Abdominal visceral fat and fasting insulin are important predictors of 24-hour GH release independent of age, gender, and other physiological factors

Numerous physiological factors modulate GH secretion, but these variables a re not independent of one another. We studied 40 younger (20-29 yr.; 21 men and 19 women) and 62 older (57-80 yr.; 35 men and 27 women) adults to dete rmine the contributions of several demographic and physiological factors to the variability in integrated 24-h GH concentrations. Serum GH was measure d every 10 min for 24 h in an enhanced sensitivity chemiluminescence assay. The predictor variables included: age group (young or old), gender, abdomi nal visceral fat (by computed tomography), total body fat mass and percenta ge body fat by dual-energy x-ray absorptiometry, serum IGF-I, fasting serum insulin, 24-h mean estradiol and testosterone, and peak oxygen uptake by g raded exercise (treadmill) testing. Multiple ordinary least squares regress ion analysis was used to quantitatively assess the individual contribution that each predictive measure made to explain the variability among values o f integrated 24-h GH concentrations while in the presence of the remaining predictors. The model explained 65% of the variance in integrated 24-h GH c oncentrations. Abdominal visceral fat (P < 0.002) and fasting insulin (P < 0.008) were consistently important predictors of integrated 24-h GH concent rations independent of age group, gender, and all other predictor variables . Although serum IGF-l was an important overall predictor of integrated 24- h GH concentrations (P = 0.002), this relationship was present only in the young subjects and was modulated by gender. The remaining variables failed to contribute significantly to the model. We conclude that abdominal viscer al fat and fasting insulin are important predictors of integrated 24-h GH c oncentrations in healthy adults, independent of age and gender. Serum IGF-l is an important predictor of integrated 24-h GH concentrations in young bu t not older subjects. Bidirectional feedback between each of these three fa ctors and GH secretion may account for the strong relationships observed.