A recent study published by the journal Springer found that low vitamin D status is related to longer axial length and an increased risk of myopia in young children.
Myopia, also referred as nearsighted vision, is diagnosed when an individual can clearly see objects up close but experience a loss of clarity from a distance. This condition occurs due to a developmental mismatch of the eye, which is most commonly seen with an elongated axial length. The axial length refers to the distance between the front and back of the eye.
Although myopia is easily remedied by glasses, contact lenses or optic surgery, a long axial length is associated with an increased risk of severe visual impairment and blindness later in life as a result of ocular complications.
The prevalence of myopia has increased significantly over recent years, affecting about 50% of young adults of European descent and up to 96% of South Koreans. Additionally, more children are developing myopia at an early age, with anywhere between 2-13% of the population being diagnosed by the age of 6 years, depending on the region.
Past research has shown that time spent outdoors may decrease the risk of developing myopia. It is hypothesized that this may be due to increased dopamine in the retina upon exposure to light, or the vitamin D production that takes place when the skin is exposed to the sun. However, it remains unclear whether its vitamin D or another product of sun exposure that is responsible for this protective effect.
Therefore, researchers recently investigated the relationship between vitamin D status, axial length and risk of myopia in 6 year old children. They also looked at the effect of time spent outdoors on these factors.
A total of 2666 children who participated in the Generation R birth-cohort study were included. Each individual received a stepwise eye examination. The researchers evaluated the participants visual acuity and axial length, measured their serum 25(OH)D levels and gathered information regarding the participants outdoor exposure via questionnaire.
Here is what the researchers determined:
- The 25(OH)D concentration of the participants averaged 27.5 ng/ml (68.8 nmol/l).
- A total of 2.3% of the children were diagnosed with myopia.
- Only 18% of those with myopia had vitamin D levels ≥ 30 ng/ml (75 nmol/l).
- Serum 25(OH)D levels were inversely related to axial length for all individuals (p <0.001). This relationship remained significant even after excluding myopic children (p < 0.02).
- After adjusting for cofounding factors, including time spent outdoors, the relationship between vitamin D status and axial length remained significant (p <0.01).
- Participants experienced a 35% decreased odds of myopia per each 10 ng/ml increase in 25(OH)D status (p = 0.01).
The researchers concluded,
“In this cohort study of young children, we found a significant association between serum 25(OH)D levels, axial length and myopia… This effect appeared independent of outdoor exposure and may suggest a more direct role for 25(OH)D in myopia pathogenesis.”
As always, it is important to note both the strengths and weaknesses of the study. The large size, use of multiple measurements of myopia and the adjustment for several potential confounders increased the validity of this study. However, the young age of the participants decreased the number of cases of elongated axial length and/or myopia due to the lack of excessive eye growth, which typically starts appearing in adolescents. Also, the design of the study only showed correlation, not causality. Therefore, the researchers call for longitudinal and clinical trials to provide a further understanding of the relationship between vitamin D, sun exposure and myopia.
Sturges, M. & Cannell, JJ. Low vitamin D status linked with myopia in young children. The Vitamin D Council Blog & Newsletter, 2016.