Vitamin A (retinol) is an essential nutrient needed in small amounts by humans for the normal functioning of the visual system; growth and development; and maintenance of epithelial cellular integrity, immune function, and reproduction. These dietary needs for vitamin A are normally provided for as preformed retinol (mainly as retinyl ester) and pro-vitamin A carotenoids (Fig. 1.1).
Preformed vitamin A is found almost exclusively in animal products, such as human milk, glandular meats, liver and fish liver oils (especially), egg yolk, and whole milk and dairy products. Preformed vitamin A is also used to fortify processed foods, that may include sugar, cereals, condiments, fats, and oils.1
Pro-vitamin A carotenoids are found in green leafy vegetables (e.g. spinach, amaranth, and young leaves from various sources), yellow vegetables (e.g. pumpkins, squash, and carrots), and yellow and orange noncitrus fruits (e.g. mangoes, apricots, and papaya). Red palm oil produced in several countries worldwide is especially rich in pro-vitamin A.2 Some other indigenous plants also may be unusually rich sources of pro-vitamin A. Such examples are the palm fruit known in Brazil as burití, that is found in areas along the Amazon (as well as elsewhere in Latin America),3 and the fruit known as gac in Vietnam, that is used to color rice, particularly on ceremonial occasions.4 Foods containing pro-vitamin A carotenoids tend to be less biologically available but more affordable than animal products. It is mainly for this reason that carotenoids provide most of the vitamin A activity in the diets of economically deprived populations.
Absorption, Storage and Transport
In the stomach, retinyl esters and other carotenoids are subjected to the action of proteolytic enzymes, are separated from the food and are aggregated into globules together with other lipids. In the intestine, retinyl esters are hydrolyzed by enzymes found in enterocytes, are incorporated into the micelles which are formed through biliary secretions, and finally absorbed.5 In physiological amounts, retinol is more efficiently absorbed than carotenoids; retinol absorption is approximately 70-90%, while the absorption of carotenoids is only 20-50%.6 By increasing the intake of these substances, we still have a highly efficient retinol absorption (60-80%) in sharp contrast to a plummeting carotenoid absorption of less than 10%.7
- Gene transcription
- Immune function
- Embryonic development and reproduction
- Bone metabolism
- Skin health
- Reducing risk of heart disease
- Antioxidant activity.
Vitamin A Deficiency and Xerophthalmia
The effects of vitamin A deficiency on the eye will be a summary of what Alcides da Silva Diniz and Leonor Maria Pacheco Santos published under the name: Vitamin A deficiency and xerophthalmia in J Pediatr (Rio J) in 2000.
The expression ‘vitamin A deficiency’ is used to describe states of subclinical deficiency of vitamin A, while ‘xerophthalmia’ designates the group of ocular signs and symptoms related to this deficiency. Night blindness was first described in Egypt around 1500 BC; the oldest medical paper known in the Western world, called Eber's papyrus (1600 BC), stated that people affected by day blindness should follow a diet high in liver—a prescription that had also been recommended by Hippocrates.8 However, a detailed description of corneal lesions and the possible nutritional origin of xerophthalmia were registered for the first time in the medical literature by Brazilian physician Manuel da Gama Lobo, who, in 1864, described typical ocular lesions in slave children in Rio de Janeiro. According to Gama Lobo, the occurrence of this ocular syndrome may be related to nutritional inadequacy; at that time he foresaw the existence of vitamins by asserting: this type of ophthalmia results from the lack of convenient and sufficient nutrition slaves were supplied with… their bodies, poor in vital components, were unable to provide the cornea with adequate nutrition.9
Yet the discovery and isolation of vitamin A only occurred very recently. In 1913, professor Elmer McCollum and his colleague Marguerite Davis discovered a fat-soluble factor in butter and egg yolk that was absolutely necessary for the growth of mice. Later on, this substance was identified as the healing factor for nutritional blindness and received the name of vitamin A.10
The main clinical manifestations of vitamin A deficiency in the ocular system chiefly occur in three ocular structures: retina, conjunctiva and cornea (Table 1.1).
