Age-related decrease in plasma growth hormone: Response to growth hormone-releasing hormone, arginine, and l-dopa in obesity

Abstract

Aging is associated with a reduction in plasma growth hormone (GH) secretion in non-obese subjects. To determine whether or not age-related changes in plasma GH secretion exist in obese subjects, we measured (a) plasma GH response to growth hormone-releasing hormone (GRH; 1 μg/kg body wt), arginine (0.5 g/kg body wt), l-dopa (500mg), and (b) plasma glucose, insulin, and free fatty acids (FFAs) in 26 fasted obese subjects of various ages ranging from 16 to 71 years. Only subjects with a body mass index (BMI; kg/m 2) between 30.0 and 39.0 were studied. Six subjects were adolescents, 9 were in their 20s, and 11 were 30 years or older. The mean peak levels of plasma GH in response to GRH, arginine, and l-dopa in obese subjects were 11.3 ± 2.1, 21.9 ± 4.4, and 5.2 ± 0.3 ng/mL in adolescents, 8.2 ± 1.6, 9.1 ± 1.5, and 3.1 ± 0.6 ng/mL in those in their 20s, and 4.5 ± 0.4, 7.3 ± 1.4, and 2.8 ± 0.3 ng/mL in those 30 years or older, respectively, showing a significant decrease in peak GH level with advancing age ( P < .05 to P < .01). There was a negative correlation between the logarithmic increase in age and the peak GH response to GRH ( r = −.635, P < .01), arginine ( r = −.564, P < .01), and l -dopa ( r = −.630, P < .01), and between the logarithmic increase in age and the integrated GH response to GRH ( r = −.564, P < .01), arginine ( r = −.612, P < .01), and l-dopa ( r = −.551, P < .01) in all subjects. Plasma glucose, insulin, and FFAs did not change with age. There was no correlation between the peak GH level or the integrated GH response to these three stimuli and the plasma levels of glucose, insulin, and FFAs, respectively. Our findings suggest that in obese subjects advancing age reduces the secretory responsiveness of pituitary somatotropes to these three stimuli.