Posted by: Hourglass PJ18 JUL 2012
Concern about the influence of low blood vitamin D levels on health continues to grow, but I was recently interested to read of a possible link between low vitamin D and myopia.
Debates on short-sightedness have generally involved the classic “nature versus nurture” dispute. On the nature side myopic parents tend to have myopic children more than non-myopic parents and recent studies have identified genetic links with myopia. On the side of nurture, excessive close work has been a suggested risk factor for myopia for at least 400 years. Evidence that myopic children spend more time in reading and other close work than non-myopic children tends to support this stereotype. However, recent large studies have shown that reading or other close work does not increase the risk of becoming myopic.
Time spent outdoors has recently become of interest in myopia research, with findings indicating that myopic children spend less time outdoors than non-myopic children. Although one might argue that the effects of spending time outdoors are just the effects of near work in reverse, no studies to date have found evidence of this trade-off behaviour. Children’s time outdoors does not appear to be negatively correlated with reading or other close work.
Several theories have been proposed for the protective effect of time spent outdoors on myopia, including inhibition of growth in the length of the eyeball and better quality retinal imaging due to more frequent fixing on distant objects outdoors. The possibility that the protective effect of time outdoors is due to higher levels of skin-synthesised vitamin D is now being studied. Interestingly, myopia progresses faster in winter, when there are fewer hours of daylight, and at a slower rate in the summer.
A recent US cross-sectional study in 22 young adults found no difference in blood vitamin D levels between myopic and non-myopic youngsters, but when the findings were adjusted for differences in diet, blood levels of vitamin D were lower in the myopic youngsters. Other studies are providing genetic clues as to how a link between myopia and vitamin D may occur.
In a recent case-control study by the same US group, single-nucleotide polymorphisms within the vitamin D receptor (VDR) were found to be associated with myopia. Results from a study from India that evaluated a different VDR polymorphism suggested that the VDR gene may not be playing a direct role in the development of myopia but might contribute indirectly to the risk through its role in calcium homeostasis and the role of intracellular calcium levels on the regulation of the ciliary muscles in the eye and on the quality of the retinal image.
All of these findings to date suggest a role for vitamin D in myopia, but whether the link is related to low blood concentrations of vitamin D due to lack of sunlight or to a defect in the vitamin D receptor gene, or perhaps both, needs further elucidation, but myopia may be yet another condition indicating the potential benefits of higher vitamin D levels in the blood.