The involvement of the retina occurs due to biochemical/functional changes (night blindness) and also structural changes (fundus xerophthalmicus). Although this type of involvement has marginal significance, it has proved to be an indicator of vitamin A deficiency as sensitive and specific as the signs presented by the anterior segment of the eyeball.11
In the visual cycle, retinal (an oxidized form of retinol) is bound to specific proteins (opsins) to form rod and cone visual pigments in the retina. In the intracellular membranes of rods, rhodopsin, is found, which is formed by the retinal–opsin complex. The photochemical reactions of vision are triggered when the retina is reached by light stimulus. In the presence of light, 11-cis retinal is turned into all-trans-retinal. These alterations change the geometric configuration of retinal and are followed by a global change in the rhodopsin molecule, which works as a molecular trigger, producing a stimulus in the nerve terminals of the optic nerve; the stimulus is therefore transmitted to the brain. 11-cis-retinal is used as a chromophore to the cones, which are essential for photopic and color vision; actually, the rhodopsin found in rods is the one intrinsically responsible for night vision.12 If vitamin A supplementation is too low, night blindness will be one of the first symptoms of xerophthalmia since rhodopsin requires high concentrations of 11-cis-retinal to create a highly sensitive visual film.13
Sommer et al, in their studies in Indonesia,38 concluded that the history of night blindness is a reliable method for the diagnosis of xerophthalmia. The word or expression used by mothers to describe the loss of night vision is an important tool to diagnose the extension of the problem. However, the authors insist that it is important to know the correct local expression in order to research about the phenomenon. The use of night blindness history as a diagnostic method in population studies is also restricted by the unavailability of reliable data gathered from very young children, who are at higher risk for nutritional blindness.15–21
The keratinizing metaplasia of the conjunctival epithelium, with the disappearance of mucin-producing cells and consequent tear film instability, cause conjunctival xerosis, in which the conjunctival surface presents brightness and transparency loss, going through a hardening and thickening process. Due to the subjective nature of clinical signs, conjunctival xerosis, as an isolated criterion, is not suitable for the diagnosis of xerophthalmia. This occurs because the ocular conjunctiva is frequently affected by other morphological changes that could remarkably reduce the power of discrimination of clinical signs in the diagnosis of xerophthalmia. Conjunctival alterations, especially concerning palpebral fissure, are often attributed to factors typically found in tropical countries such as environmental and racial characteristics. The most commonly described manifestations include thickening of the conjunctiva, pingueculae, pterygia, perilimbal pigmentation areas, conjunctival and episcleral melanosis, etc. On the other hand, well-defined nosological entities such as the dry eye syndrome, trachoma, and ocular pemphigoid should be considered in the differential diagnosis of xerophthalmia.22
There is formation of Bitot's spots in the areas of the conjunctiva where xerosis is more intense. These spots are depositions of spumous or caseous material that result from the accumulation of desquamated epithelial cells, meibomium gland phospholipids and saprophytic microorganisms (Corynebacterium xerosis).21 These lesions are asymptomatic and easily removable, except in some caseous cases that present pronounced adherence to the conjunctiva; they are oval or triangular, condensed or scattered, usually adjacent to the corneoscleral limbus, in the temporal and nasal regions of the bulbar conjunctiva, corresponding to the interpalpebral fissure. Nasal Bitot‘s spots, although less perceptible, suggest a narrower relationship with vitamin A deficiency.23 The specificity of Bitot's spots, as a clinical sign of vitamin A deficiency, has been questioned, since the spots observed in adults and school-age children are usually adherent and persistent after treatment and may indicate sequelae of previous xerophthalmia.23 Therefore, Bitot's spots should be associated with night blindness in order to maximize clinical sign reliability in the diagnosis of xerophthalmia.22
The decline in the production of the mucus that interfaces the aqueous component of the precorneal tear film with the hydrophobic surface of the cornea, resulting in early tear film rupture, gives the cornea a rough, dry, wrinkled and brightless aspect, expressed by the clinical sign of corneal xerosis. The keratinized epithelium is extremely vulnerable, and the lower corneal region, which is more exposed and unprotected, may undergo an erosive process, causing the destruction of the corneal epithelium, and the exposure of Bowman's layer.24 The stage of corneal erosion, combined with intense photophobia, is the clinical border where all subjacent corneal lesions present opacity as cicatricial sequela.
The involvement of Bowman's membrane and subjacent stroma is the most severe lesion caused by xerophthalmia. A usually single corneal ulcer, round or oval in shape, with clearly defined edges, is initially formed. The rupture of the anatomical barrier integrity caused by ulceration favors the release of proteolytic enzymes. These enzymes produce liquefactive necrosis of the corneal stroma, which characterizes keratomalacia.21
Despite severe ocular involvement, the eye remains undisturbed, hyporeactive, with no significant inflammatory signs, except in the presence of concomitant secondary infection.16,24 Sommer,24 in a randomized clinical assay, showed that the frequent use of antibiotic therapy failed during corneal scarring; daily practice has shown that only a treatment with vitamin A can reverse cases of xerophthalmia.
If corneal involvement is restricted to xerosis, the ocular surface is totally rehabilitated (with no sequelae) after specific vitamin treatment. If the lesion affects Bowman's membrane and/or the subjacent stroma, corneal opacity of variable intensity (nebula, macula or leukoma), appears as a sequela, depending on the intensity of the process.21 In cases of ulcer or keratomalacia, the cicatricial lesion that results from corneal stroma loss is called descemetocele. If the cornea is perforated, with loss of ocular contents, there is atrophy of the eye (phthisis bulbi). In other cases, in which the anterior chamber is partially restored after perforation, adherent leukoma develops. On the other hand, the development of staphyloma18 is established if the anterior chamber is obliterated by iridocorneal synechiae, with consequent increase in intraocular pressure, due to the partial destruction of the cornea.22
It is important to consider that corneal involvement may precede retinal and conjunctival involvement, especially in underfed and seriously ill infants.16
VITAMIN B1 (THIAMINE)
Thiamine is a water-soluble vitamin of the B complex (vitamin B1) (Fig. 1.2).
