The ABC's of cornea health

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The cornea, the ocular window to the world, plays a crucial role in vision. In addition to protecting the eye from outside infiltration and UV radiation, the cornea is responsible for approximately 65% to 75% of the refraction of light as it passes through the eye.¹ The cornea performs the initial refraction onto the lens, which further focuses the light onto the retina. The tear layer is comprised of lipid, aqueous, and mucin components necessary for the maintenance of corneal health, as they offer protection as well as maintain a moist and clean interface. The cornea does not contain any blood vessels, since transparency is needed for the eye’s function. It receives nutrients through diffusion on its external side from tear fluid and the aqueous humor internally.¹ Therefore, maintaining the integrity of the cornea is essential for clear vision, as we expand on the impact of nutrition. In this article, I will detail how nutrition contributes to the homeostasis of corneal health.

Vitamin A

Vitamin A is vital for maintaining the health of the corneal surface. It is considered a fat-soluble vitamin. It helps in the production of mucus, which keeps the cornea moist and free from infections. Deficiency in vitamin A can lead to dry eye syndrome (DES); night blindness; and, in severe cases, corneal ulcers and blindness.² Sources of vitamin A include carrots, sweet potatoes, spinach, mango, eggs, cod liver oil, fortified cereals and milk, and pumpkin.

Testing can reveal vitamin A deficiencies via serum retinol and serum beta-carotene. Zinc is also a necessary nutrient to free vitamin A from the liver.

Vitamin C (ascorbic acid)

Vitamin C is an antioxidant that helps protect the cornea from oxidative stress and supports collagen synthesis, essential for corneal structure and repair. Vitamin C is also key in glutathione production (necessary for cellular health). A diet rich in vitamin C can help reduce the risk of cataracts and other age-related eye diseases while also promoting healing.³ Citrus fruits, strawberries, sweet red pepper, and broccoli are excellent sources of vitamin C.

Testing can include serum vitamin C and white blood cell ascorbic acid. Deficiencies are commonly seen with alcoholism, restricted diets such as ketogenic, gastric bypass issues, and those with increased risk of infection.

Vitamin E

Vitamin E, another powerful fat-soluble antioxidant, protects the eye from oxidative damage caused by free radicals. It also works synergistically with other antioxidants like vitamin C and beta-carotene to enhance their protective effects. Vitamin E has also been shown to have a protective effect against the corneal and conjunctival damage caused by vitamin A deficiency.⁴ Olive oil, nuts, seeds, eggs, dark leafy greens, and avocados are rich in vitamin E.

Testing can include alpha-, gamma-, and delta-tocopherols. Deficiencies in vitamin E can result in peripheral neuropathy, muscle weakness, retinopathy, and cataracts.

Vitamin D

Vitamin D, a fat-soluble vitamin, affects a range of physiological functions including the control of metabolism, bone formation, and immunity.⁵ In addition, keratoconus (KC) prevalence is positively associated with vitamin D deficiency. In a recent study, a positive correlation was linked with vitamin D supplementation on systemic biomarkers of collagen degradation, inflammation, oxidative stress, and copper metabolism in adolescent patients with KC.⁶ Vitamin D supplementation was found to inhibit systemic collagen degradation by significantly reducing matrix metalloproteinase-9, a key collagenolytic enzyme, in plasma after vitamin D supplementation.

Vitamin D can be tested through blood work. Deficiencies in vitamin D can be caused from little exposure to sunlight or poor absorption. Studies have shown that proper use of a daily broad-spectrum sunscreen with high UV-A protection will not compromise vitamin D status in healthy people.⁷ Therefore, many who do not get adequate sun exposure are candidates for supplementation.

Zinc

Antioxidant zinc is crucial for the metabolism of vitamin A and the maintenance of a healthy retina and cornea. It is particularly concentrated in the corneal epithelium and posterior stroma.⁸ Deficiency in zinc can lead to impaired vision and increased susceptibility to infections with impaired wound healing. Foods high in zinc also have high levels of phytates, which bind zinc. These include oysters, meat, shellfish, herring, root vegetables, legumes, seeds, and nuts. However, zinc supplementation must be done cautiously because excessive zinc can interfere with the metabolism of copper.

Common tests to reveal low zinc levels include plasma zinc or Rutherford backscattering spectrometry zinc.

Omega-3 essential fatty acids

Omega-3 polyunsaturated fatty acids, particularly docosahexaenoic acid and eicosapentaenoic acid, are essential for maintaining the health of cell membranes, including those in the cornea. They contain anti-inflammatory properties that can help manage conditions like DES with a positive relationship between the systemic omega-3 index and corneal nerve status.⁹ They also play a key role in neural development, maintenance, and function, as their metabolites have neuroprotective properties and can confer benefit in neurodegenerative disease.¹⁰ Fish such as salmon, mackerel, and sardines are rich sources of omega-3s. For vegetarians, flaxseeds, chia seeds, and walnuts are good alternatives.

