title: Serum levels of bioactive IGF1 and physiological markers of ageing in healthy adults.
abstract: OBJECTIVE
  Senescent changes in body composition and muscle strength are accompanied by reduced production of GH and IGF1, but the causal relationship remains elusive. We speculate that serum bioactive IGF1, measured by the IGF1 kinase receptor activation assay, is closer related to human physiological ageing than total IGF1 measured by immunoassay.
  
  
  METHODS
  We conducted a cross-sectional study in 150 adult males and females, between 20 and 70 years. After an overnight fasting, serum levels of bioactive IGF1, total IGF1 and IGF-binding protein 1 (IGFBP1) and IGFBP3 were assessed. Furthermore, body composition and muscle strength was measured.
  
  
  RESULTS
  Total IGF1 levels were higher in females (P=0.048). Bioactive IGF1 were identical in males and females (P=0.31), decreasing with age. Total IGF1 tended to decrease more with age compared with bioactive IGF1 (-1.48 vs -0.89 percent/year, P=0.052). Total body fat (TBF) was lower and BMI was higher in males (P<0.001 and P=0.005), and both increased with age. Knee extension and elbow flexion force were higher in males (P=0.001 and P=0.001), but decreased with age in both genders.  Total but not bioactive IGF1 was positively correlated to TBF, knee extension and muscle function in males. In multiple linear regression, only age predicted total IGF1, whereas age and IGFBP1 predicted bioactive IGF1.
  
  
  CONCLUSIONS
  Bioactive IGF1 tends to decrease to a lesser extent than total IGF1 with age and was not correlated with measures of body composition or muscle strength. Therefore, levels of circulating bioactive IGF1 does not appear to be a better biomarker of physiological ageing than total IGF1.
authors.affiliation: Department of Endocrinology and Internal Medicine, Aarhus University Hospital, Nørrebrogade 44, DK-8000 Aarhus C, Denmark.
authors.forename: Poul Frølund
authors.initials: PF
authors.lastname: Vestergaard
authors.forename: Mette
authors.initials: M
authors.lastname: Hansen
authors.forename: Jan
authors.initials: J
authors.lastname: Frystyk
authors.forename: Ulrick
authors.initials: U
authors.lastname: Espelund
authors.forename: Jens S
authors.initials: JS
authors.lastname: Christiansen
authors.forename: Jens Otto Lunde
authors.initials: JO
authors.lastname: Jørgensen
authors.forename: Sanne
authors.initials: S
authors.lastname: Fisker
mesh_terms: D000328:Adult; D000368:Aged; D000375:Aging / Q000097:blood*; D001823:Body Composition / Q000502:physiology; D015992:Body Mass Index; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000097:blood; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000097:blood; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D053580:Muscle Strength / Q000502:physiology; D012737:Sex Factors
chemical_list: C583557:IGF1 protein, human; C032231:IGFBP1 protein, human; C515497:IGFBP3 protein, human; D018970:Insulin-Like Growth Factor Binding Protein 1; D018972:Insulin-Like Growth Factor Binding Protein 3; D007334:Insulin-Like Growth Factor I