Common sources for thiamine are dried peas, beans, cereal bran, and dried yeast. Fresh glandular tissue is also a good source for thiamine and other members of the vitamin B water-soluble complex, but is seldom used in modern commercial fish diets.
Thiamine can be easily lost by holding diet ingredients too long in storage or by preparing the diet under slightly alkaline conditions. Wet or frozen diets pose a problem because moisture content increases the chance of enzymatic hydrolysis and subsequent destruction of thiamine.
The importance of thiamine (in its pyrophosphate form) for cellular functions stems from the role it plays as a cofactor for transketolase, α-ketoglutarate dehydrogenease and pyruvate dehydrogenase, enzymes important in carbohydrate metabolism. Further, because thiamine bridges the glycolytic and the pentose phosphate metabolic pathway that is critical for creating chemical reducing power in cells, the vitamin is also considered as having a role in reducing oxidative stress.25
Thus, low intracellular levels of thiamine will lead to impairment in energy metabolism and a propensity for oxidative injury. In addition, a reduction in intracellular thiamine has been shown to lead to apoptotic cell death.26
Clinically, thiamine deficiency (which occurs in disease conditions such as alcoholism, diabetes mellitus and celiac disease;27 leads to a plethora of abnormalities that include cardiovascular and neurological disorders.28
Impaired cellular thiamine accumulation that occurs in the autosomal recessive disorder thiamine-responsive megaloblastic anemia (TRMA), leads to (among other things) abnormalities in the retina and visual disturbances. Optic nerve atrophy and retinal dystrophy have been reported in a small number of patients, cone-rod dystrophy and retinal dystrophy may form part of the syndrome.29
In contrast to the negative effects that thiamine deficiency causes in occular (and other) tissues, thiamine supplementation appears to be of potential benefit in preventing diabetic retinopathy.30
Vitamin B2 (riboflavin)
Vitamin B2, the second water-soluble vitamin discovered, is an easily absorbed micronutrient with a key role in maintaining health in humans and animals. Vitamin B2 is required for a wide variety of cellular processes. Like the other B vitamins, it plays a key role in energy metabolism, and is required for the metabolism of fats, ketone bodies, carbohydrates, and proteins. Riboflavin occurs in the free form only in the eye, whey and urine (Fig. 1.3).
Milk, cheese, leafy green vegetables, liver, kidneys, legumes such as mature soybeans, yeast, mushrooms and almonds are good sources of vitamin B2. Keeping feeds from sunlight or intense artificial light is necessary to minimize loss of the vitamin by conversion to lumiflavin.
VITAMIN B3 (NIACIN)
Niacin, also known as vitamin B3 or nicotinic acid, is a water-soluble vitamin that prevents the deficiency disease, pellagra. It is an organic compound with the molecular formula C6H5NO2. It is a derivative of pyridine. The terms niacin, nicotinamide, and vitamin B3 are often used interchangeably to refer to any one of this family of molecules, since they have a common biochemical activity (Fig. 1.4).
Niacin is found in most animal and plant tissues. Rich sources are yeast, liver, kidney, heart, legumes, and green vegetables. Wheat contains more niacin than corn and the vitamin is also found in milk and egg products. The vitamin is very stable since it is generally found in coenzyme form in raw materials. Niacin added to the diet as a supplement remains relatively unaltered during diet manufacture, processing, and storage.
VITAMIN B6 (PYRIDOXINE)
Vitamin B6 is one of the most important micronutrients that we obtain from foods (Fig. 1.5). It helps maintain normal brain functions; therefore, vitamin B6 deficiency leads to a range of neurological dysfunctions, such as nervousness, depression, seizures and severe central neuropathy.31 Several in-vitro and in-vivo experimental studies have demonstrated that vitamin B6 exerts neural protective effects. It has been shown, for example, that vitamin B6 attenuates glutamate-induced neurotoxicity,32 as well as protects against ischemia or glucose-deprivation-induced neuronal death and glutamic-acid-induced seizures.33 Furthermore, vitamin B6 has been reported to promote the neuronal survival and improve cognitive function like memory.34
Good sources of pyridoxine are yeast, whole cereals, egg yolk, liver and glandular tissues. Pyridoxine compounds in phosphorylated form present in agricultural products are fairly stable but are sensitive to ultraviolet radiation. Some pyridoxal phosphate will be lost on exposure to air. Free forms or pyridoxal and pyridoxamine are rapidly destroyed by air, light and heat when moist.
VITAMIN B9 (FOLIC ACID)
Folic acid (also known as vitamin B9 or folacin) and folate (the naturally occurring form) are forms of the water-soluble vitamin B9. Vitamin B9 (folic acid and folate inclusive) is essential to numerous bodily functions ranging from nucleotide biosynthesis to the remethylation of homocysteine (Fig. 1.6). It is especially important during periods of rapid cell division and growth. Both children and adults require folic acid to produce healthy red blood cells and prevent anemia. Folate and folic acid derive their names from the Latin word folium (which means “leaf”).
