Effect modification of time spent outdoors on the association between early childhood overweight and myopia: a one-year follow-up study

Background This study examined the moderating role of outdoor time on the relationship between overweight and myopia. Methods The data for this study was obtained from a prospective study in Shanghai, where non-myopic children wore wristwear and were followed up for 1 year. Eye examinations were performed at each visit. The modification effect was assessed on the additive scale using multivariable logistic regression, and relative excess risk due to interaction was used to calculate the modification effect. Results A total of 4683 non-myopic children were included with 32.20% being overweight at baseline. Following a 1-year period, 17.42% of children had myopia. When compared to those who spent < 90 minutes outdoors, children who spent > 120 had a relative risk of myopia onset that was reduced to 0.61. As time spent outdoors decreased, more risks of myopia onset were identified among overweight children than among normal children, the modification effect on the additive scale was -0.007, with similar to 70% of this effect attributed to the modifying influence of outdoor time. Conclusions Increasing outdoor time can reduce myopia more among overweight children than normal. Future interventions should focus on outdoor activities among overweight children to reduce myopia risks.