Low levels of vitamin D may be associated with an enhanced risk of diabetic retinopathy, according to a new study. It is just the latest in a history of work to identify a connection between the two conditions, though the potential mechanism linking the two remains mysterious.

The new study, published in Clinical Nutrition ESPEN, analyzed the data from 402 adults with type 2 diabetes, half of whom had diabetic retinopathy (DR). The two groups were very similar in most ways: There was no real difference in average age, weight, A1C, cholesterol, or use of diabetic medications. The researchers found three differences common to those who had developed diabetic retinopathy:

• Higher systolic blood pressure (142 mmHg vs. 130 mmHg). Blood pressure is known to be one of the major causes of diabetic retinopathy, in concert with high blood sugar.
• Longer duration of diabetes (8.5 years vs. 6.0 years). Duration of diabetes is another known risk factor for diabetic retinopathy and other complications; the longer the body endures diabetes, the more time high blood sugar levels have to cause damage and dysfunction.
• Higher likelihood of vitamin D deficiency. 58.9 percent of those with DR had vitamin D deficiency, compared with only 33.3 percent of those without DR. Those with DR were also less than half as likely to have normal vitamin D status.

Dozens of similar studies have found similar connections. In a large 2022 review, researchers considered 36 studies to have investigated the link between DR and vitamin D, and found that 30 of them identified a correlation between low vitamin D levels and the complication — including studies of people with both type 1 and type 2 diabetes. Only a handful of the 36 studies reported no association at all.

Nevertheless, in a statement to the American Academy of Ophthalmology’s EyeNet Magazine, a spokesman from the team of researchers said that they were reluctant to draw conclusions as the research has been hampered by small sample sizes and a lack of standardized protocols. Larger and more robust experiments are still needed.

How might it work? The mechanism by which low vitamin D levels could hasten retinopathy is still a matter of conjecture. A 2023 review outlines some of the possibilities. Among other theories: Vitamin D has anti-inflammatory properties, which could help quell an overactive immune system in the eyes, and the nutrient may help prevent the formation of abnormal new blood vessels that scar the retina in late vision-threatening DR.

As yet, though, this is all conjecture. Vitamin D deficiency is not listed as a cause of DR in an American Academy of Ophthalmology article on the subject. And there is also a possibility that the causation moves in the opposite direction: Perhaps people with diabetic retinopathy are less likely to spend time outside in the sun, and that’s the cause of their lower vitamin D levels?

Vitamin D and Diabetes

The broader connections between vitamin D and diabetes have been a focus of intense research for many years. People with type 1 and type 2 diabetes generally have lower levels of vitamin D than the general public, and vitamin D deficiency has been investigated as a potential contributing factor for both conditions.

Vitamin D is important for a huge number of bodily functions and has a very complex relationship with the metabolic system. Many different studies have found that vitamin D supplementation can improve glucose management, but diabetes authorities have been extremely reluctant to validate vitamin D as an effective treatment. Despite an apparent wealth of evidence in favor of prioritizing vitamin D levels, most experts believe that science remains largely unconvincing.

In 2022, after a major study showed that vitamin D supplementation had no impact on bone health — one of the few vitamin D benefits that was widely accepted as legitimate — The New England Journal of Medicine published an editorial (PDF) arguing that doctors should stop screening vitamin D levels and that people should “stop taking vitamin D supplements to prevent major diseases or extend life.”

To be clear, severe vitamin D deficiency can have real consequences, such as rickets and osteomalacia, and supplementation may still be important for certain people, such as infants, breastfeeding mothers, or those with conditions that cause nutrient malabsorption. People with darker skin produce less vitamin D in sunlight and therefore are more likely to have lower amounts of vitamin D in circulation — Black Americans, for example, are at a sharply elevated risk of vitamin D deficiency.

Major authorities such as the Centers for Disease Control and Prevention and the National Institutes of Health continue to treat vitamin D as a nutrient of concern, making broad recommendations for vitamin D intake. On the other hand, there’s also a downside to excessive vitamin D supplementation: vitamin D toxicity.

The Bottom Line

Studies linking low vitamin D levels with diabetes and diabetic retinopathy keep piling up, but we still don’t know if the connection is real. Many experts recommend that people both with and without diabetes should prioritize vitamin D intake — whether through healthy eating, getting sunlight, or using supplements. The evidence of benefits, however, is shaky. If you’re concerned, consider asking your doctor if you should be monitoring your vitamin D levels.