Yeast, green vegetables, liver, kidney, glandular tissue, fish tissue, and fish viscera are good sources of folic acid. Activity is lost during extended storage and when material is exposed to sunlight. Therefore, dry feeds should be carefully protected during manufacture and moist diet rations should be carefully preserved. Both types of fish diets should be fed soon after manufacture to assure minimal loss of folic acid activity.
VITAMIN B12 (COBALAMIN)
Vitamin B12 is a water soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins. It is normally involved in the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production.
Vitamin B12 is the name for a class of chemically-related compounds, all of which have vitamin activity. It is structurally the most complicated vitamin and it contains the biochemically rare element cobalt. Biosynthesis of the basic structure of the vitamin can only be accomplished by bacteria, but conversion between different forms of the vitamin can be accomplished in the human body. A common synthetic form of the vitamin, cyanocobalamin, does not occur in nature, but is used in many pharmaceuticals and supplements, and as a food additive, due to its stability and lower cost. In the body it is converted to the physiological forms, methylcobalamin and adenosylcobalamin, leaving behind the cyanide, albeit in minimal concentration. More recently, hydroxocobalamin, methylcobalamin and, adenosylcobalamin can also be found in more expensive pharmacological products and food supplements. The utility of these is presently debated (Figs 1.7A and B).
Historically, vitamin B12 was discovered from its relationship to the disease pernicious anemia, which is an autoimmune disease that destroys parietal cells in the stomach that secrete intrinsic factor. Intrinsic factor is crucial for the normal absorption of B12, therefore, a lack of intrinsic factor, as seen in pernicious anemia, causes a vitamin B12 deficiency. Many other subtler kinds of vitamin B12 deficiency, and their biochemical effects, have since been elucidated.
Rich sources of vitamin B12 are found in fish meal, fish viscera, liver, kidney, glandular tissues, and slaughter house wastes. Since vitamin B12 is labile on storage, and in mild acid solution is easily destroyed by heating, care must be exercised in diet preparation containing flesh or meat scraps.
VITAMIN C (ASCORBIC ACID, ASCORBATE)
Vitamin C is an essential nutrient for the biosynthesis of collagen, L-carnitine, and the conversion of dopamine to norepinephrine.35 Under physiological conditions, it functions as a potent reducing agent that efficiently quenches potentially damaging free radicals produced by normal metabolic respiration of the body (Fig. 1.8).36 Though most animals are able to synthesize large quantities of vitamin C endogenously, humans lost this capability as a result of a series of inactivating mutations of the gene encoding gulonolactone oxidase (GULO),37 a key enzyme in the vitamin C biosynthetic pathway.37,38 These mutational events were estimated to have occurred about 40 million years ago, rendering all descending species, including humans, ascorbic acid deficient.38
Acute lack of vitamin C leads to scurvy, manifest by blood vessel fragility, connective tissue damage, fatigue, and, ultimately, death.
Ascorbate is found in many fruits and vegetables.39 Citrus fruits and juices are particularly rich sources of vitamin C but other fruits including cantaloupe, honeydew melon, cherries, kiwi fruits, mangoes, papaya, strawberries, tangelo, watermelon, and tomatoes also contain variable amounts of vitamin C. Vegetables such as cabbage, broccoli, brussels sprouts, bean sprouts, cauliflower, kale, mustard greens, red and green peppers, peas, tomatoes, and potatoes may be more important sources of vitamin C than fruits. This is particularly true because the vegetable supply often extends for longer periods during the year than does the fruit supply.
Historically, there has been a wide interest in ascorbic acid because of its role as an essential nutrient. Some of the highest naturally occurring levels have been found in the eye.40
Ascorbic acid is found throughout the eye of many species in concentrations that are high relative to most other tissues.41–45 The variation of ascorbic acid concentrations in different ocular tissues is species-dependent with the variation being greatest for the aqueous humor and the least for the retina. Diurnal animals have the highest concentrations, with the ascorbic acid concentration in some ocular tissues being 20-70 times that of the plasma.44 Concentrations up to 3 mmol/L have been reported in the aqueous humor and the lens; cow, man, and horse have concentrations of 1 mmol/L. Nocturnal animal species have very low aqueous humor and lens ascorbic acid concentrations.
Ascorbic acid is supplied to the eye from the plasma. It is transported across the blood-aqueous barrier by the ciliary body into the aqueous humor.46–48 It is generally thought that the aqueous humor serves as a source of ascorbic acid for all the other ocular tissues.
Results of experiments that examined retina,49 retinal pigment epithelial cells,50 and corneal epithelial51 cells in culture indicate all have mechanisms to accumulate ascorbic acid. The transport in most cases has been shown to be energy dependent and carrier mediated. In contrast to these cell types, Khatami et al52 reported that transport of ascorbate into cultured retinal pericytes was a carrier-mediated, facilitated diffusion process with no accumulation of ascorbic acid. Transport was not sodium or energy dependent but was still inhibited by glucose.
