Effect of dietary restriction and repeated growth hormone-releasing factor injections on growth hormone response to growth hormone-releasing factor in obese subjects

Abstract

The response of serum growth hormone (GH) to a test bolus of growth hormone-releasing factor (GRF) (1 μg/kg, IV) was examined in eight obese subjects (166% ± 8% ideal body weight [IBW]) during a weight-maintaining control period and after 2 weeks of hypocaloric feeding (750 kcal). During the period of hypocaloric feeding subjects received either repetitive GRF 1 μg/kg, IV (GRF group, n = 6) at 9 am, 1 pm, and 5 pm three times a week, or saline injections (placebo group, n = 6) at the same intervals. Four subjects were studied twice, several months apart, each receiving GRF and placebo treatments. Nitrogen balance was determined daily throughout the study. We also examined GH response to the GRF test in seven normal-weight subjects (106% ± 6% IBW). During the control period, obese subjects demonstrated a blunted GH response to GRF expressed as peak height or GH area under the curve (0 to 120 minutes) compared with normal subjects ( P < .005). At the end of the hypocaloric feeding, mean GH peak height after the GRF test was unchanged in the placebo group compared with the control period. GH release was significantly increased only during the second hour of testing ( P < .025). However, the GRF group demonstrated an increase in both mean GH peak height ( P < .025) and GH release during the first and second hours of testing ( P < .025) compared with the control period. Weight loss and blood glucose, triglycerides, FFA, amino acids, insulin, and T 3 changes were similar in both groups. Nitrogen balance was also similar in both groups, indicating that neither the peak nor the total GH release increase demonstrated by the GRF group was sufficient to induce a significant nitrogen retention during hypocaloric feeding. Repeated GRF injections during hypocaloric feeding can enhance both the peak and integrated GH response to GRF in obese subjects. This suggests that at least during a hypocaloric regimen, somatotrophs may be primed by GRF in some obese subjects.