title: Igf-I bioactivity in an elderly population: relation to insulin sensitivity, insulin levels, and the metabolic syndrome.
abstract: OBJECTIVE
  There is a complex relationship between IGF-I, IGF binding proteins, growth hormone, and insulin. The IGF-I kinase receptor activation assay (KIRA) is a novel method for measuring IGF-I bioactivity in human serum. We speculated that determination of IGF-I bioactivity might broaden our understanding of the IGF-I system in subjects with the metabolic syndrome. The purpose of our study was to investigate whether IGF-I bioactivity was related to insulin sensitivity and the metabolic syndrome.
  
  
  METHODS
  We conducted a cross-sectional study embedded in a random sample (1,036 elderly subjects) of a prospective population-based cohort study. IGF-I bioactivity was determined by the IGF-I KIRA. Categories of glucose (in)tolerance were defined by the 2003 American Diabetes Association criteria. Insulin sensitivity was assessed by homeostasis model assessment. The Adult Treatment Panel III definition of the metabolic syndrome was used.
  
  
  RESULTS
  In subjects with normal fasting glucose and impaired fasting glucose, IGF-I bioactivity progressively increased with increasing insulin resistance, peaked at fasting glucose levels just below 7.0 mmol/l, and dropped at higher glucose levels. Mean IGF-I bioactivity peaked when three criteria of the metabolic syndrome were present and then declined significantly when five criteria of the metabolic syndrome were present.
  
  
  CONCLUSIONS
  We observed that IGF-I bioactivity was related to insulin sensitivity, insulin levels, and the metabolic syndrome. Our study suggests that there exists an inverse U-shaped relationship between IGF-I bioactivity and number of components of the metabolic syndrome. This observation contrasts with previous results reporting an inverse relationship between total IGF-I and components of the metabolic syndrome.
authors.affiliation: Department of Internal Medicine, Erasmus Medical Center, Rotterdam, The Netherlands.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Brugts
authors.forename: Cornelia M
authors.initials: CM
authors.lastname: van Duijn
authors.forename: Leo J
authors.initials: LJ
authors.lastname: Hofland
authors.forename: Jacqueline C
authors.initials: JC
authors.lastname: Witteman
authors.forename: Steven W J
authors.initials: SW
authors.lastname: Lamberts
authors.forename: Joseph A M J L
authors.initials: JA
authors.lastname: Janssen
mesh_terms: D000328:Adult; D000368:Aged; D015992:Body Mass Index; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D005260:Female; D018149:Glucose Intolerance / Q000097:blood / Q000453:epidemiology; D006801:Humans; D007328:Insulin / Q000097:blood*; D018969:Insulin-Like Growth Factor Binding Proteins / Q000097:blood; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008297:Male; D024821:Metabolic Syndrome / Q000453:epidemiology*; D008875:Middle Aged; D017526:Receptor, IGF Type 1 / Q000097:blood*
chemical_list: D007328:Insulin; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1