Studies on the transport of ascorbate into all ocular tissues or cells suggest a close relationship between glucose and ascorbate transport. Understanding the effect of high glucose on ascorbate transport in all of the ocular tissues may be very important in the study of the ocular complications ofdiabetes.52 This may be particularly crucial for retinal pericytes, which are lost in diabetes.53
Numerous studies in many cell types and tissues have defined roles for ascorbic acid in protein and catecholamine biosynthesis; in collagen, lipid, and iron metabolism; in hormone activation, and as an antioxidant.54–56 In many cases the exact role of ascorbic acid is still not clear, and it has even been suggested that the most important role of ascorbic acid in cells may not yet be known54 or is only that of a reductant.55 Ascorbic acid has a role in recycling vitamin E in membranes57, 58 and interacts with selenium.59
The role(s) of ascorbic acid in ocular tissues has not been as widely studied and is even less well understood. The millimolar concentrations present in each of the ocular tissues certainly argues for an important function.40
This high concentration of ascorbic acid in ocular tissues combined with its well-known properties as a strong reductant and scavenger of radicals60 such as superoxide have been used as arguments that the major function of ascorbic acid in the eye is that of a protector against oxidative damage, particularly light-induced damage. Only a few studies have addressed other possible functions of ascorbic acid in the eye, functions that are certain to be important and that may be unique for each ocular tissue.
Trabecular meshwork cells are immersed in aqueous humor. It was found by using trabecular meshwork cells in culture that ascorbic acid modulates the production of fibronectin and laminin, which are two components of the basal lamina61 and synthesis of glycosaminoglycans.62
The focus of studies in the cornea has been on the promotion of wound healing by ascorbic acid.63,64 It affects the metabolism of arachidonic acid in the iris, ciliary body, and cornea.65
Most of the functions ascribed to ascorbic acid are related to its redox properties. It has been postulated that ascorbic acid—dehydroascorbic acid operates as a redox system in ocular tissues66 and is linked to the activity of the hexose monophosphate shunt, thus contributing to the maintenance of reduced pyridine nucleotide levels.67, 68 Ascorbic acid decreases the membrane damage found in lenses of rats made diabetic.69 Brewitt's70 in vitro studies indicated an important role for ascorbic acid in lens development and maintenance of transparency during development. Ascorbic acid is also thought to rid the lens of O2, thus decreasing the probability of oxidative insult.71, 72
Most studies on the role of ascorbic acid in retina and lens have focused on the protective effects of the molecule. Ringvold73 proposed that it provided protection against (UV) ultraviolet irradiation. The high concentration of ascorbate in the aqueous humor of diurnal animals relative to that of nocturnal animals is strong support for Reiss,44, 74 proposal that the high concentration of ascorbate in the eye may be an adaptation that protects the eye against solar radiation. Ascorbic acid clearly provides protection against light-induced loss of retinal pigment epithelial cells and photoreceptor cells.75–87 In the lens, ascorbic acid prevents the riboflavin-mediated, light-induced damage to the cation pump70, 79 and decreases the photoperoxidation of the membranes;80 in both cases damage is thought to be the result of superoxide generation. Supplementation of guinea pig diets with ascorbic acid appeared to decrease UV- and heat-induced damage to lens proteins.81, 82
The relationship between cataracts and ascorbate is unclear. As discussed by Health42 cataracts are not generally associated with scurvy or experimentally induced scurvy and early attempts to prevent cataracts by administering ascorbic acid were unsuccessful. However, recently, it was shown that injection of ascorbic acid provided some protection against development of selenite induced cataracts in Sprague-Dawley rats,83 and ascorbic acid supplementation to Wistar rats, a species that cannot synthesize ascorbic acid, decreased the incidence of cataracts in animals made diabetic.69 Epidemiologic studies suggest a correlation between low intake of ascorbic acid and certain types of cataracts.84–86 The levels of ascorbic acid are low or absent in cataractous lenses42, 87, 88 and increased levels of dehydroascorbate and ascorbate free radical are found in human lens with progression of senile cataracts.87, 89 These latter observations may, however, represent both a cause and an effect of cataracts.
It is well accepted that oxidation is the cause of many of the modifications that lens proteins accumulate throughout life. These modifications are thought to lead to formation of highmolecular-weight aggregates that scatter light, thereby contributing to cataractogenesis. A number of studies have implicated ascorbate as a causative factor in many of these modifications. Ascorbate protects against oxidation by its interaction with or scavenging of radicals, but under the appropriate conditions it can act as a prooxidant and participates in reactions that generate radicals.90 These reactions can be light-induced or result from metal-catalyzed oxidation. In the presence of metals, in particular copper or iron, and oxygen, ascorbic acid becomes oxidized, forming dehydroascorbate, hydrogen peroxide, and reduced metal. The hydrogen peroxide may then interact with the reduced metal, generating hydroxyl radical, ferryl radical, or other reactive oxygen species. When these reactions involve copper or iron bound to proteins, the radicals generated oxidatively modify amino acids at or near the metal-binding site.91, 92 These reactions become important when cells lose the ability to sequester the metal, making it available to react, and/or when cells lose the ability to maintain ascorbic acid in the reduced state.
