Vitamin D helps the body absorb calcium and phosphorus, making your bones and teeth healthy. Besides this well-known role, there is significant evidence that the sunshine vitamin is actively involved in regulating immunity – and thus helps you fight a range of infectious diseases, reduce the severity of asthma and allergies; and at the same time lowers the risk of autoimmune disorders.
Emerging research has also found a close link between vitamin D deficiency and conditions like cardiovascular disease, cancer, diabetes and also complications associated with diabetes.
In this blog, we are going to explore the role of vitamin D in eye health. It appears that its anti-inflammatory properties, along with many other effects it has on the body, may help in reducing the risk of all kinds of eye diseases such as age-related macular degeneration, diabetic retinopathy, myopia (near-sightedness) and dry eyes.
Even more importantly, low levels of vitamin D has been closely implicated with a higher risk of developing these conditions. So, what is this connection between vitamin D and eye health? Let’s find out.
1. Vitamin D and macular degeneration
Macular degeneration is the leading cause of vision loss in the elderly. In this condition, the macula – a small spot in the centre of the retina – is damaged. It is also called age related macular degeneration (AMD).
The macula is made of millions of light-sensitive cells (rods and cones), which are responsible for sharp, detailed vision along with central vision. Damaged macula results in blurry or distorted central vision. This impacts your ability to perform simple day to day activities like reading, writing, driving, cooking and recognizing faces. AMD also reduces your ability to see colors.
In order to understand the role of vitamin D, it is important to understand various stages of AMD:
- Marked by the presence of small or medium sized drusen – round yellow deposits of fats and proteins that build up in the eye.
- Does not cause any symptoms or vison loss
- Number and size of drusen increases
- Symptoms are noticeable and include blurred spots in the central vision and a need for more light.
- Larger drusen size increases the risk of developing wet macular degeneration, a more advanced form that can lead to sudden loss of central vision.
- Marked by gradual breakdown of the light-sensitive cells in the macula and supporting tissue. The blurred spot in the central vison becomes bigger and darker. This may lead to loss of vision over time. This is called Dry AMD. The majority of the patients are diagnosed with dry AMD.
- For some people, dry AMD may progress into a more advanced form of AMD, which is marked by the growth of new blood vessels (angiogenesis) and scarring of retinal tissue (fibrosis). This is called Wet AMD. Vision loss caused by such damage is permanent and can no longer be reversed with surgery or injections. Symptoms include blurred vision and the presence of grey spot in the central field of view. People with wet AMD also find straight lines seem wavy or curvy.
While age, genetics and environmental factors like smoking are involved in the development of AMD, recent research suggest that inflammation may play a big role in its development. Inflammatory molecules have been found within drusen, supporting the central role of inflammation.
Studies show that low levels of vitamin D increase one’s risk of early and/or late AMD.   In addition, vitamin D is believed to play a tremendous role in fighting inflammation, oxidative damage, angiogenesis (growth of new blood vessels) and fibrosis. These properties may help in preventing the development and progression of AMD.   . In addition, a 2015 study suggested that maintaining healthy vitamin D levels may help reduce some of the risk, if you are genetically prone to develop macular degeneration. 
Unfortunately, AMD is incurable. Since people typically don’t experience any vision loss initially, it is important to be regular with your eye exam. This is especially important if you are at risk.
2. Vitamin D and diabetic retinopathy
People with diabetes are at an increased risk of developing microvascular complications, such as diabetic retinopathy (DR). In fact, DR is one of the most common complications associated with diabetic patients with poorly controlled blood glucose levels. If not treated well and in time, it can even lead to blindness.
High blood sugar levels are known to damage tiny blood vessels in the retina. These vessels are responsible for providing retina with oxygen and nutrients.
- In the initial stages, these blood vessels develop small areas of swelling, making them prone to leaking fluid in the retina.
- As the disease progresses, the blood vessels begin to swell and lose their ability to supply oxygen rich blood to the retina.
- In later stages, retina releases growth factors that trigger the formation of abnormal blood vessels. This process is called neovascularization and it is the body’s way of restoring oxygen supply to the retina. However, these fragile new vessels often rupture, leaking the blood into the vitreous gel, fluid that fills the middle of the eye. All these dangerous changes within the eye can lead to blurred vision and even complete vision loss if left untreated. This situation can be prevented by controlling blood sugar levels and having your eyes regularly screened.
What’s more, emerging research shows that low vitamin D status is closely related to DR.  This 2017 meta-analysis found that vitamin D deficiency may increase the risk of diabetic retinopathy in patients with type 2 diabetes. 
