The daily amount of time spent being physical active was inversely associated with a refractive change toward myopia. The estimates of the multiple regression analysis suggest that the protective effect of 1 hour of physical activity per day is equal in magnitude to the detrimental effect of 3 hours of study per day. Other researchers have described an association between high levels of outdoor activity in children and a more hyperopic refractive error in cross-sectional data.
3 Recently, Jones et al.
5 reported that a lower number of hours spent in sports and outdoors activities per week in third grade was associated with future myopia. Limitations to physical activities due to the wearing of glasses or an actual protective effect of physical activity are some of the possible explanations of these observations, although the common use of contact lenses makes the first explanation less likely. It is not possible to conclude whether the association between physical activity and myopia is causal or merely represents an association. Thus, the association could be confounded by factors related to physical activity. As most university students in Denmark ride the bicycle to get around, it constitutes a considerable amount of the person’s physically active time. Since riding the bicycle implies being outside, the protective effect may be related to the outdoor environment rather than physical activity. On the other hand, it is possible that the association between physical activity and myopia
is causal. It has been suggested that the association could be caused by some effect of distant gazing, or just being away from reading.
2 However, our results indicate that the latter does not explain the effect of physical activity, as the impact of physical activity seems stronger than the impact of studying. Another possibility is that physical activity influences the refraction by nonvisual means. Physical activity influences the body and brain via several different mechanisms, including effects on body fatness, blood pressure, lipid and lipoprotein metabolism, vascular function, central and peripheral growth factors, inflammation, oxidative stress, and insulin resistance.
21 22 Thus, growth hormones or other systemic mechanisms induced as a result of physical activity may be involved in the regulation of the growth of the eyes.
Although it could be suspected that students spending more time studying consequently spent less time being physically active, this was not the case. In fact, the trend was the reverse.