That these reactions may occur in the lens is supported by the following observations. Increased levels of copper and iron have been reported in aging lenses and in cataracts.93 Dccreased activity of ascorbate free-radical reductase, a mechanism thought to maintain reduced ascorbic acid, has been correlated with increased insolubilization of protein in certain cataracts.94 Furthermore, reaction of lens proteins in vitro with ascorbic acid and trace amounts of copper or iron induces many modifications seen in cataracts. Ascorbate forms covalent adducts with crystallins in vitro.95, 96 These adducts are thought to undergo Amadori rearrangement, forming browning products such as those found in brunescent lenses. There is a conversion of the crystallins to more acidic forms and an introduction of nontryptophan fluorescence.97, 98 In addition these reactions cause nondisulfide covalent crosslinking and the insolubilization of lens proteins.99 The products of the metal-catalyzed oxidation of ascorbic acid are also cytotoxic to lens and cornea cells in culture.100, 101
These observations certainly indicate a potential role for ascorbic acid in the protein modifications that occur during cataract formation. They do not argue that ascorbic acid actually causes cataracts. These reactions are likely to occur when the overall oxidative stress of the lens exceeds the capacity of the antioxidant mechanisms, there is a change in metal metabolism, or when the mechanisms that maintain reduced ascorbic acid are compromised.40
The term “vitamin D” refers to several different forms of this vitamin. Two forms are important in humans: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Vitamin D2 is synthesized by plants. Vitamin D3 is synthesized by humans in the skin when it is exposed to ultraviolet-B (UVB) rays from sunlight. Foods may be fortified with vitamin D2 or D3 (Fig. 1.9).
Vitamin D is an important nutrient in the maintenance of bone health. The primary functions of vitamin D are the regulation of intestinal calcium absorption and the stimulation of bone resorption leading to the maintenance of serum calcium concentration.102
Sources of vitamin D include sunlight, diet, and supplements.103 Vitamin D occurs naturally in a limited number of foods in highest amounts in fatty fish and in low amounts in meats and other animal food products; it is also available in fortified foods (including milk and milk products, margarines, and breakfast cereals). Fortified foods constitute the major dietary food sources of vitamin D in the United States.104 The majority of Americans do not achieve adequate vitamin D levels. In fact, it is estimated that 90% of adults between 51 and 70 years of age do not get enough vitamin D from their diet.103
Sunlight exposure, or ultraviolet B (UVB) radiation, is absorbed by the 7-dehydrocholesterol that resides in the skin to form previtamin D3. Previtamin D3, an unstable compound, is quickly converted to vitamin D3 via heat.105 Vitamin D3 moves out into the extracellular space and is drawn into the capillaries by vitamin D-binding protein (DBP).106 Once in the capillaries, the vitamin D is transported to the liver where it undergoes hydroxylation to form 25-hydroxyvitamin D [25(OH)D]. 25-hydroxyvitamin D is again bound by DBP and taken to the kidney where it is transported and released into the renal tubule cell and hydroxylated to form 1,25-dihydroxyvitamin D [1,25(OH)2D].107 This is the biologically active form of vitamin D, which is responsible for calcium homeostasis.
Dietary sources of vitamin D are absorbed into the lymphatic system via chylomicrons, where they enter the circulation and are bound by DBP.108 From here, they are taken to the liver and kidneys, as explained above, for the formation of the active form of vitamin D [1,25(OH)2D].
Vitamin D and the Eye
Vitamin D may reduce the risk of AMD by its anti-inflammatory properties. Several putative mechanisms support the anti-inflammatory role of vitamin D. Studies have reported that vitamin D decreases proliferation of T helper cells,109 T cytotoxic cells, and natural killer cells110 and enhances T suppressor cell activity.111 Vitamin D also decreases the production of proinflammatory agents such as IL-2,112,113 IL-6,114 IL-8,115 and IL-12.116 In addition, a recent study has shown that vitamin D intake reduces C-reactive protein, a marker of systemic inflammation.117
There is laboratory and epidemiologic evidence of inflammation underlying AMD pathology. A common polymorphism in complement factor H, a key regulator of the alternate complement pathway identified in a region of a gene responsible for binding heparin and C-reactive protein, was associated with higher risk for AMD in several previous studies.118,119–122 Using histological methods, Anderson et al123 identified immuno-proteins entrapped within drusen, implying local inflammation, and Hageman et al124 proposed a mechanism by which local inflammation may contribute to drusen development. Associations between markers of inflammation (such as C-reactive protein) and AMD have been observed in some125,126 but not all127 previous epidemiological studies. Anti-inflammatory drug use was significantly related to AMD in one study128 but not other previous studies.129,130 Recently results of the Beaver Dam Eye Study131 indicated an association between histories of gout and emphysema, two diseases associated with inflammation, and intermediate and late stages of AMD.131
Alternatively, vitamin D might protect against AMD by virtue of its antiangiogenic properties. There is recent evidence of vitamin D being a potent inhibitor of angiogenesis by its effects on endothelial cells132–134 and by interrupting signaling pathways that are key to angiogenesis, specifically in tumorigenesis. By virtue of its antiangiogenic role, vitamin D may protect against “wet” advanced AMD, which involves growth of new blood vessels in the retina.
