title: Longitudinal changes in lean mass predict pQCT measures of tibial geometry and mineralisation at 6-7 years.
abstract: BACKGROUND
  Studies in childhood suggest that both body composition and early postnatal growth are associated with bone mineral density (BMD). However, little is known of the relationships between longitudinal changes in fat (FM) and lean mass (LM) and bone development in pre-pubertal children. We therefore investigated these associations in a population-based mother-offspring cohort, the Southampton Women's Survey.
  
  
  METHODS
  Total FM and LM were assessed at birth and 6-7 years of age by dual-energy x-ray absorptiometry (DXA). At 6-7 years, total cross-sectional area (CSA) and trabecular volumetric BMD (vBMD) at the 4% site (metaphysis) of the tibia was assessed using peripheral quantitative computed tomography [pQCT (Stratec XCT-2000)]. Total CSA, cortical CSA, cortical vBMD and strength-strain index (SSI) were measured at the 38% site (diaphysis). FM, LM and bone parameters were adjusted for age and sex and standardised to create within-cohort z-scores. Change in LM (ΔLM) or FM (ΔFM) was represented by change in z-score from birth to 7 years old and conditioned on the birth measurement. Linear regression was used to explore the associations between ΔLM or ΔFM and standardised pQCT outcomes, before and after mutual adjustment and for linear growth. The β-coefficient represents SD change in outcome per unit SD change in predictor.
  
  
  RESULTS
  DXA at birth, in addition to both DXA and pQCT scans at 6-7 years, were available for 200 children (48.5% male). ΔLM adjusted for ΔFM was positively associated with tibial total CSA at both the 4% (β=0.57SD/SD, p<0.001) and 38% sites (β=0.53SD/SD, p<0.001), cortical CSA (β=0.48SD/SD, p<0.001) and trabecular vBMD (β=0.30SD/SD, p<0.001), but not with cortical vBMD. These relationships persisted after adjustment for linear growth. In contrast, ΔFM adjusted for ΔLM was only associated with 38% total and cortical CSA, which became non-significant after adjustment for linear growth.
  
  
  CONCLUSIONS
  In this study, gain in childhood LM was positively associated with bone size and trabecular vBMD at 6-7 years of age. In contrast, no relationships between change in FM and bone were observed, suggesting that muscle growth, rather than accrual of fat mass, may be a more important determinant of childhood bone development.
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK; Paediatric Endocrinology, University Hospital Southampton NHS Foundation Trust, Southampton SO16 6YD, UK.
authors.forename: Rebecca J
authors.initials: RJ
authors.lastname: Moon
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK; Rheumatology, Salisbury Hospital NHS Foundation Trust, Salisbury, UK.
authors.forename: Zoe A
authors.initials: ZA
authors.lastname: Cole
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK.
authors.forename: Sarah R
authors.initials: SR
authors.lastname: Crozier
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK.
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Curtis
authors.affiliation: Paediatric Endocrinology, University Hospital Southampton NHS Foundation Trust, Southampton SO16 6YD, UK.
authors.forename: Justin H
authors.initials: JH
authors.lastname: Davies
authors.affiliation: Musculoskeletal Research Unit, School of Clinical Sciences, University of Bristol, Learning & Research Building, Southmead Hospital, Bristol BS10 5NB, UK.
authors.forename: Celia L
authors.initials: CL
authors.lastname: Gregson
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK.
authors.forename: Sian M
authors.initials: SM
authors.lastname: Robinson
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK.
authors.forename: Elaine M
authors.initials: EM
authors.lastname: Dennison
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK; NIHR Southampton Biomedical Research Centre, University of Southampton and University Hospital Southampton NHS Foundation Trust, Tremona Road, Southampton SO16 6YD, UK.
authors.forename: Keith M
authors.initials: KM
authors.lastname: Godfrey
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK.
authors.forename: Hazel M
authors.initials: HM
authors.lastname: Inskip
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK; NIHR Southampton Biomedical Research Centre, University of Southampton and University Hospital Southampton NHS Foundation Trust, Tremona Road, Southampton SO16 6YD, UK; NIHR Musculoskeletal Biomedical Research Unit, University of Oxford, Nuffield Orthopedic Centre, Headington, Oxford OX3 7HE, UK.
authors.forename: Cyrus
authors.initials: C
authors.lastname: Cooper
authors.affiliation: MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton General Hospital, Southampton SO16 6YD, UK; NIHR Southampton Biomedical Research Centre, University of Southampton and University Hospital Southampton NHS Foundation Trust, Tremona Road, Southampton SO16 6YD, UK. Electronic address: nch@mrc.soton.ac.uk.
authors.forename: Nicholas C
authors.initials: NC
authors.lastname: Harvey
mesh_terms: D015502:Absorptiometry, Photon; D000273:Adipose Tissue; D001823:Body Composition; D015519:Bone Density; D001846:Bone Development / Q000502:physiology*; D002648:Child; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D013977:Tibia / Q000000981:diagnostic imaging* / Q000254:growth & development*; D014057:Tomography, X-Ray Computed
keywords: Body composition; Childhood; Epidemiology; Growth; Osteoporosis; pQCT