The macula is the central part of the retina, located at the back of our eyes. It is responsible for fine, detailed, central vision and its layers are densely packed with light-sensitive cells (photoreceptors, rods and cones) that transmit nerve impulses through the optic nerve to the brain, where a visual image is formed. This electrochemical transmission of light is how we see.

Retinal photoreceptors, and more specifically the fovea is subjected to oxidative stress throughout our lifetime. Oxidative stress or oxidative damage occurs during the chemical reaction of oxygen with DNA, proteins and lipids.  Harmful and unstable free radicals of oxygen and nitrogen cause damage to healthy cells as these paramagnetic entities move through the body in search of an electron to make them more stable. Research has shown that the damage caused by oxygen and free radicals can lead to increased risk of cancer, heart disease, and eye disease. We are constantly forming free radicals as a normal part of aging, and we increase production of them when we are exposed to harmful environmental challenges from air pollution, pesticides, cigarette smoke,” electromagnetic smog” and solar ultraviolet rays.

Vitamin C, also known as ascorbic acid, is a powerful antioxidant, so one of its most vital roles in our bodies is to help prevent cellular damage from oxidative stress. Vitamin C, like other antioxidants, can stop the cycle of free radical damage by donating an electron to the free radical, neutralizing it, and preventing it from causing further damage. As we age, the body's natural antioxidant production can decline, so it is important to ensure that we are getting enough.

Vitamin C and Age-related Macular Degeneration

The macula is highly sensitive to free radicals and oxidative stress. Unfortunately, the macula degenerates with age and in some people this process of deterioration happens more quickly. This is known as age-related macular degeneration (AMD). As AMD progresses, it can lead to loss of the all-important photoreceptors, known as geographic atrophy, or damaging new blood vessels can begin to grow, leak fluid and scar, known as neovascular or wet AMD. Vitamin C can prevent cellular damage to the photoreceptors in the retina and macula by reacting with, and deactivating, the free radicals. This can prevent deterioration of the macula, leading to devastating vision loss.

Vitamin C, when taken with other essential nutrients, has also been shown to slow down the progression of age-related macular degeneration (AMD). One of the most important studies related to AMD and nutrition is the Age-Related Eye Disease Study (AREDS) study, which showed that when study participants took 500 mg of Vitamin C daily, in combination with beta-carotene, Vitamin E and zinc, their progression of advanced AMD slowed by 25% and vision loss slowed by 19%. [1]

Vascular endothelial growth factor (VEGF) is a driver of increased leakage of fluid through permeable layers of tiny blood vessels in the retina and macula. Some treatments for macular diseases include injections of anti-VEGF drugs inside the eye to decrease the amount of fluid leakage. Early research studies have shown that Vitamin C, or ascorbic acid, can help prevent VEGF-induced increases in blood vessel permeability and leakage. [6]

Vitamin C and Diabetic Macular Edema

People with diabetes are at an increased risk for vision problems related to their retina and macula. Diabetic retinopathy is a complication of diabetes that affects the blood vessels of the retina. High blood glucose levels, over time, can lead to cell death and blockage of the tiny blood vessels that nourish the retina. When the blood supply is cut off, the eye will attempt to grow new blood vessels to deliver needed blood to the retina. Unfortunately, the newly formed blood vessels are poorly developed, very weak and may easily leak blood (hemorrhage) and other liquid (plasma) into the retina. When this fluid leakage occurs into the area of the macula it is called diabetic macular edema, which poses a significant risk for visual loss.

