Time spent outdoors in childhood is associated with reduced risk of myopia as an adult

Abstract

Myopia (near-sightedness) is an important public health issue. Spending more time outdoors can prevent myopia but the long-term association between this exposure and myopia has not been well characterised. We investigated the relationship between time spent outdoors in childhood, adolescence and young adulthood and risk of myopia in young adulthood. The Kidskin Young Adult Myopia Study (KYAMS) was a follow-up of the Kidskin Study, a sun exposure-intervention study of 1776 children aged 6-12 years. Myopia status was assessed in 303 (17.6%) KYAMS participants (aged 25-30 years) and several subjective and objective measures of time spent outdoors were collected in childhood (8-12 years) and adulthood. Index measures of total, childhood and recent time spent outdoors were developed using confirmatory factor analysis. Logistic regression was used to assess the association between a 0.1-unit change in the time outdoor indices and risk of myopia after adjusting for sex, education, outdoor occupation, parental myopia, parental education, ancestry and Kidskin Study intervention group. Spending more time outdoors during childhood was associated with reduced risk of myopia in young adulthood (multivariable odds ratio [OR] 0.82, 95% confidence interval [CI] 0.69, 0.98). Spending more time outdoors in later adolescence and young adulthood was associated with reduced risk of late-onset myopia (≥ 15 years of age, multivariable OR 0.79, 95% CI 0.64, 0.98). Spending more time outdoors in both childhood and adolescence was associated with less myopia in young adulthood.

Figure 1

Pathways of the CFA models used to generate factor scores for the total sun exposure index (a), childhood sun exposure index (b) and recent sun exposure index (c). Double-headed arrows indicate covariance terms; small circles indicate error terms. Parent: Parent-reported time spent outdoors. Self: self-reported time spent outdoors. 25-D 25-hydroxyvitamin D, CUVAF conjunctival ultraviolet autofluorescence, Y years, Back nevi average yearly change in number of nevi on the backs between ages 6–12 years, Skin score skin damage score.

Figure 2

Correlation between each of the sun-exposure indices and observed measures of sun exposure. Pearson correlation coefficient is presented in lower half of table; correlations involving skin damage score are the polyserial correlation coefficient due to the ordinal nature of skin damage score. In the upper half, the size and darkness of the circles represent the strength of the correlation with larger and darker circles representing a higher correlation coefficient. All correlations ≥ 0.13 are significantly greater than 0 (i.e. p < 0.05). 25(OH)D 25-hydroxyvitamin D, CUVAF conjunctival ultraviolet autofluorescence. Figure drawn using the ‘corrplot’ package in R v3.5.1 (R foundation for statistical computing, Vienna, Austria, https://www.R-project.org).

Figure 3

Plots of spherical equivalent over childhood (left) and recent (right) sun indices. The blue line represents the simple linear regression line and grey shading represents 95% confidence intervals. Data from three individuals with moderate to high hyperopia (grey points) were excluded when calculating the linear regression equation and 95% confidence interval. Figure drawn using the ‘ggplot2’ package in R v3.5.1 (R foundation for statistical computing, Vienna, Austria, https://www.R-project.org).