Feed the Eye to Heal the Eye

INTRODUCTION Nutrition is of paramount importance for the visual system. Proper nutrition helps make better use of your eyesight and reduces the appearance of degenerative and nondegenerative eye diseases. Therefore, nutrition can and should be refined on the basis of ocular pathologies that are in the process of appearing or already present and to promote their treatment.1,2 MICROBIOTA AND MICROBIOME Eating, however, means modifying our microbiota and, consequently, our microbiome. The microbiota indicates the population, in the order of a few billion, of microorganisms (archaea, bacteria, yeasts, viruses, and fungi) present throughout our body, on the dermis and in all our cavities and which, in the right balance between symbionts (microorganisms with beneficial action) and pathobionts (microorganisms that cause diseases), necessary for our well-being; on the other hand, the microbiome refers to the totality of the genetic heritage possessed by the microbiota, i.e., the genes that the latter is able to express. The microorganisms of the gut microbiota help us assimilate food, protect us from many diseases, and produce substances that are very useful for our metabolism including vitamins such as Vitamin D, short-chain fatty acids (SCFAs), certain hormones, and neurotransmitters. Today, the gut microbiota is an important therapeutic target for many inflammatory, autoimmune, neurodegenerative, and many other diseases that scientific studies are progressively identifying. The composition of the individual microbiota and the gut microbiota in particular is influenced by pregnancy, natural or cesarean delivery, breastfeeding or artificial feeding, weaning, excessive hygiene, stress, exercise, drug therapies, and above all nutrition. With reference to drugs, it is very important to underline that antibiotics, if on the one hand they prevent the proliferation of pathogens and the development of infectious diseases, on the other hand they compromise the normal bacterial population, which resides mainly in the intestine, where they play a fundamental role in maintaining the health of the host organism. However, nutrition exerts one of the main influences on the balance of the microbiota, since the diet represents a source of essential nutrients for the human body not only to support growth, vital, and reproductive functions, but also to modulate and support the various microbial communities and, in particular, the gut microbiota. The choice of a type of food, its quality characteristics, and even its origin shape our gut microbes, influencing their composition and function, with repercussions on host-microbe interactions Figure 1.3-5Figure 1: Graphic abstractTHE MEDITERRANEAN DIET In particular, the Mediterranean diet (MedDiet) has a high prebiotic potential, as it involves a high consumption of legumes, vegetables, whole grains, and fruits, the same “healthy” carbohydrates, in particular nondigestible carbohydrates such as fiber and resistant starch. Prebiotics are, in fact, nondigestible substances naturally contained in some foods, mainly water-soluble and nongeling fibers including nonstarchy polysaccharides or beta-glucans, fructans, fructooligosaccharides, inulins, lactitol, lactosucrose, lactulose, pyrodextrins, and soy oligosaccharides, which promote the growth, in the colon, of one or more bacterial species useful for the development of probiotic microflora.6-10 The MedDiet (from the Greek “δίαιτα” = lifestyle), which became UNESCO's definitive intangible cultural heritage in 2010, is currently considered the best in terms of health, based on countless experimental evidence. The Mediterranean model is characterized by a relatively high consumption of total fats, especially extra virgin olive oil, a moderate consumption of wine with meals, low in saturated fats but rich in vegetables, fruits, nuts, legumes, and, in particular, fermented dairy products not processed from cereals, nutrients and dietary fiber, antioxidant compounds and bioactive elements with anti inflammatory effects ensure a number of beneficial effects. For example, it allows one to maintain a low glycemic index in general and all those healthy properties that contribute to the achievement and maintenance of a healthy body weight, increasing longevity, reducing the risk of chronic diseases, including cardiovascular disease and metabolic syndrome; therefore, type 2 diabetes, obesity, certain cancers, and last but not least, cognitive impairment. 