Assessing Vitamin C (ascorbic acid) levels in the blood is one of the best indicators of the amount of oxidative stress occurring in the body. People with diabetic complications, often as a result of high levels of oxidative stress, have been shown to have 60% less ascorbate concentrations in their blood plasma than what they would normally be. People with type 2 diabetes that are experiencing diabetic microvascular disease (damage to small blood vessels throughout the body) and retinopathy have demonstrated lower ascorbate levels than people not having these complications. One study found that people with diabetic retinopathy and Type 1 diabetes had ascorbate concentrations (Vitamin C) in their retina that were only 23% of what is considered to be normal, confirming that the retinas in people with diabetes are under significant amounts of potentially damaging oxidative stress. [2]

An early laboratory study showed that Vitamin C can help to prevent cell death caused by high glucose, an effect that was not noted for other antioxidants in the same study. [4] It was also noted that the Vitamin C tightened a layer (endothelial layer) of the tiny blood vessels that were leaking due to high glucose. [5] Researchers have also conducted animal studies, giving diabetic rats the AREDS antioxidant formulation including vitamin C, and found that the number of poorly developed blood vessels (retinopathy) decreased by nearly 50%. [3] Further studies are underway to confirm these findings in people with diabetes.

How much Vitamin C do I need for macular health?

The daily recommended amount of Vitamin C as per the U.S. Food and Drug Administration for healthy individuals is 90 mg/day for men and 75 mg/day for women. For people that have increased risk factors for certain diseases or disorders, or those under different stressors (smokers, alcoholics, pregnant women, diabetics), more daily Vitamin C is often recommended. As noted, in the AREDS study, people were given the supplement that contained 500 mg of Vitamin C and noted slowing of AMD progression and vision loss.
Our bodies are not able to store Vitamin C, so any excess is excreted in the urine. Some studies have shown that a daily upper limit of Vitamin C of 2000 mg is not likely harmful, but may cause unpleasant symptoms such as diarrhea, nausea, heartburn, cramps, headache and insomnia in some individuals.  

What are the Best Sources of Vitamin C for Eye Health?

Great dietary sources of Vitamin C include colorful fruits and vegetables including kiwi, blackcurrants, peppers (green chili peppers and yellow bell peppers), kale and leafy greens, broccoli, and citrus fruits.

 
Sometimes getting enough Vitamin C from our diets alone can be challenging so you can consider adding a supplement to your daily routine to help to reach the recommended amount to meet your needs. It is important to carefully read product labels and confirm that the supplement is of the highest quality, from a reputable company, and contains appropriate nutrient amounts and no unwanted ingredients such as dyes or fillers.

When considering a nutritional supplement, it is always important to check with your physician or health care professional.

REFERENCES:

[1] Age-Related Eye Disease Study Research Group.  A randomized, placebo-controlled, clinical trial of high-dose supplementation with vitamins C and E, beta carotene, and zinc for age-related macular degeneration and vision loss: AREDS report no. 8.  Arch Ophthalmol. 2001 Oct;119(10):1417-36. https://www.ncbi.nlm.nih.gov/pubmed/11594942

[2] Barba I, Garcia-Ramírez M, Hernández C, Alonso MA, Masmiquel L, García-Dorado D, Simó R. Metabolic fingerprints of proliferative diabetic retinopathy: an 1H-NMR-based metabonomic approach using vitreous humor. Invest Ophthalmol Vis Sci. 2010 Sep; 51(9):4416-21. https://www.ncbi.nlm.nih.gov/pubmed/20375324

[3] Kowluru RA, Kanwar M, Chan PS, Zhang JP. Inhibition of retinopathy and retinal metabolic abnormalities in diabetic rats with AREDS-based micronutrients. Arch Ophthalmol. 2008 Sep; 126(9):1266-72. https://www.ncbi.nlm.nih.gov/pubmed/18779489

[4] May JM. Ascorbic Acid Repletion: A Possible Therapy for Diabetic Macular Edema? Free Radic Biol Med. 2016 May; 94: 47–54. https://www.ncbi.nlm.nih.gov/pubmed/26898503

[5] May JM, Jayagopal A, Qu ZC, Parker WH. Ascorbic acid prevents high glucose-induced apoptosis in human brain pericytes. Biochem Biophys Res Commun. 2014 Sep 12; 452(1):112-7. https://www.ncbi.nlm.nih.gov/pubmed/25152398

[6] Ulker E, Parker WH, Raj A, Qu ZC, May JM. Ascorbic acid prevents VEGF-induced increases in endothelial barrier permeability. Mol Cell Biochem. 2016 Jan;412(1-2):73-9. https://www.ncbi.nlm.nih.gov/pubmed/26590088