title: Time outdoors, blood vitamin D status and myopia: a review.
abstract: BACKGROUND
  Myopia is a major public health concern throughout the world and the prevalence has been increasing rapidly in recent years, especially in urban Asia. The "vitamin D hypothesis" has been raised recently because vitamin D may be a link between less time outdoors and increased risk of myopia.
  
  
  METHODS
  We reviewed all studies published in English which examined the association of time outdoors and blood vitamin D status with myopia.
  
  
  RESULTS
  The protective effect of time spent outdoors on the risk of myopia onset has been well-established with numerous observational studies and three trials published. Five studies reporting the association between the blood vitamin D status and the risk of myopia and two studies examining the variations in the vitamin D receptor as potential risk factors for myopia development were identified. Most of the current evidence was cross-sectional in nature and had not properly controlled important confounders in its analyses. The evidence supporting that vitamin D played a role in myopia development is weak and the mechanisms are unclear.
  
  
  CONCLUSIONS
  At the current stage, it is still unclear whether blood vitamin D status regulates the onset or progression of myopia. Blood vitamin D status may only serve as a biomarker of outdoor exposure, which is the real protective factor for myopia.
authors.affiliation: Jiangsu Key Laboratory of Preventive and Translational Medicine for Geriatric Diseases, School of Public Health, Medical College of Soochow University, Suzhou, China.
authors.forename: Chen-Wei
authors.initials: CW
authors.lastname: Pan
authors.affiliation: Jiangsu Key Laboratory of Preventive and Translational Medicine for Geriatric Diseases, School of Public Health, Medical College of Soochow University, Suzhou, China.
authors.forename: Deng-Juan
authors.initials: DJ
authors.lastname: Qian
authors.affiliation: Singapore Eye Research Institute, Singapore. seang_mei_saw@nuhs.edu.sg and Saw Swee Hock School of Public Health, National University of Singapore, Singapore.
authors.forename: Seang-Mei
authors.initials: SM
authors.lastname: Saw
mesh_terms: D006370:Heliotherapy; D006801:Humans; D009216:Myopia / Q000097:blood* / Q000517:prevention & control*; D018167:Receptors, Calcitriol / Q000097:blood; D012307:Risk Factors; D013472:Sunlight; D014807:Vitamin D / Q000097:blood*
chemical_list: D018167:Receptors, Calcitriol; C579457:VDR protein, human; D014807:Vitamin D