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 trea ted with growth hormone (GH) or insulinlike growth factor I (IGF-I) or both is associated with alterations in energy expenditure or macronut rient oxidation or both. Surgically stressed male rats (approximate to 235 g) were given total parenteral nutrition (TPN) and treated with r ecombinant human GH (rhGH) (800 mu g/d), rhIGF-I (800 mu g/d), rhGH rhIGF-I (800 mu g/d of each), or TPN alone for 3 d. Treatment with GH or IGF-I or both resulted in significantly greater body weight gain, n itrogen retention, and serum total IGF-I concentrations compared with TPN alone (P < 0.0001). Assessment of respiratory gas exchange and mot or activity for 23 h on day 3 indicated no significant differences bet ween groups in either total or activity-related rates of energy expend itures (kJ/kg(0.75)). 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 si gnificantly reduced (P < 0.0001) by these growth factors. GH and IGF-I combined further enhanced fat oxidation while reducing protein catabo lism. Serum insulin concentrations were significantly increased by GH but decreased by IGF-I. GH significantly decreased serum total triiodo thyronine concentrations and IGF-I significantly decreased serum corti costerone concentrations. These results suggest that treatment with GH or IGF-I can increase fat oxidation and spare protein for growth with out altering energy expenditure in surgically stressed rats maintained with TPN.