RELATIONSHIP BETWEEN GROWTH-HORMONE (GH) STATUS, SERUM LEPTIN AND BODY-COMPOSITION IN HEALTHY AND GH DEFICIENT ELDERLY SUBJECTS

OBJECTIVE Growth hormone (GH) secretion declines with age and is affec ted by body composition, The signal that mediates the latter relations hip remains ill-defined. Leptin, the protein product of the adipocyte specific ob gene, is thought to accurately reflect fat mass and could therefore be a candidate to influence GH secretion, We have therefore investigated the relationship between GH status, leptin and body compo sition in normal and GH-deficient elderly subjects. DESIGN GH secretio n was assessed by 20-minute sampling over 24 hours and serum leptin co ncentrations were measured in a single morning, fasted sample. PATIENT S Twenty-one GH deficient elderly patients (61-83 years) and 22 gender - and BMI-matched controls (61-88 years), MEASUREMENTS Body compositio n was assessed by dual-energy X-ray absorptiometry (DEXA). GH was meas ured in an ultrasensitive chemiluminescent assay and serum leptin was determined by radioimmunoassay. RESULTS Leptin was correlated with per centage body fat in both sexes (male r = 0.75, female r = 0.89, both P < 0.001). Male patients had increased fat mass (FM) (P < 0.01) and le ptin concentrations (P < 0.05) but similar lean mass (LM) compared wit h controls, However, leptin concentration per unit FM was identical in both groups (P = 0.3), In contrast, female patients had lower LM (P < 0.05) but similar FM to controls, yet their leptin concentration per unit FM was twice that of the controls (P < 0.05). In multiple linear regression (MLR) leptin was determined positively by FM and negatively by LM (controls r(2) = 76%; patients r(2) = 73%, both P < 0.0001). Wh en controlled for gender, GH secretion in the controls was correlated negatively with leptin (r = -0.68, P < 0.01) and negatively with perce ntage body fat (r = -0.73, P < 0.01). In MLR, using leptin as a marker of body composition, 66% of the variability in GH secretion in the co ntrols could be explained by gender (38%) and by leptin (28%), CONCLUS IONS Both decreased lean mass and increased fat mass raise serum lepti n concentrations in normal and growth hormone-deficient elderly subjec ts, Leptin is therefore a marker of body composition rather than fat m ass alone. The influence of body composition on growth hormone secreti on in the elderly may be mediated through leptin, acting as a peripher al signal from adipose tissue to decrease GH secretion.