11 MedDiet has a high prebiotic potential, with its high consumption of legumes, vegetables, whole grains, and fruits, the same “healthy” carbohydrates, especially nondigestible carbohydrates such as fiber and resistant starch, affect microbial composition and diversity, can stimulate the growth of beneficial bacterial species involved in the production of butyrate and methane, which scientific data have associated with an improved cardiometabolic profile.12 As far as fats are concerned, EVO oil, obtained from the first cold pressing of the ripe fruit, is the only one suggested by MedDiet; it contains hydroxytyrosol and tyrosol, oleocanthal, resveratrol and many other bioactive phenolic dietary compounds with antioxidant and phytochemical properties, tocopherols, polyphenols and phytosterols with antioxidant and anti-inflammatory action.13 These properties represent a nutritional approach of choice to combat various diseases, including eye diseases, with an inflammatory component, mainly because recent validated scientific data highlight the ability of specific nutrients to cross the retinal barrier.14 As far as proteins are concerned, it is advisable to prefer white meats and fish, eggs, and some cheese. Fresh and frozen fish, possibly 2–3 times a week, checking its origin to rule out the possibility that it may contain pollutants and harmful metals, such as mercury. For frozen foods, ensure that they are purchased from places where there is a cold chain guarantee. For meat, white meats, such as chicken, turkey (preferably free-range), and rabbit, should certainly be preferred, minimizing the consumption of red meats, such as beef and veal, and almost always eliminating visible fat. In fact, excessive consumption of red meat (more than three to four times a month), such as the use of carnitine in gyms, causes the production of trimethylamine in the intestine, a molecule that passes into the liver where it is metabolized into trimethylamine N oxidase, which is very harmful to the cardiovascular system, renal, and cerebral. Its high level is considered a predictive biomarker of cardiometabolic diseases, cancer, and even myocardial infarction.12 On the other hand, we can also consume 2–3 eggs a week (except for individual pathological conditions), spread over different days; possibly from free-range hens to have more Vitamin D in the yolk, without worrying too much about a possible increase in cholesterol, which, however, should not fall below 190 mg/ml of plasma, as it is necessary at the level of the lipid bilayer of cell membranes, as a precursor of steroid hormones, to produce Vitamin D at the epidermal level from solar ultraviolet rays. If you drink milk, it is recommended to choose skimmed or semi-skimmed milk and possibly prefer goat's milk, with A2 casein and not cow's milk with proinflammatory A1 casein. Consume cheese in small amounts, due to its high fat content, and prefer low-fat cheeses, such as grilled cheese. Ultra-processed, processed, packaged, and shelf-stable foods should be avoided most of the time because they are high in additives, preservatives (often harmful synthetics), and process contaminants. However, they are poor in vitamins and other essential micronutrients, but also in fiber and phytoestrogens, which perform various protective functions, and contain large amounts of sugars, salts, and fats. These foods are harmful to our health, although they are still very popular, also because they are more practical and cheaper. Their frequent consumption is also associated with increased salt excretion and alters the Na/K ratio, increasing it.12-15 THE EYE DIET For both overall eye health and specific diseases such as macular degeneration, there are many useful vitamins and minerals. For those suffering from macular degeneration, studies on age-related eye diseases recommend dietary supplementation with minerals and antioxidants, including lutein, zeaxanthin, Vitamin C, Vitamin E, zinc, and copper. To maintain overall eye health, a diet rich in minerals and antioxidants is recommended. The best sources for these are leafy greens, such as kale, spinach, kale, and turnips, as well as fruits, berries, and other colorful vegetables. Supplements with zinc, selenium, lutein, and Vitamins A, C, and E are also good. A diet rich in ω-3 fatty acids and low in ω-6 fatty acids is also beneficial, especially for those suffering from dry eyes, as ω-6 are proinflammatory and can contribute to the inflammatory component of dry eyes. The latter are generally found in meat, oils, margarines, and carbohydrates, which make up a large part of most Western diets. ω-3 have anti-inflammatory properties and can help the tear glands produce high-quality tears. These can be taken as a supplement or through a diet rich in nuts and fish, such as salmon. Although flaxseed oil supplements are good, a high-quality fish oil with a high concentration of EPA and DHA is the best option, as it is easier for our body to use.16-19 The use of aclolics, which causes a delay between the brain and the eyes due to