Lutein and zeaxanthin

These carotenoids are found in high concentrations in the retina and the lens of the eye. They help filter harmful high-energy blue wavelengths of light and act as antioxidants to protect the eye tissues. A new study showed a statistically significant reduction in ocular surface damage and inflammation of the tear film in those suffering with dry eye disease.¹¹ Lutein and zeaxanthin can be found in green leafy vegetables like kale, spinach, and collards, as well as in egg yolks, turmeric, and corn.

Hydration

Proper hydration is crucial for maintaining the moisture balance in the cornea. The cornea is made up of several layers, the largest being the stroma, which occupies approximately 90% of the total corneal thickness.¹² Plus, the cornea is composed chiefly of water, collagen, proteoglycans, and keratocytes.¹² Therefore, drinking adequate fluids ensures proper integrity of the cornea. It is recommended to consume half the number of pounds of one’s body weight in ounces of water daily, and to increase fluid intake during moderate exercise or heat exposure.

Oxygen

Corneal heath relies on avascular oxygen supply. Under normal conditions, oxygenation of the anterior cornea is achieved by exposure to the atmosphere when the eye is open and by exposure to the palpebral conjunctiva when the eye is closed. Insufficient oxygen exposure can result in hypoxia and corneal edema.¹³

Collagen peptides

The cornea is a highly collagenous structure in which several types of collagens are distributed across 5 primary layers.¹⁴ Currently, riboflavin-mediated collagen cross-linking, first approved by the FDA in 2016, remains the only approved treatment to halt progressive corneal thinning associated with KC by improving the biomechanical properties of the stroma.¹ Collagens represent the major component of the extracellular matrix (type I) and basement membranes (type IV) contributing to 90% of the structural basis necessary for corneal stability. It plays a key role in organ development, wound healing, and tissue repair and is found throughout the many layers of cornea tissue. Type I collagen mimetic peptides have been shown to specifically target areas of collagen disruption associated with tissue wounds with statistically significant reduced healing times. Although therapeutic agents are currently unavailable, the research remains promising.

Future treatments and nutritional supplementation are continually being studied with promising results for corneal disease including but not limited to nicotinamide (derivative of vitamin B3), stem cell–based therapy, cannabidiol, and the amino acid arginine.^15-18

As we consider the advancements in the treatment of corneal disease, we have been fortunate to utilize amniotic-derived membranes, hyaluronic acid–based treatments, plasma and autologous serums, and manuka honey–based cleansers, while even more holistic treatments are coming down the line.

Conclusion

As we witness a shift into a “food is medicine” practice mindset, promoting a well-balanced diet rich in fruits, vegetables, lean proteins, and healthy fats provides a comprehensive range of nutrition to support overall eye health, including the cornea. Limiting the intake of processed foods, sugars, and unhealthy fats can also reduce the risk of developing chronic diseases that can adversely affect eye health through inflammatory pathways.

Nutrition has a positive influence in healing the eye following an infection or surgical procedure, so prescribing proper nutrition can improve healing times when the integrity of the cornea is compromised. A diet rich in vitamins A, C, and E and minerals like zinc, combined with omega-3 fatty acids and carotenoids like lutein and zeaxanthin, can significantly contribute to the health and longevity of the cornea, while not forgetting proper hydration as a key fundamental element in supporting eye health. By encouraging informed dietary choices, patients can ensure improved vision quality.

References:

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12. Hayes S, White T, Boote C, et al. The structural response of the cornea to changes in stromal hydration. J R Soc Interface. 2017;14(131):20170062. doi:10.1098/rsif.2017.0062

13. Leung BK, Bonanno JA, Radke CJ. Oxygen-deficient metabolism and corneal edema. Prog Retin Eye Res. 2011;30(6):471-492. doi:10.1016/j.preteyeres.2011.07.001

14. Baratta RO, Schlumpf E, Del Buono BJ, DeLorey S, Calkins DJ. Corneal collagen as a potential therapeutic target in dry eye disease. Surv Ophthalmol. 2022;67(1):60-67. doi:10.1016/j.survophthal.2021.04.006

15. Tran VN, Strnad O, Šuman J, et al. Cannabidiol nanoemulsion for eye treatment – anti-inflammatory, wound healing activity and its bioavailability using in vitro human corneal substitute. Int J Pharm. 2023;643:123202. doi:10.1016/j.ijpharm.2023.123202

16. Li Z, Duan H, Jia Y, et al. Long-term corneal recovery by simultaneous delivery of hPSC-derived corneal endothelial precursors and nicotinamide. J Clin Invest. 2022;132(1):e146658. doi:10.1172/JCI146658

17. Li Z, Duan H, Li W, et al. Nicotinamide inhibits corneal endothelial mesenchymal transition and accelerates wound healing. Exp Eye Res. 2019;184:227-233. doi:10.1016/j.exer.2019.04.012

18. McKay TB, Priyadarsini S, Rowsey T, Karamichos D. Arginine supplementation promotes extracellular matrix and metabolic changes in keratoconus. Cells. 2021;10(8):2076. doi:10.3390/cells10082076