J Physiol (Lond) September 15-2005 567: 1035-1045

No effect of growth hormone administration on substrate oxidation during exercise in young, lean men

Mette Hansen1, Rikke Morthorst1, Benny Larsson2, Rolf Dall3, Allan Flyvbjerg3, Michael Højby Rasmussen4, Hans Ørskov3, Michael Kjær1 and Kai Henrik Wiborg Lange1



1 Institute of Sports Medicine, Copenhagen, Bispebjerg Hospital, DK-2400 Copenhagen NV, Denmark
2 Team Danmark Test Center, Bispebjerg Hospital, DK-2400 Copenhagen NV, Denmark
3 Medical Research Laboratories, Aarhus University Hospital, DK-8000, Aarhus C, Denmark
4 Medical and Science Development Projects, Novo Nordisk A/S, DK-2880 Bagsværd, Denmark The purpose of this study was to examine the effects of increased fat availability induced by growth hormone (GH) administration on the oxidative metabolism during exercise. Seven well-trained males (age 25 ± 2 years (mean ± S.E.M.); peak oxygen consumption : 62 ± 1 ml min–1 kg–1 (completed four randomised trials: 120 min bicycling at 55% 4 h after receiving either 7.5 IU (2.5 mg) GH or placebo (Plc), and during rest after receiving either GH or Plc. In all studies a standardized meal was given 2 h after GH or Plc injection. GH administration resulted in an 60-fold increase in serum GH concentration at rest (P < 0.0001) and during exercise (P < 0.0001). The increase in serum GH was followed by an increase in circulating glycerol at rest (8%, P < 0.0001). When combined with exercise the increase in plasma glycerol was more pronounced (GH: 716% of baseline versus Plc: 328%, P < 0.0001). However, this increase in fat mobilization did not increase fat oxidation during exercise (indirect calorimetry). In conclusion, GH administration combined with aerobic exercise increased lipolytic parameters substantially more than exercise alone, but did not further augment whole body fat oxidation.

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