the slow pace of communication between neurotransmitters in brain coordination, weakens the eye muscles. Excess alcohol consumption can also cause dry eyes and eyelid twitching. The long-term effects of alcohol abuse can have detrimental consequences on vision and eye health, such as toxic amblyopia, which is the result of a toxic reaction in the optic nerve that causes permanent vision loss. However, excessive alcohol can also increase the risk of age-related macular degeneration: The World Health Organization report revealed that a person's average daily consumption is 33 g of pure alcohol, which is equivalent to two glasses of wine, and an Australian study revealed that drinking more than 20 g of alcohol per day leads to a 20% increase in the chances of developing early macular degeneration compared to those who do not consume alcohol. Poor diet and alcohol may also be related to the development of cataracts. Not forgetting that prolonged alcohol abuse will eventually affect our vision through vitamin deficiency, as the liver can only process a certain amount of alcohol at a time, and excessive consumption can affect the absorption of vitamins in the liver, which are necessary to maintain healthy eyes and good vision.20-23 Seasonality, biodiversity, nutrient density, and the use of a variety of traditional and local food products, as well as culinary traditions, are important elements of the MedDiet, which, being predominantly plant-based, is sustainable on an individual level, but also sustainable for the planet, ensuring well-being for future generations, as it implies a lower impact on water and energy, as well as lower land consumption and lower greenhouse gas emissions compared to other diet models.24-29 In addition, it is necessary to take into account the correct cooking practices and treatments (soaking, cooking at 100°C, sprouting, etc.) useful to eliminate the antinutrients present in the various foods, as well as considering the already known knowledge on the effects of different types of nutrition on the main eye diseases Table 1.Table 1: Impact of diet on the main eye diseasesIt should also be remembered that diet influences epigenetics, i.e. those changes not in DNA and chromatin, which are heritable and closely related to our well-being. For this reason, before opting for this or that diet, it is certainly more advantageous to focus on a correct lifestyle, choosing the right diet, abstaining from excessive smoking and alcohol consumption, allowing nutrients to better modulate the biological processes of our body.24 In this scenario, calorie restriction plays an increasingly important role, that is, the reduction of about 40% of normal caloric intake contributes to the control of weight loss, the control of cholesterol and triglyceride values, and the reduction of age-related diseases. Moreover, MedDiet seems to be the only one that can mimic the effects of calorie restriction.30-34 THE OCULAR MICROBIOME Numerous bacterial microhabitats coexist in the human eye, the composition of which reflects the levels of exposure to the external environment. Studies published in the literature have shown that the genus most represented on the ocular surface is Corynebacterium, followed by Staphylococcus, Streptococcus, Acinetobacter, and Pseudomonas. In particular, a study,35 recently published in the ocular surface, analyzed the structure and distribution of bacterial communities in the various microhabitats of the human eye, to identify similarities and differences. The four ocular sites analyzed were: Eyelid margin tissue of patients with eyelid abnormalities Conjunctival tissue of the fornices and limbus of patients with pterygia Swabs of the ocular surface (conjunctival) swabs Swabs from the skin of the face. The study proposed a classification of the ocular bacterial distribution into three groups: Group 1: Consisting of organisms that reside on the skin and eyelid margin and then move to the ocular surface, as indicated by the low relative abundance at this site. The genera belonging to this group are Corynebacterium and Staphylococcus. The former was, in fact, constantly present on the outer surface of the skin, and its relative abundance progressively decreased as it moved from the skin (12.2%), to the eyelid margin (7.6%), and to the ocular surface (4.0%), but was rarely present in conjunctival tissue samples. Similarly, common skin-resident Staphylococcus was abundant on the skin surface (15.1%), decreased at the eyelid margin (1.9%), and was present in relatively small amounts on the ocular surface (3.1%) Group 2: Consisting of microorganisms located mainly on the ocular surface and detected at low levels on the skin, eyelid margin, and conjunctiva. 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