title: Does vitamin D mediate the protective effects of time outdoors on myopia? Findings from a prospective birth cohort.
abstract: OBJECTIVE
  More time outdoors is associated with a lesser risk of myopia, but the underlying mechanism is unclear. We tested the hypothesis that 25-hydroxyvitamin D (vitamin D) mediates the protective effects of time outdoors against myopia.
  
  
  METHODS
  We analyzed data for children participating in the Avon Longitudinal Study of Parents and Children (ALSPAC) population-based birth cohort: noncycloplegic autorefraction at age 7 to 15 years; maternal report of time outdoors at age 8 years and serum vitamin D2 and D3 at age 10 years. A survival analysis hazard ratio (HR) for incident myopia was calculated for children spending a high- versus low-time outdoors, before and after controlling for vitamin D level (N = 3677).
  
  
  RESULTS
  Total vitamin D and D3, but not D2, levels were higher in children who spent more time outdoors (mean [95% confidence interval (CI)] vitamin D in nmol/L: Total, 60.0 [59.4-60.6] vs. 56.9 [55.0-58.8], P = 0.001; D3, 55.4 [54.9-56.0] vs. 53.0 [51.3-54.9], P = 0.014; D2, 5.7 [5.5-5.8] vs. 5.4 [5.1-5.8], P = 0.23). In models including both time outdoors and sunlight-exposure-related vitamin D, there was no independent association between vitamin D and incident myopia (Total, HR = 0.83 [0.66-1.04], P = 0.11; D3, HR = 0.89 [0.72-1.10], P = 0.30), while time outdoors retained the same strong negative association with incident myopia as in unadjusted models (HR = 0.69 [0.55-0.86], P = 0.001).
  
  
  CONCLUSIONS
  Total vitamin D and D3 were biomarkers for time spent outdoors, however there was no evidence they were independently associated with future myopia.
authors.affiliation: Centre for Myopia Research, School of Optometry, Hong Kong Polytechnic University, Kowloon, Hong Kong Special Administrative Region, China.
authors.forename: Jeremy A
authors.initials: JA
authors.lastname: Guggenheim
authors.affiliation: School of Social and Community Medicine, University of Bristol, Bristol, United Kingdom.
authors.forename: Cathy
authors.initials: C
authors.lastname: Williams
authors.affiliation: School of Social and Community Medicine, University of Bristol, Bristol, United Kingdom.
authors.forename: Kate
authors.initials: K
authors.lastname: Northstone
authors.affiliation: School of Social and Community Medicine, University of Bristol, Bristol, United Kingdom MRC Integrative Epidemiology Unit (IEU) at the University of Bristol, Bristol, United Kingdom.
authors.forename: Laura D
authors.initials: LD
authors.lastname: Howe
authors.affiliation: School of Social and Community Medicine, University of Bristol, Bristol, United Kingdom MRC Integrative Epidemiology Unit (IEU) at the University of Bristol, Bristol, United Kingdom.
authors.forename: Kate
authors.initials: K
authors.lastname: Tilling
authors.affiliation: MRC Integrative Epidemiology Unit (IEU) at the University of Bristol, Bristol, United Kingdom.
authors.forename: Beate
authors.initials: B
authors.lastname: St Pourcain
authors.affiliation: MRC Integrative Epidemiology Unit (IEU) at the University of Bristol, Bristol, United Kingdom.
authors.forename: George
authors.initials: G
authors.lastname: McMahon
authors.affiliation: School of Social and Community Medicine, University of Bristol, Bristol, United Kingdom MRC Integrative Epidemiology Unit (IEU) at the University of Bristol, Bristol, United Kingdom.
authors.forename: Debbie A
authors.initials: DA
authors.lastname: Lawlor
mesh_terms: D000293:Adolescent; D002648:Child; D004739:England / Q000453:epidemiology; D015444:Exercise; D005260:Female; D006801:Humans; D015994:Incidence; D007899:Leisure Activities; D008137:Longitudinal Studies; D008297:Male; D009216:Myopia / Q000453:epidemiology*; D011446:Prospective Studies; D016019:Survival Analysis; D014807:Vitamin D / Q000097:blood*
chemical_list: D014807:Vitamin D
keywords: epidemiology; light levels; myopia; refractive error; vitamin D