Human growth hormone response to repeated bouts of aerobic exercise

We examined whether repeated bouts of exercise could override growth hormone (GH) auto-negative feedback. Seven moderately trained men were studied on three occasions: a control day (C), a sequential exercise day (SEE; at 1000, 1130, and 1300), and a delayed exercise day (DEB; at 1000, 1400, and 1800). The duration of each exercise bout was 30 min at 70% maximal O-2 consumption ((V) over dot o(2 max)) on a cycle ergometer. Standard meals were provided at 0600 and 2200. GH was measured every 5-10 min for 24 h (0800-0800). Daytime (0800-2200) integrated GH concentrations were similar to 150-160% greater during SEE and DEB than during C: 1,282 +/- 345, 3,192 +/- 669, and 3,389 +/- 991 min.mu g.l(-1) for C, SEE, and DEB, respectively [SEE > C (P < 0.06), DEB > C (P < 0.03)]. There were no differences in GH release during sleep (2300-0700). Deconvolution analysis revealed that the increase in 14-h integrated GH concentration on DEB was accounted for by an increase in the mass of GH secreted per pulse (per liter of distribution volume, l(v)): 7.0 +/- 2.9 and 15.9 +/- 2.6 mu g/l(v) for C and DEB, respectively (P < 0.01). Comparison of 1.5-h integrated GH concentrations on the SEE and DEB days (30 min exercise + 60 min recovery) revealed that, with each subsequent exercise bout, GH release apparently increased progressively, with a slightly greater increase on the DEB day [SEE vs. DEB: 497 +/- 162 vs. 407 +/- 166 (bout 1), 566 +/- 152 vs. 854 +/- 184 (bout 2), and 633 +/- 149 vs. 1,030 +/- 352 min.mu g.l(-1) (bout 3), P < 0.05]. We conclude that the GH response to acute aerobic exercise is augmented with repeated bouts of exercise.