title: Bioactive insulin-like growth factor-I in obesity.
abstract: BACKGROUND
  In obesity, total IGF-I is not reduced to the degree predicted by low GH levels, and free IGF-I levels are normal to high. Total and free IGF-I may not reflect IGF-I biological activity because immunoassays cannot account for the modifying effects of IGF binding proteins on interactions between IGF-I and its receptor.
  
  
  OBJECTIVE
  The aim of the study was to investigate the biological activity of IGF-I in obesity.
  
  
  METHODS
  We conducted a cross-sectional study at a General Clinical Research Center.
  
  
  METHODS
  Thirty-four healthy women (11 lean, 12 overweight, and 11 obese) of comparable age (overall mean, 30.7 +/- 1.3 yr) participated in the study.
  
  
  METHODS
  There were no interventions.
  
  
  METHODS
  We measured bioactive IGF-I (as measured by a kinase receptor activation assay), IGFBP-1, and GH using 6-h pools of serum collected every 10 min for 24 h, and fasting IGF-I and IGFBP-3.
  
  
  RESULTS
  Mean 24-h GH (R = -0.76; P < 0.0001), total IGF-I (R = -0.36; P = 0.040), and IGFBP-1 (R = -0.41; P = 0.017) levels were inversely associated with BMI, whereas bioactive IGF-I and IGFBP-3 levels were not. Mean bioactive IGF-I was similar in the groups [2.72 +/- 0.22 (lean), 3.10 +/- 0.32 (overweight), and 2.43 +/- 0.23 [corrected] (obese) microg/liter; overall P = 0.22]. Percentage bioactive IGF-I [(bioactive/total IGF-I) x 100] was higher in obese subjects than both lean and overweight subjects (P = 0.039).
  
  
  CONCLUSIONS
  Despite low GH secretion in obesity and decreasing IGFBP-1 with increasing BMI, 24-h mean bioactive IGF-I levels are not reduced in obese women and do not correlate with BMI or IGFBP-1 levels. This argues against elevated bioactive IGF-I as the etiology of reduced GH secretion through a feedback mechanism in obesity.
authors.affiliation: Neuroendocrine Unit, Bulfinch 457B, Massachusetts General Hospital, Boston, MA 02114, USA.
authors.forename: J
authors.initials: J
authors.lastname: Frystyk
authors.forename: D J
authors.initials: DJ
authors.lastname: Brick
authors.forename: A V
authors.initials: AV
authors.lastname: Gerweck
authors.forename: A L
authors.initials: AL
authors.lastname: Utz
authors.forename: K K
authors.initials: KK
authors.lastname: Miller
mesh_terms: D000328:Adult; D015992:Body Mass Index; D003430:Cross-Sectional Studies; D005260:Female; D019382:Human Growth Hormone / Q000097:blood; D006801:Humans; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000097:blood; D007334:Insulin-Like Growth Factor I / Q000032:analysis*; D009765:Obesity / Q000378:metabolism*
chemical_list: D018970:Insulin-Like Growth Factor Binding Protein 1; D019382:Human Growth Hormone; D007334:Insulin-Like Growth Factor I