Context. Children with obesity (BMI > +2 SDS) and children with constitutional tall stature (CTS, height > +2 SDS) have normal-high serum IGF-I levels, associated with a low and a broad range of GH secretion, respectively. This suggests increased sensitivity to GH, whereas children with idiopathic short stature (ISS, height < -2 SDS) are believed to have decreased GH sensitivity.

Objective, design, and main outcome measure. To compare the responsiveness to GH in 62 prepubertal children (43 F, 19 M) with obesity, CTS, or ISS, and 26 controls (15 F, 11M,; height and BMI -2 to +2 SDS), we used an IGF-I generation test and studied the IGF-I concentration 24 h after a single injection of GH (2 mg/m²).

Patients. Twenty patients with obesity, 20 with CTS, 22 with ISS, and 26 controls were studied. The mean age was 8.3 ± 2.9 yr, with no difference in age or gender between groups.

Results. Compared with controls, the mean IGF-I increment was 80% higher in obese children and 36% higher in tall children (P < 0.05 obese or tall vs. control children; P = 0.05 obese vs. tall children). Conversely, the IGF-I increment was similar in short compared with control children, despite a mean baseline IGF-I 62% lower in short children (P < 0.05 vs. controls). In all groups, the IGF-I increment was correlated with the BMI SDS or the fat mass percentage (r = 0.51-0.58, P < 0.05).

Conclusion. Obese children tend to have greater GH responsiveness than tall children, and both have greater GH responsiveness than controls. GH responsiveness was similar in controls and short children, despite a lower baseline IGF-I in short children. Whether the differences in the IGF-I response to GH between these children reflect differences in the respective anabolic (growth promotion) and metabolic (i.e. insulin action modulation) roles of circulating IGF-I is unknown.