Certain nutrients like vitamin B, vitamin A, polyunsaturated fatty acids and more show some association with myopia development, but findings need to be replicated across the board. Photo: Getty Images.

In a new systematic review, investigators sought to tackle the question of whether dietary habits can influence the process of emmetropization in childhood development. To do so, researchers collected evidence from study outcomes that formerly analyzed the association between nutrition and myopia.

After looking through several databases, the study researchers included 27 articles to review. Most of these included non-interventional studies looking at specific nutrients and dietary elements, often showing inconsistencies in their association with myopia and most showing no association. Nine studies showed association of diverse nutrients and dietary elements with an increase or decrease in myopia development risk. However, the majority of these had minimal odds ratios and wider or overlapping confidence intervals, indicating weaker associations. Of the interventional trial, the three nutrients and dietary elements assessed all had myopia control implications, but two of these trials only saw minimal effect.

Despite this apparent lack of consistency in the literature, the authors do offer multiple theories about how some nutrients may influence the development and progression of myopia. Macronutrients—mainly high glycemic load carbohydrates—may lead to ocular elongation, as one study postulates, due to increased consumption promoting insulin resistance. This in turn leads to increased free-circulating concentration of insulin-like growth factor-1, causing a cascade resulting in scleral tissue proliferation, and thus elongation. This hypothesis was supported by three studies’ findings. Similarly, increased insulin level was associated with increased myopia odds in one study.

Micronutrients, as they pertain to myopia, included vitamin D, vitamin A and antioxidative trace elements. Vitamin D is proposed to affect myopia development through many means, including anti-proliferative effects on scleral growth, its role in retinoic acid metabolism, its influence on ciliary muscle functioning through the calcium pathway and its impact on dopamine release. The studies in this review mainly analyzed serum concentration of vitamin D, which was lower in those with myopia. Other studies, though, saw no association when looking at dietary intake of the nutrient, so it remains if there is a causal relationship between vitamin D and myopia or if it is just a biomarker for time spent outdoors, as a previous meta-analysis indicated.

Vitamin A has been associated with myopia development in animal models, but human studies have not replicated this finding, as all studies here indicated no relationship between the two. Trace elements of zinc, copper and selenium were associated with myopia in a few studies, theorized to counter myopia development that occurs from oxidative stress.

The dietary element of polyunsaturated fatty acids (PUFA) may be beneficial to visual health, as a recent study reported that omega-3 PUFA supplementation increased choroidal blood flow, thought to inhibit scleral hypoxia in its effects and resultant scleral remodeling that can lead to myopia. One study included in this review did conclude breastfeeding had a reduced risk of myopia.

The caffeine metabolite 7-MX is thought to directly act on the sclera or modulate muscarinic dopamine or acetylcholine receptors at a retinal level, thus triggering an emmetropization cascade. However, 7-MX showed only a minimal effect on axial elongation control in two different clinical trials.

Overall, the studies included in this review did show some specific associations of nutrients with myopia, but these were not seen across other studies. Based on these conflicting findings, the authors believe that “given the vast, diverse and complex nature of nutrition, more systematic investigation is warranted to comprehend the extent to which these specific nutrients and dietary elements are associated with myopia through longitudinal studies by subduing the limitations in the existing literature.”