Growth hormone or insulin-like growth factor I increases fat oxidation and decreases protein oxidation without altering energy expenditure in parenterally fed rats.

We assessed whether the increased growth in parenterally fed rats treated with growth hormone (GH) or insulin-like growth factor I (IGF-I) or both is associated with alterations in energy expenditure or macronutrient oxidation or both. Surgically stressed male rats (approximately 235 g) were given total parenteral nutrition (TPN) and treated with recombinant human GH (rhGH) (800 micrograms/d), rhIGF-I (800 micrograms/d), rhGH+rhIGF-I (800 micrograms/d of each), or TPN alone for 3 d. Treatment with GH or IGF-I or both resulted in significantly greater body weight gain, nitrogen retention, and serum total IGF-I concentrations compared with TPN alone (P < 0.0001). Assessment of respiratory gas exchange and motor activity for 23 h on day 3 indicated no significant differences between groups in either total or activity-related rates of energy expenditures (kJ/kg0.75). Estimates based on the nitrogen-free respiratory quotient (RQ) revealed fat oxidation to be significantly increased by GH (P < 0.001) and IGF-I (P < 0.03), whereas protein oxidation was significantly reduced (P < 0.0001) by these growth factors. GH and IGF-I combined further enhanced fat oxidation while reducing protein catabolism. Serum insulin concentrations were significantly increased by GH but decreased by IGF-I. GH significantly decreased serum total triiodothyronine concentrations and IGF-I significantly decreased serum corticosterone concentrations. These results suggest that treatment with GH or IGF-I can increase fat oxidation and spare protein for growth without altering energy expenditure in surgically stressed rats maintained with TPN.