title: Serum levels of insulin-like growth factor-I are related to age and not to body composition in healthy women and men.
abstract: BACKGROUND
  Aging is accompanied by decreased bone and lean body mass, increased fat mass, and reduced growth hormone (GH) axis function, reflected in diminished levels of insulin-like growth factor-I (IGF-I). Similar changes in body composition occur in nonelderly, GH-deficient adults and are reversible with GH administration, suggesting that diminished GH/IGF-I axis activity may contribute to such age-related changes. To determine the precise pattern of IGF-I decline with age, and to test the hypothesis that this decline is related to concomitant changes in body composition and bone metabolism independent of age, we conducted a cross-sectional survey in 351 healthy participants in the Baltimore Longitudinal Study of Aging.
  
  
  METHODS
  We evaluated relationships among IGF-I, age, and total and regional adiposity, as assessed by body mass index (BMI) and waist-to-hip ratio (WHR); lean body mass, as estimated from urinary creatinine excretion (Crex/ht); bone mineral density (BMD), as assessed by single and dual photon absorptiometry scanning; and circulating levels of parathyroid hormone (PTH), 1,25-(OH)2 D3, 25-OHD, and osteocalcin.
  
  
  RESULTS
  Serum IGF-I levels declined with age (p < .0001) in both men (r = -.51) and women (r = -.67). In men, the decline was linear, whereas IGF-I levels decreased faster in women < 45 years of age than in older women (p < .01) or in men (p < .001). IGF-I was inversely related to BMI (p < .005), WHR (p < .001), and PTH (p < .01) in women. IGF-I was positively related to BMD of the hip and radius in both genders (p < .0003) and to Crex/ht (p < .0005) and osteocalcin (p < .0001) in men. With increasing age, Crex/ht and BMD decreased (p < .0001) and WHR, PTH, and osteocalcin increased (p < .005) in both genders, whereas BMI increased only in women (p < .005). After adjustment for age, IGF-I was not significantly related to BMI, WHR, Crex/ht, or BMD in either gender. IGF-I was positively related to 1,25-(OH)2 D3 (p < .01) independently of age in women.
  
  
  CONCLUSIONS
  Advancing age, rather than declining serum levels of IGF-I, appears to be a major determinant of life-time changes in body composition and BMD in women and men.
authors.affiliation: Endocrine Section, Gerontology Research Center, National Institute on Aging (NIH), Baltimore, USA.
authors.forename: K G
authors.initials: KG
authors.lastname: O'Connor
authors.forename: J D
authors.initials: JD
authors.lastname: Tobin
authors.forename: S M
authors.initials: SM
authors.lastname: Harman
authors.forename: C C
authors.initials: CC
authors.lastname: Plato
authors.forename: T A
authors.initials: TA
authors.lastname: Roy
authors.forename: S S
authors.initials: SS
authors.lastname: Sherman
authors.forename: M R
authors.initials: MR
authors.lastname: Blackman
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000097:blood*; D001823:Body Composition / Q000502:physiology*; D015992:Body Mass Index; D015519:Bone Density; D002112:Calcifediol / Q000097:blood; D002117:Calcitriol / Q000097:blood; D005260:Female; D019382:Human Growth Hormone / Q000097:blood; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D015675:Osteocalcin / Q000097:blood; D010281:Parathyroid Hormone / Q000097:blood; D012016:Reference Values; D012044:Regression Analysis; D012727:Sex Characteristics
chemical_list: D010281:Parathyroid Hormone; D015675:Osteocalcin; D019382:Human Growth Hormone; D007334:Insulin-Like Growth Factor I; D002117:Calcitriol; D002112:Calcifediol