Vitamin E is the collective name for a set of 8 related α-, β-, γ-, and δ-tocopherols and the corresponding four tocotrienols, which are fat-soluble vitamins with antioxidant properties.135,136 Of these, α-tocopherol (also written as alpha-tocopherol) has been most studied as it has the highest bioavailability.137 It has been claimed that α-tocopherol is the most important lipid-soluble antioxidant, and that it protects cell membranes from oxidation by reacting with lipid radicals produced in the lipid peroxidation chain reaction.135,138 This would remove the free radical intermediates and prevent the oxidation reaction from continuing. The oxidised α-tocopheroxyl radicals produced in this process may be recycled back to the active reduced form through reduction by other antioxidants, such as ascorbate, retinol or ubiquinol139 (Fig. 1.10).
Food Sources of Vitamin E
High levels of vitamin E can be found in the following foods:140
- Nuts, such as almonds or hazelnuts
- Red palm oil
- Spinach and other green leafy vegetables
- Vegetable oils — Canola, corn, sunflower, soybean, cottonseed, olive oil, rice
- Wheat germ
- Whole grain
Vitamin E and the Eye
The relation between vitamin E and the eye was described by Kaya Nusret Engin in Molecular Vision in 2009; under the title of Alpha-tocopherol: looking beyond an antioxidant.
Vitamin E is an important natural antioxidant, and its most common and biologically active form is α-tocopherol. In addition to this, specific regulatory effects of vitamin E have been revealing. The body exerts a certain effort to regulate its tissue levels with specific tocopherol transport proteins and membrane receptors. Antiproliferative and protein kinase C-supressing effects of alpha-tocopherol have been previously demonstrated, which have not been mimicked by betatocopherol or probucol. Protein kinase C promises to be an important area of interest in the means of glaucoma and cataractogenesis. It has been shown in different models that retinal vascular dysfunction due to hyperglycemia could be prevented by alpha-tocopherol via the diacylglycerol-protein kinase C pathway. Glutamate transporter activity has been shown to be modulated by protein kinase C. This pathway is also important in intraocular pressure-lowering effects of prostaglandin and its analogs in glaucoma therapy. Filtran surgery became another possible area of usage of alphatocopherol since its antiproliferative effect has been demonstrated in human Tenon's capsule fibroblasts. Prevention of posterior capsule opacification is another area for future studies. It is evident that when correct and safe modulation is the objective, alpha tocopherol merits a concern beyond its mere antioxidant properties.141
While the recommended daily allowance (RDA) for vitamin E is 8 mg (12 IU) for females and 10 mg (15 IU) for males, Packer142 recommends up to 1,000–1,200 IU intake of vitamin E in some pathologies including cataract.
Regarding the pharmacodynamics of tocopherols, it has been reported in a study conducted in human eyes that the retinal levels of vitamin E are higher than those of the choroids or vitreous and is correlated with serum levels of vitamin E.143 It is known that vitamin E can only reach its theraupetic levels in aqueous humor and lens via topical application and is accumulated within the retina when applied via the oral or parenteral route.144 Moreover, it is reported in animal studies that when 100 mg/kg α-tocopherol is applied via oral or parenteral route, it causes a similar threefold to sixfold increase to its serum levels, though the retinal and vitreal increases are somewhat slower via the oral route.145 Based on the common knowledge summarized above, vitamin E is occasionally prescribed in ophthalmology clinics.
Discovery of α-tocopherol specific membrane receptors146 and cytosolic transfer proteins strengthen the thesis that vitamin E possesses properties beyond a mere antioxidant function.147 Specifically for the eye, scavenger receptor class B type I at the inner blood-retinal barrier has been described in vitro, which is responsible for α-tocopherol uptake from the circulating blood and plays a key role in maintaining α- tocopherol in the neural retina.148
Protective effects of vitamin E have been shown in almost all eye tissues within clinical, in vitro, and in vivo studies. For instance, vitamin E is known to double the rabbit corneal endothelial cell survival time149 and enhances retinal cell survival via its effect on mitochondrial activity.150 Also, α-tocopherol can protect the retina from light injury for up to 24 hr of exposure.151 Vitamin E plays an important prophylactic role against several serious light induced diseases and conditions of the eye (cataractogenesis and retinal photodeterioration) and skin (erythrocyte photohemolysis, photoerythema, photoaging, and photocarcinogenesis) that are mediated by photooxidative damage to cell membranes.152 These findings do not have to be explained with antioxidant mechanisms, especially for α-tocopherol.
