The ocular surface is a system, consisting of various components, which are structurally and functionally connected. Among its constituent elements, the cornea, conjunctiva, lacrimal and meibomian glands, nasolacrimal duct and eyelids are in direct contact with the external environment and act synergistically to protect key structures of visual function. In this context, the production of an efficient tear film plays a very important role. In particular, the lacrimal glands secrete the aqueous component of tears, along with other substances, including proteins protective; the corneal and conjunctival epithelia release mucins, which turn the tears into a muco-aqueous gel and increase lubrication; the Meibomian glands secrete fatty substances, which form the outermost layer of the tear film, preventing its evaporation. Any factor that can disturb the balance of the ocular surface system can interfere with the composition and function of the film tear, resulting in damage to the tissues of the eye.
This is what happens in the dry eye syndrome (DED), a multifactorial disease of the ocular surface system that affects 5% to 30% of individuals over the age of 50 and causes dry eyes. The DED has a more elavoided in the elderly, post-menopausal women, contact lens wearers and patients with autoimmune diseases. DED is characterised by a reduced production or excessive evaporation of tear fluid, causing symptoms such as blurred vision, hypersensitivity to light, irritation, burning and itching, which can limit daily activities, with a negative impact on quality of life.
The first-line treatment of dry eye syndrome involves the application of so-called 'artificial tears', medical devices in eye drops that typically work by improving the condition of the tear film and alleviating symptoms. However, over time, many studies have shown that dietary modifications and the use of nutritional supplements can be adopted to prevent and treat ocular surface conditions such as DED. Specific micronutrients, derived from both food intake and dietary supplements for the eyes, can influence the morphology and function of ocular surface components and metabolic pathways, with a positive impact on ocular surface health.
Let's get to know some of them better.
Essential fatty acids: Omega-3
Omega-3 polyunsaturated fatty acids are essential structural components of cell membranes and precursors for the synthesis of numerous biologically active substances. Omega-3 fatty acids have anti-inflammatory, anticoagulant and antihypertensive properties and help regulate lipid metabolism, glucose tolerance and central nervous system functions. Some of the earliest clinical evidence of the effects of omega-3 fatty acids on ocular surface health came from a large study involving over 30,000 women that demonstrated a relationship between low dietary intake of omega-3 fatty acids and increased risk of DED. Since then, several clinical studies have demonstrated the efficacy of omega-3 fatty acid supplementation in improving DED. The therapeutic utility of omega-3 supplementation has been further confirmed by two recent studies, which concluded that these fatty acids are effective in improving the signs and symptoms of dry eye syndrome.
Vitamin A
The term vitamin A designates both retinol, the most biologically active form obtained from animal sources, and carotenoids, vitamin A precursors found in a wide variety of fruits and vegetables. Vitamin A is necessary for healthy mucous membranes, for the transmission of light impulses by the retina, for bone metabolism and for the health of the immune and reproductive systems. In particular, vitamin A is involved in the metabolism, growth and differentiation of the cells that make up the ocular surface. To understand its importance, one only has to consider that vitamin A deficiency due to malnutrition is one of the main causes of preventable blindness in developing countries. Vitamin A supplementation has been shown to not only improve the health and regeneration of ocular surface tissues, but also to improve tear quality in patients with DED.
Vitamin C
Vitamin C is a water-soluble vitamin required for the functioning of a wide range of enzymes and is found in fruits such as citrus fruits, strawberries, cherries, and vegetables such as tomatoes and broccoli. The tear film contains high levels of vitamin C, reflecting the ocular surface's high need for antioxidant defence, as it is in constant contact with the external environment and exposed to the sun's UV rays, which promote the production of free radicals harmful to the eye. In addition, vitamin C appears to play an important role in corneal wound healing processes. In diabetic patients, vitamin C, in combination with vitamin E, has been shown to improve tear production and tear film stability. and to contribute to the reduction of oxidative stress on the ocular surface.
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