title: Fat mass exerts a greater effect on cortical bone mass in girls than boys.
abstract: BACKGROUND
  It is unclear whether fat mass (FM) and lean mass (LM) differ in the way they influence cortical bone development in boys and girls.
  
  
  OBJECTIVE
  The aim of the study was to investigate the contributions of total body FM and LM to parameters related to cortical bone mass and geometry.
  
  
  METHODS
  We conducted a longitudinal birth cohort study, the Avon Longitudinal Study of Parents and Children.
  
  
  METHODS
  A total of 4005 boys and girls (mean age, 15.5 yr) participated in the study.
  
  
  METHODS
  We measured cortical bone mass, cortical bone mineral content (BMC(C)), cortical bone mineral density, periosteal circumference (PC), and endosteal circumference by tibial peripheral quantitative computed tomography.
  
  
  RESULTS
  LM had a similar positive association with BMC(C) in boys and girls [regression coefficients with 95% confidence interval (CI); P for gender interactions: boys/girls, 0.952 (0.908, 0.997); P = 0.85]. However, the mechanisms by which LM influenced bone mass differed according to gender because LM was positively associated with PC more strongly in girls [boys, 0.579 (0.522, 0.635); girls, 0.799 (0.722, 0.875); P < 0.0001], but was only associated with cortical bone mineral density in boys [boys, 0.443 (0.382, 0.505); girls, 0.014 (-0.070, 0.097); P < 0.0001]. There was a stronger positive association between FM and BMC(C) in girls [boys, 0.227 (0.185, 0.269); girls, 0.355 (0.319, 0.392); P < 0.0001]. This reflected both a greater positive association of FM with PC in girls [boys, 0.213 (0.174, 0.253); girls, 0.312 (0.278, 0.347); P = 0.0002], and a stronger negative association with endosteal circumference(PC) [boys, -0.059 (-0.096, 0.021); girls, -0.181 (-0.215, -0.146); P < 0.0001].
  
  
  CONCLUSIONS
  Whereas LM stimulates the accrual of cortical bone mass to a similar extent in boys and girls, FM is a stronger stimulus for accrual of cortical bone mass in girls, reflecting a greater tendency in females for FM to stimulate periosteal growth and suppress endosteal expansion.
authors.affiliation: Academic Rheumatology, Avon Orthopaedic Centre, Southmead Hospital, Bristol BS10 5NB, United Kingdom.
authors.forename: Adrian
authors.initials: A
authors.lastname: Sayers
authors.forename: Jonathan H
authors.initials: JH
authors.lastname: Tobias
mesh_terms: D000273:Adipose Tissue / Q000502:physiology*; D000293:Adolescent; D001823:Body Composition; D015519:Bone Density; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D012727:Sex Characteristics; D013851:Thinness / Q000378:metabolism; D014057:Tomography, X-Ray Computed