Diabetic Retinopathy: What's Vitamin D Deficiency Got to Do With It?

Diabetes mellitus is one of the most common systemic diseases co-managed by optometrists due to the risk of ocular complications, including diabetic retinopathy. Up to 80% of type 2 diabetic patients can develop some form of sight-threatening diabetic retinopathy, making it one of the leading causes of blindness among middle-aged adults worldwide.¹

Many risk factors for diabetic retinopathy have been established, including poor glycemic control, longer duration with the disease, smoking, hypertension, and inflammation. An additional risk factor, vitamin D deficiency, is now gaining more attention in the literature and worldwide.

Vitamin D deficiency (VDD) has recently become an epidemic, affecting more than one billion children and adults worldwide³

Vitamin D is a fat-soluble metabolite responsible for human growth and development. The most currently accepted level of vitamin D [25(OH)D] sufficiency is a blood level of at least 30 ng/mL. It is produced in skin exposed to UV sunlight and metabolized into its active form of calcitriol.² Calcitriol plays an important role in many cellular processes, therefore researchers are paying close attention to the relationship between VDD and cancer, infectious disease, autoimmune disease, neurological disorders, cardiovascular disease, and diabetes mellitus.

There is a high prevalence of VDD in type 2 diabetic patients, suggesting that it may play a role in the pathogenesis of diabetic retinopathy.

VDD disrupts the normal release of insulin from the pancreatic beta cells, leading to higher levels of calcium and reactive oxygen species, ultimately causing death to the beta cells and setting the stage for diabetes to develop.⁴ Vitamin D receptors are also found within the retina. Research suggests that calcitriol inhibits retinal endothelial cell proliferation–a major cause of severe retinopathy including retinal neovascularization.^1,2

The association between VDD and diabetic retinopathy has been examined in the past years, but individual studies have had inconclusive results. Meta-analyses^1,5 were then conducted in 2017 to further examine existing results to find any significant correlation between VDD and diabetic retinopathy. After examining multiple published randomized controlled trials, cross-sectional, cohort, and case-controlled studies, here’s what they found:

There is increasing evidence that VDD (serum 25(OH)D levels < 20 ng/mL) is a strong risk factor for developing type 2 diabetes, as there was a statistically significant lower serum vitamin D level in patients with diabetes compared to patients without.
There is still conflicting evidence of whether the prevalence or level of VDD significantly differs between type 2 diabetic patients with versus without diabetic retinopathy.
The prevalence of type 2 diabetic patients with VDD increased with the severity of diabetic retinopathy rated by the ETDRS classification. In addition, VDD combined with a longer duration of diabetes, persistent hyperglycemia, or compromised kidney function increased the risk of more severe retinopathy beyond the risk associated with VDD alone.

The definite relationship between VDD and diabetes mellitus must be explored further, however, from these studies we can conclude that VDD does have an impact on our diabetic patients and the severity of retinopathy. Optometrists have a responsibility to counsel patients on diabetes and the importance of lifestyle and nutritional changes.

“As healthcare professionals, we can encourage vitamin D supplementation in our diabetic patients for possible prevention of retinopathy development.”

Patients require different doses of vitamin D based on age and weight, so the appropriate dosage should be determined by their primary care physician. Patients can also attempt to increase their vitamin D levels with a healthy amount of daily sun exposure.

In addition to increasing vitamin D levels, consider recommending these particular diets, suggested by Dr. Kevin Cornwell, for type 2 diabetic patients' nutritional health.

References

Luo BA, Gao F, & Qin LL. The association between vitamin d deficiency and diabetic retinopathy in type 2 diabetes: a meta-analysis of observational studies. Nutrients 2017;9(307):doi:10.3390/nu9030307.
Patrick PA, Visintainer PF, Shi Q, Weiss IA, et al. Vitamin d and retinopathy in adults with diabetes mellitus. Arch Ophthalmol 2012;130(6):756-760.
Holick MF. The vitamin d deficiency pandemic: approaches for diagnosis, treatment and prevention. Rev Endocr Metab Disord 2017;18:153-165.
Berridge MJ. Vitamin d deficiency and diabetes. Biochemical Journal 2017;474:1321-1332.
Zhang J, Upala S, & Sanguankeo A. Relationship between vitamin d deficiency and diabetic retinopathy: a meta-analysis. Can J Ophthalmol 2017;52(2):219-224.