The early effects of human GH administration in GH-deficient (GHD) adults on protein, electrolyte homeostasis, and body composition were investigated in a metabolic ward study. Four patients were studied. In addition to a constant caloric and nitrogen (N)-sufficient diet, the patients received GH for 15 days in dosages of 12.5-25 micrograms/kg.day, with a maximum of 1.48 mg (4 IU)/day. GH replacement therapy was well tolerated by all patients. There was a slowly increasing effect on IGF-I levels, which reached a maximum after 8-12 days. The lowered IGFBP-3 levels normalized quicker, reaching maximum circulating concentrations 3 days after the start of GH treatment. Insulin concentrations maximally increased after 5 days, after which they leveled off. Insulin-like growth factor-binding protein-1 levels were maximally suppressed after 2 days of treatment. N balance became positive in all patients (mean, +2.8 +/- 0.2 g/day). Maximal N retention occurred after 2-5 days of GH administration, after which adaptation occurred. This degree of N retention represents a formation of 20 g muscle/day, which would mean an increase of 3.6 kg muscle over a period of 6 months of GH replacement therapy. A rapidly occurring positive sodium balance was observed within 24-72 h. Maximal sodium retention amounted to 61 mmol/day. It slowly decreased spontaneously over the subsequent 12 days. In parallel, rapid changes in bioelectrical impedance analysis (BIA) were observed. There was a close parallel between the net cumulative sodium retention and the decrease in BIA in these patients during the first 15 days of GH therapy. This suggests that the calculation of body composition compartments on the basis of BIA measurements during the initial phase of GH replacement does not represent actual changes in fat mass. This was substantiated with measurements of body composition using dual energy x-ray absorptiometry. In conclusion, measurements of early metabolic changes in GHD adults during the first 15 days after the start of GH replacement indicate that IGF-I values reach maximal levels only after 8-12 days, that the measurements of changes in IGFBP-1 and IGFBP-3 levels probably do not contribute to a determination of the optimal GH replacement dose, that maximal N-retaining effects occur within 2-5 days, after which adaptation occurs, that massive sodium retention occurs during this period, which spontaneously levels off, and that cumulative sodium retention closely correlates during this period with changes in BIA.(ABSTRACT TRUNCATED AT 400 WORDS)