both in Test 1 (24.9+5.4 vs 12.6 + 2.75) respectively and in Test 2 (17.7+6.7 vs 6.64+0.24) for females and males respectively.

Discussion/ Conclusions

These data suggest that regular aerobic exercise affects resting GH levels by enhancing them in both sexes. Subsequently, there was a gender difference upon GH response, which was independent of the influence of exercise training, with the females showing a more pronounced serum GH concentration at rest, either with training or without training, as compared to their age and fitness matched male counterparts.

References:

1. Bunt J. et al. (1986) J. Appl. Physiol. Sex and training differences in human Growth Hormone levels during prolonged exercise. 61:1796-1801.
   
2. Chakmakjian ZH & Bethune JE. (1968) J. Lab. Clin. Med. Study of hGH response to insuline, vasopressin, exercise and estrogen administration. 21(3): 429-437.