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Androgens May Mediate a Relative Preservation of IGF-I Levels in Overweight and Obese Women Despite Reduced Growth Hormone Secretion

Neuroendocrine Unit, Department of Medicine (A.L.U., A.Y., K.K.M.), and Department of Pathology (P.S.), Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114; and Clinical Research Center (J.B.), Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Address all correspondence and requests for reprints to: Karen K. Miller, Neuroendocrine Unit, Bulfinch 457B, Massachusetts General Hospital, Boston, Massachusetts 02114. E-mail: KKMiller{at}Partners.org.

Context: Obesity is characterized by reduced GH secretion, but data regarding IGF-I levels and their determinants are conflicting.

Objectives: The objectives were to determine whether IGF-I levels are reduced and to investigate determinants of GH and IGF-I in healthy overweight and obese women.

Design: A cross-sectional study was performed.

Setting: The study was conducted at a General Clinical Research Center.

Study Participants: Thirty-four healthy women without pituitary/hypothalamic disease participated, including 11 lean [body mass index (BMI) <25 kg/m²], 12 overweight (BMI 25 kg/m² and <30 kg/m²), and 11 obese (BMI 30 kg/m²) women of comparable age (overall mean age, 30.7 ± 7.8 yr).

Intervention: There was no intervention.

Main Outcome Measures: The main outcome measures were frequent sampling (every 10 min for 24 h) for GH, peak GH after GHRH-arginine stimulation, IGF-I, IGF binding protein-3, estrone, estradiol, testosterone, free testosterone, SHBG, homeostasis model assessment of insulin resistance, and abdominal fat.

Results: Mean 24-h GH and peak stimulated GH were lower in overweight than lean women and lowest in obese women. Mean IGF-I levels trended lower in obese, but not overweight, compared with lean women. Free testosterone was positively associated with IGF-I (R = 0.36, P = 0.04) but not with GH measures. Visceral fat was the only determinant of mean 24-h GH (R² = 0.66, P < 0.0001) and of peak stimulated GH (R² = 0.63, P < 0.0001), and mean 24-h GH accounted for 39% of the variability of IGF-I (P = 0.0002), with an additional 28% (P < 0.0001) attributable to free testosterone levels.

Conclusions: Despite a linear decrease in GH secretion and peak stimulated GH levels with increasing BMI in healthy overweight and obese women, IGF-I levels were not commensurately reduced. Androgens may contribute to this relative preservation of IGF-I secretion in overweight and obese women despite reduced GH secretion.