Time outdoors, blood vitamin D status and myopia: a review

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.