Gender differences in growth hormone response to exercise before and after rhGH administration and the effect of rhGH on the hormone profile of fit normal adults

Summary Objectives  Exercise is a potent physiological stimulus of GH secretion. We hypothesized that exogenous recombinant human growth hormone (rhGH) administration through an increase in GH and IGF-I levels would blunt the GH response to exercise. The aim of the study was to examine and compare the impact of rhGH on the exercise-induced GH response in healthy normal men and women. Design and Measurements  Sixty-nine subjects (36 men, 33 women) were randomized to receive low-dose rhGH (0·1 U/kg/day), high dose rhGH (0·2 U/kg/day), or placebo. Subjects were matched for age (24 ± 3·1), and body mass index (BMI). rhGH was given as a single subcutaneous (s.c.) injection for the first 28 days. All subjects exercised to exhaustion (maximal oxygen consumption – VO2max) before rhGH treatment (Test 1), and on day 28 (Test 2). GH was measured before exercise (time 0), immediately after exercise (time 0′) and at 15, 30, 60, 90 and 120 min postexercise. Baseline IGF-I levels were measured before exercise on days 0 and 28. Results  Baseline IGF-I levels showed no gender differences (42·3 women vs. 38·8 nmol/l men) but basal GH values were higher in women (9·9 vs. 1·8 mU/l, P < 0·001). The areas under the GH response curve, for Test 1 were similar in men and women. Peak GH values were higher in women than men (37·9 vs. 23·5 mU/l, but this did not quite reach statistical significance (P = 0·055). In men, administration of rhGH resulted in a significant increase in IGF-I levels over the basal state in both the LD and HD groups (P < 0·0001). In women, the increase in lGF-I levels reached significance only in the HD group (P < 0·0001). On day 28, GH secretion in response to exercise was calculated from the areas under the GH response curve correcting for an exogenous rhGH component (ΔAUC). In men, the ΔAUC, for Test 2 were similar in all three groups. In women, the ΔAUC was higher in the placebo group, than in the HD group (P < 0·02). Free T4 levels decreased significantly in men, and free T3 increased in both men and women, in HD group after the rhGH administration. TSH levels were suppressed only in women. No changes in sex hormones were found in men or women in any of the treatment groups. Conclusions  In terms of IGF-I, men are more responsive to rhGH treatment than women. In addition, as men, but not women, were able to overcome the negative feedback control of the elevated IGF-I levels, it seems that exercise may be a more robust stimulus to GH release in men compared to women.