An association between α-tocopherol and some ocular pathologies has also been demonstrated previously. For example, retinitis pigmentosa is shown to be related to an H101Q mutation in the α-tocopherol transfer protein gene.153 The combination of cryotherapy with vitamin E prophylaxis appeared to decrease the severity and sequelae of threshold retinopathy of prematurity.154 Average levels of α-tocopherol were shown to be lower in people with exudative macular degeneration.155
Retinal pigment epithelium cells migrating through the damaged retina play an important role in the pathogenesis of proliferative vitreoretinopathy. Majon et al156 found that α-tocopherol inhibits proliferation of human retina pigment epithelium (RPE) cells in culture without exerting cytotoxic effects. Maximal inhibition was achieved with 100 µM α-tocopherol. It has been found that α-tocopherol succinate inhibits proliferation and migration of retinal pigment epithelial cells in vitro.157 α-Tocopherol and α-tocopheryl acid succinate in saline solution presented a retardation of proliferative vitreoretinopathy in retinal detachments.158
The protective function of α-tocopherol against the process of cataractogenesis in humans is reported in epidemiologic studies.159 In the Beaver Dam Eye Study, it is shown that age-related lens opacities in humans are linked inversely to vitamin E status.160
Glaucoma is another possible area of usage for vitamin E. Failure in glaucoma surgery is primarily due to fibrocellular scar formation, derived from Tenon's capsule fibroblasts. It has been found that δα-tocopherol (vitamin E) was able to inhibit proliferation of in vitro human Tenon's capsule fibroblasts.161 Following this, filtran surgery became another model in which an antiproliferative effect has been shown in vivo. α-Tocopherol derivatives showed antiproliferative properties in the experimental models of filtering surgery and showed better intraocular pressure (IOP) control and bleb survival.162,163 Cell culture studies further illuminated this effect, and comparative studies with other antimetabolites have been performed.164
On the other hand, dual effects of α-tocopherol and PKC on the eye are of interest in the means of glaucoma therapy. Kunisaki et al in 1995165 and Lee et al in 1999166 showed in different models that retinal vasculer dysfunction due to hyperglycemia could be prevented by α-tocopherol via a diacylglycerol-PKC pathway. In a study performed by Engin et al,167 60 glaucomatous eyes from 30 patients were divided into three groups. While group A patients recieved no tocopherol, group B and group C patients were given 300 and 600 mg/day of oral α-tocopheril acetate, respectively. Visual fields and retinal blood flows of ophthalmic and posterior ciliary arteries with Doppler ultrasonography were evaluated in the beginning of the study, as well 6 and 12 months after treatment. Compared with group A, differences of pulsatilty and resistivity indexes of ophthalmic and posterior ciliary arteries were lower in groups B and C 6 and 12 months after treatment. Posterior ciliary artery differences of resistivity indexes in the 6th and 12th months and ophthalmic artery differences of pulsatilty indexes reductions in the sixth month were statistically significant. Differences of mean deviations with visual fields in groups B and C were significantly lower than that of group A.
Among other signaling pathways that have been shown to be affected by α-tocopherol, PKC promises to be an important area of interest.141
Beside its role in retinal vasoregulation mentioned above, the PKC pathway boasts a decisive factor in the pathogenesis for and on the clinical course of glaucoma. Glaucoma is one of the neurodegenerative conditions arising from a compromised glutamate homeostasis. Glutamate transporter activity has been shown to be modulated by PKC.168 PKC have been shown to affect nonvascular smooth muscle cells such as the iris sphincter.169 Wiederholt et al170 have reported that various pathways and ion channels affect PKC isomers producing different responses in eye nonvascular smooth muscle cells, but in general, PKC inhibitors relax trabecular meshwork while leaving the ciliary muscle comparatively unaffected.
Alexander and Acott171 have reported that the PKC pathway is crucial in glaucoma therapy for the intraocular pressure-lowering effects of prostaglandin F 2α (PGF2α) and its analog, latanoprost. The cytokine, Tumor Necrosis Factor α (TNFα), is a strong modulator of trabecular meshwork matrix metalloproteinase (MMP) and tissue inhibitor (TIMP) expressions. TNFα treatment triggered some PKC isoform translocations. Exposure of trabecular cells to TNFα for 72 hr differentially downregulated several PKC isoforms. Treatment with a phorbol mitogen that stimulates most PKC isoforms produced strong increases in these MMPs. Effects of TNFα on MMP and TIMP expressions were completely blocked by only one PKC inhibitor.
In a study performed concerning cat iris sphincter smooth muscle cells, the relaxing effects of PGF2α and carbachol have been shown to be produced by mitogen activated protein (MAP) kinases in a PKC-dependent manner.172 On the other hand, PKC activators strongly stimulate the phosphorylation of AQP4 (aquaporin in the ciliary body) and inhibit AQP4 activity in a dose-dependent manner.173
The prevention of posterior capsule opacification (PCO) is another area for future studies. Despite recent advances in cataract extraction, lens epithelial cells remaining in the capsule proliferate and eventually cause opacification within days of surgery.174 Although inhibition of lens epithelial cells can be observed with various agents, toxic side effects to the ciliary body, cornea epithelium, and iris limit their use in human subjects.175–177 PCO is a process mainly involving proliferation and migration of the lens epithelium,178 and PKC is a signaling pathway that is known to result in major effects on this process (Fig. 1.11).
PKC activity exists in the cytosol and particulate fractions of bovine lens epithelial cells,179 and its role in both cell differentiation180 and proliferation181 have been shown in rabbit lens epithelial cells.
PKC also plays a role in cataractogenesis by phosphorylating proteins from calf lens fiber membranes182 and activating neutral proteases.183 The PKC-inhibiting effect of vitamin E is known to exist in epithelial cells.184 Intramuscular vitamin E supplementation is sufficient in protecting histopathologic changes in the lens epithelium.185
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