Another study concluded that “A low serum 25-OHD level was an independent predictor of HbA1c, diabetic neuropathy, and diabetic retinopathy in patients with DM2.” 
Vitamin D and diabetes
Poorly controlled blood glucose levels and high blood pressure are strong risk factors for retinopathy. And vitamin D’s role in controlling blood glucose and blood pressure is well-established.
This study published in Cardiovascular Diabetology concluded that a healthy vitamin D status (≥75 nmol/L) may be effective in reducing the risk of diabetic retinopathy. The researchers speculated that vitamin D’s ability to regulate blood glucose may be the reason for this effect. 
A more recent study suggested that higher the levels of vitamin D in the blood, lower is the risk of developing diabetes.  While this is not the first study to bring across this association, researchers believe more studies are needed to confirm the role of vitamin D in reducing the risk of diabetes.
A 2018 study found that lower vitamin D levels are associated with higher blood glucose in Asian Indian women with pre-diabetes. People with pre-diabetes – with abdominal obesity, insulin resistance and metabolic syndrome – are at an increased risk of developing type 2 diabetes as well as cardiovascular disease. 
Another 2018 meta-analysis concluded that “Vitamin D supplementation and improved vitamin D status improved glycemic measures and insulin sensitivity and may be useful as part of a preventive strategy for type 2 diabetes.” 
Diabetic retinopathy may not produce any noticeable symptoms in early stages. It is, therefore, even all the more important to detect the condition early before it leads to complete loss of vision. Going by the link between low vitamin D levels and increased risk of DR, screening for vitamin D levels may be a useful tool in catching the problem early enough so as to treat it on time.
3. Vitamin D and myopia
People with myopia can see close objects clearly, meaning they can read things up close like a book or computer screen. But farther objects appear blurred. Myopia is also known as near-sightedness or short-sightedness.
In this condition the eye grows too long from front to back. As a result, the rays of light can’t focus properly on the retina – a thin layer of light-sensitive issue in the back of the eye. Normally, cornea and lens focus rays of light on retina, which senses the light and converts it into signals that are carried to the brain. Your brain then interprets these signals as the images you finally see. In myopia, the eye is too long, and the lens focuses light in front of the retina, instead of focusing it on the retina. This results in a blurry image.
Genetics play a major role in myopia but certain factors like spending too much time indoors and doing near work also increase the risk. So, people who read a lot and spend a lot of time on computers or phones are likely to develop myopia.
How is vitamin D deficiency related to myopia?
Low vitamin D levels are associated with an increased risk of myopia, as shown by a number of studies. 
Can playing outdoors help reduce myopia risk? New research suggests that increased outdoor time can prevent the onset of myopia in young children. There are many theories on why playing outdoors may help. For example, playing in the sunlight helps the body make more vitamin D, which may play a role in preventing abnormal eye growth in children. Another theory suggests it is the brighter light that is the helpful element. Brighter light outdoors stimulates the cells in the retina to release dopamine, that in some ways helps in slower, normal eye growth. 
Another study showed that myopic children spent more time indoors, had lower vitamin D status, had a higher body mass index and played less sports than children who didn’t have myopia. 
This 2016 study showed that low vitamin D levels in blood are linked with longer axial length and increased risk of myopia in a group of 6-year-old children. Interestingly, this effect was found to be unrelated to outdoor exposure, suggesting that a lack of vitamin D may have a direct role in the development of myopia. 
4. Vitamin D and dry eye
Dry eye is a condition where your eyes do not have enough moisture or tears. What usually happens is that when you blink, a layer of tears spreads over the surface of your eye, known as the cornea. Tears keep the eye’s surface moist and also wash away any dust or micro-organisms that could increase the risk of eye infections. You need a healthy layer of tears at all times for healthy eyes and clear vision.
If you have symptoms like dryness, redness and burning sensation in the eyes, increased sensitivity to light, and blurred vision, chances are you may have dry eyes. Another important symptom of a dry eye to which you should pay attention is if your eyes are becoming tired at a faster pace than previously.
Recently, many studies have suggested that vitamin D deficiency may cause dry eye syndrome.     Since inflammation is often the underlying cause of dry eyes, this association makes sense. Studies show that low vitamin D levels may cause dry eye and impaired tear function . And this same study further reported that vitamin D may play a protective role here due its ability to reduce inflammation as well as enhance tear film functions. 
This 2016 study concluded that vitamin D supplementation is an effective strategy for patients with dry eye syndrome that is not responding to conventional treatment.  Detailing the mechanisms, it reported that vitamin D supplementation:
- Stimulated the secretion of tears
- Reduced tear instability
- Lowered inflammation
- Improved the symptoms of dry eye syndrome
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