Endogenous GH plays a very limited metabolic role during the daily feed/fast cycle but is essential for the increased lipolytic rate found with more prolonged fasting.

To better characterize the 24 hr GH secretion pattern in humans, we studied frequently sampled 24 hr GH profiles in 93 young (18-45 years of age) healthy, fed volunteers: men (n=67) and women (n=26) with BMI<26 kg/m(2). Analysis of the composite GH series in men revealed 3 significant GH "waves" with peaks occurring at midnight (p<0.0001), at noon (p<0.02) and at 1800 h (p<0.0001). In women, similar pattern was seen, with three GH "waves" peaking at midnight (p<0.0001), 1100 h (p<0.02) and at 1600 h (p<0.002). We conclude that the 24 hr rhythmicity of GH secretion is far more complex than currently appreciated. The attribution of the two daytime GH "waves" to food consumption is unlikely but cannot be excluded at the present time. The complex temporal pattern of pulsatile GH secretion may have important effects on regulation of target cell function.

These findings indicate that, in obese subjects, discrete components of GH secretory pattern may differentially affect IGF-I generation and lipolytic responses.

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