Growth hormone (GH) substitution for one year normalizes elevated GH-binding protein levels in GH-deficient adults secondary to a reduction in body fat. A placebo-controlled trial

Abstract

The high affinity growth hormone binding protein (GHBP) in human serum derives from the extracellular domain of the GH receptor. It is well known that fat mass correlates positively to GHBP levels, but it is uncertain whether GH secretory status influences GHBP levels. Since body composition is known to change during GH substitution in adult GHD patients, we determined the relation between GHBP and body composition during GH substitution in GHD adults. Twenty-five GHD adults aged 45.0 +/- 1.8 years, were examined before and after 12 months of placebo-controlled GH substitution (2 IU/m2) in a parallel design. A group of 27 healthy age- and gender-matched normal-weight adults provided reference data. The participants underwent anthropometric measurements [body mass index (BMI), waist/hip ratio (W/H)], computer-tomography (CT-scan) of femoral and abdominal regions, dual-energy X-ray absorptiometry (DEXA-scan), and bioimpedance (BIA), as well as blood sampling. At baseline, the GHBP levels were increased compared to controls (1.63 +/- 0.14 nmol/l vs 1.12 +/- 0.1 nmol/l, P = 0.01). During 12 months of GH substitution, GHBP levels decreased to the levels of the control subjects. GHBP correlated positively to indices of adiposity in GHD patients at baseline: intra-abdominal fat (r = 0.54, P = 0.005), subcutaneous abdominal fat (r = 0.59, P < 0.002), body fat (BIA) (r= 0.41, P= 0.044), BMI (r= 0.58, P = 0.002), and total body fat (DEXA scan) (r= 0.61, P < 0.001). After 12 months of GH substitution, different estimates of body fat were significantly decreased in the GH treated group, but the positive relationship between GHBP and these estimates of body fat was maintained. In multiple linear regression analyses, fasting insulin levels were also a significant determinant of GHBP levels. We conclude that GHBP levels are increased in GHD patients and decrease to normal levels during 12 months of GH substitution. Furthermore, GHBP is predominantly correlated to indices of adiposity also in GHD patients.