A large number of studies, both in vitro and in vivo, have demonstrated the beneficial effect of certain dietary components (which can be taken through diet or supplements) on ocular surface health. Indeed, there is much evidence to support the possible role of various micronutrients and nutraceuticals in the treatment of ocular surface diseases.
Ocular surface and dry eye syndrome
The ocular surface is a delicately balanced system made up of various components that are interconnected both structurally and functionally. It represents a direct connection between the eye and the external environment and, therefore, all its components act synergistically to keep the eye healthy and protect the key structures of vision from external pathogens. Both of these goals are achieved, inter alia, through the production of an efficient tear film. Any factor that disrupts the homeostasis of the ocular surface system can alter the stability and osmolarity of tears, leading to tissue damage through osmotic, mechanical and inflammatory processes.
One of the conditions in which ocular surface homeostasis is altered is dry eye syndrome, a multifactorial ocular surface disease that affects up to 30% of individuals over the age of 50. It has been, in fact, detected a higher incidence in older people, postmenopausal women, contact lens wearers and patients with autoimmune diseases.
Dry eye syndrome can be due to decreased tear production or increased evaporation. Both of these conditions lead to hyperosmolarity and subsequent inflammation of the ocular surface.
Symptoms of dry eye syndrome, such as blurred vision, photo-sensitivity, irritation, burning and itching, can limit daily activities and have a negative impact on quality of life.
The role of essential fatty acids in ocular surface health
Strong scientific evidence demonstrates the effectiveness of omega-3 fatty acid intake, either through diet or supplements, in improving dry eye syndrome symptoms and ocular surface health. One of the first clinical trials came from a large cross-sectional study involving over 30,000 women that demonstrated a relationship between low dietary intake of omega-3 fatty acids and an increased risk of dry eye syndrome.
Omega-3s are key structural components of cell membranes and precursors for the synthesis of numerous biologically active substances. The main omega-3 fatty acids include short-chain alpha-linoleic acid and long-chain eicosapentaenoic acid, docosapentaenoic acid and docosahexanoic acid. While short-chain omega-3 fatty acids are obtained from plant sources, long-chain omega-3 fatty acids are obtained from oily fish and can be synthesised by elongating short-chain fatty acids. Omega-3s exhibit anti-inflammatory properties that they exert through competitive inhibition with arachidonic acid as a substrate for the enzymes cyclooxygenase and 5-lipoxygenase.
It has been reported that omega-3 fatty acids may also have a neuroprotective effect, which is of great clinical interest to ophthalmologists. Indeed, corneal nerves are essential for tear production, for the protective blink reflex and for the release of trophic neuromodulators that maintain the vitality and metabolism of ocular surface tissues.
The role of vitamins in ocular surface health
Vitamin A
The term vitamin A includes retinol, the most biologically active form, of animal origin, and carotenoids, precursors found in a wide variety of fruits and vegetables. Vitamin A is necessary for a whole range of functions and, in the eye, for mucosal health, retinal phototransduction, metabolism, growth and differentiation of the ocular surface epithelium. In fact, vitamin A deficiency, due to malnutrition, is one of the main causes of blindness in developing countries. A recent clinical study showed that taking vitamin A, either through diet or supplements, improves tear quality in patients with dry eye syndrome, promoting ocular surface health.
Vitamin B-12
Vitamin B12 is a cofactor in DNA synthesis and is involved in the metabolism of fatty acids and amino acids. It is found in animal products including meat, milk, eggs, fish and shellfish. This micronutrient plays a key role in myelin synthesis and its deficiency is associated with myelopathy, peripheral neuropathy, neuropsychiatric syndromes and optic atrophy. In recent years, the role of neurosensory abnormalities in the pathophysiology of dry eye syndrome has been recognised, and two recent studies have demonstrated an improvement in symptoms in patients with a severe form following vitamin B12 supplementation in the form of eye drops.
Vitamin C
Vitamin C is a water-soluble vitamin required for the functioning of a wide range of enzymes and is found in fruits and vegetables such as citrus fruits, strawberries, cherries, tomatoes and broccoli. The tear film contains high levels of vitamin C, reflecting the high demand for this nutrient by the ocular surface for antioxidant defence. In addition, vitamin C appears to play an important role in corneal wound healing processes.
Vitamin D
Vitamin D is a fat-soluble vitamin that can be acquired through specific food intake or produced in the skin following exposure to sunlight. Vitamin D deficiency has recently been associated with the pathogenesis of dry eye syndrome. Indeed, vitamin D plays an immunomodulatory role and regulates cell proliferation, differentiation and apoptosis, thus enhancing the corneal epithelial barrier. Moreover, by promoting surfactant production, it increases the lipid component of the tear film. Finally, it modulates systemic calcium absorption, which plays a crucial role in maintaining fluid secretion from the lacrimal glands. Serum vitamin D levels have shown significant correlations with tear production and dry eye syndrome symptoms. For all these reasons, vitamin D has been investigated as a potential therapy for this condition.
Dry eye syndrome and other micro-nutrients
Selenium and lactoferrin
Selenium is an essential micronutrient found mainly in meat, fish, seafood and cereals. The human body contains several selenoproteins that are essential for development and metabolism. In particular, glutathione peroxidase protects cells from oxidative stress and its expression is reduced in patients with dry eye syndrome. In contrast, selenoprotein P is a selenium transport protein produced by the lacrimal gland and secreted in tears to supply selenium to the corneal epithelium. In dry eye syndrome, levels of selenoprotein P in the tears are also reduced. Lactoferrin is an iron-binding glycoprotein present in most exocrine fluids, including tears, which serves to protect the corneal epithelium from ultraviolet irradiation. The concentration of lactoferrin has also been found to be reduced in dry eye syndrome, and oral supplementation in patients results in a significant improvement in symptoms.
Curcumin
Curcumin is a polyphenol isolated from Curcuma longa, which is widely used as a spice. It is known to have anti-inflammatory, antioxidant, anti-angiogenic, healing and antimicrobial properties. Curcumin helps restore ocular surface homeostasis by reducing reactive oxygen species, decreasing the expression of inflammation mediators and increasing neurotrophic factors. Therefore, studies have identified curcumin as a promising candidate for the treatment of dry eye syndrome.
Bibliography:
Dr. Carmelo Chines
Direttore responsabile
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