Topic of the Month

Micronutrients and eye health

March 1, 2014

Good eyesight requires the thousands of cells which compose the eye to all function well. The cells, in turn, rely on an adequate supply of micronutrients to support and maintain their function. Several vitamins are needed as co-factors for the enzymes involved in sight. Vitamins are also important to the optical nerve that transmits light signals from the eye to the brain; to the muscle cells that facilitate visual acuity; and to the blood vessels that are responsible for supplying the eyes with blood and nutrients. The carotenoids found in the macula lutea, the yellow spot at the center of the retina, filter short-wave light like natural sunglasses and protect this sensitive area of the eye against UV damage. In addition, omega-3 fatty acids protect the retina and are indispensible for sensory cell function and hence for vision.

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Older people, in particular, often suffer from failing eyesight. Age-related degene- rative eye conditions are in fact the most common cause of severe loss of sight in industrialized countries. In the condition known as age-related macular degeneration (AMD) the optical cells at the center of the retina, which mediate acute vision and the perception of color, perish. This results in a gradual loss of visual acuity in the center of the visual field. With cataracts, the lens becomes cloudy and light can no longer pass through it unimpeded and be dispersed. Vision becomes blurry, with a lack of contrast, and colors appear duller. Sufferers also become more sensitive to glare. A balanced, healthy diet with lots of fruit and green leafy vegetables which contain abundant cell-protective antioxidants can help prevent eye disorders well into old age, or at least slow their progress.

Micronutrients in the prevention of

Cataract

A cataract is a progressive clouding of the lens that is accompanied by an age-related, gradual, painless loss of visual acuity. Several types of cataract are distinguished according to the area of the lens that becomes cloudy: nuclear cataracts develop slowly in the center of the lens and cortical cataracts start at the outer rim of the lens, while subcapsular cataracts usually affect the back of the lens and progress relatively rapidly. With advanced cataracts, the cloudiness may be visible as a gray coloration behind the pupil. Those affected suffer from increasingly blurred vision and are more sensitive to glare (especially at night), because the clouding of the lens causes diffuse refraction of the light. The perception of contrasts is also reduced, so that the world appears to be wrapped in fog. Initially symptoms are mild, becoming only slowly more severe. As a result, the condition is often diagnosed relatively late. If untreated, cataracts can lead to blindness, but with a small operation, in which the cloudy lens is removed and replaced by a plastic one, vision can be success- fully improved.

Cataracts are not only a consequence of the aging process, but the result of many factors, including long- term irradiation with UV light (sunlight), smoking and various general conditions like diabetes and arterios- clerosis. An insufficient intake of antioxidant nutrients can also play a part. All these factors encourage oxidative processes in which reactive oxygen molecules (free radicals) can damage or even destroy the cells and tissues of the eye (1). Several studies confirm that targeted consumption of antioxidant micronutrients, which combat free radicals, can have a positive influence on the progress of cataracts (2, 3). This applies especially to vitamins C and E, beta-carotene and other carotenoids (lutein and zeaxanthin) and to the trace elements selenium and zinc (4, 5). As a water-soluble antioxidant, vitamin C can act in the aqueous fluid of the optical lens to prevent or slow the development of cataracts. Epidemiological and clinical trials have shown an association between a low intake or low plasma levels of vitamin C and the incidence of this dise- ase (6, 7). In laboratory experiments and animal studies, vitamin C was shown to be particularly effective in preventing UV-light-induced oxidative damage (8).

In synergy with vitamin C, fat-soluble vitamin E acts to combat the occurrence and progression of cataracts in the lipid -containing cell walls of eye tissue. When concentrations of reactive oxygen molecules are high, the vitamin is completely oxidized, although vitamin C can regenerate it. The retina, especially, contains very high concentrations of vitamin E. One large clinical study revealed a marked reduction in the incidence of cataracts and cataract operations through improved provision of vitamins E and C in the diet (9). Dietary and intervention studies provided evidence that the targeted administration of dietary supplements containing vitamin E could stop the advance of a cataract (10, 11). Beta-carotene, lutein and zeaxanthin are other fat- soluble antioxidants which support the body’s own ability to defend itself against oxidative stress (12) and according to several studies can delay the formation of cataracts by absorbing UV light (11, 4). Other rando- mized controlled studies confirmed this preventive effect (13, 14). The antioxidant trace elements selenium and zinc can also help prevent cataracts (4, 5). Moreover, they complement vitamin activity: selenium strengthens the effectiveness of vitamin E and zinc enhances the bioavailability of vitamin A. Studies have also demonstrated preventive effects after targeted consumption of vitamin A and some B-vitamins (B1, B2, B9), as well as after consumption of multivitamin products (15, 16). All the currently available investigations into the possible preventive effect of micronutrients on cataracts indicate that they work best in combination (17).

Age-related macular degeneration

Age-related macular degeneration, or AMD, is the most common cause of sight impairment in people over 60 in the Western world. With AMD, the macula, an area at the center of the retina responsible for visual acuity, becomes damaged. Two forms of macular degeneration are distinguished: with the dry form, for which there is no known effective cure, vision often remains stable over a long period of time and usually deteriorates slowly. The treatable wet form leads to a sudden swelling in the middle of the retina due to formation of ab- normal blood vessels. This can cause a drastic loss of visual acuity, perception of contrasts and color vision, as well as sensitivity to glare. Since AMD affects only the center, and not the outer areas, of the retina, even in advanced stages of the disease only central and not peripheral vision is affected. The macula contains the highest concentration of light receptors (photoreceptors) and a high density of the carotenes lutein and zea- xanthin, as well as meso-zeaxanthin (18). As macular pigments, these act like natural sunglasses to protect the eye against UV irradiation by filtering in particular short-wave light and through their anti-inflammatory and antioxidant properties. However, in the course of biological aging, the macular pigments undergo progressive decomposition due to UV irradiation and can no longer fully perform their protective functions (19). Moreover, in old age small whitish or yellow deposits called drusen can develop underneath the retina and cause impairment to the pigment layer. In addition, genetic predisposition, high blood pressure and high levels of homocysteine in the blood, and in particular smoking, are considered to be risks factors for the incidence of AMD.

Adequate intakes of certain micronutrients can play an important part in preventing and treating dry AMD. One effective strategy could be to build up stores of the depleted carotenes lutein and zeaxanthin in the macula through dietary intake or supplementation. Further, specific protective nutrients which are naturally abundant in the retina, such as the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), could be taken at the same time. One randomized controlled study (LUTEGA) was able to de- monstrate that in patients with dry AMD, dietary supplementation with lutein, zeaxanthin and omega-3 fatty acids brought about a marked increase in the concentration of macular pigments and stopped or at least delayed the advance of the dry AMD, with measurably improved visual performance (20, 21). This confirmed the results of a large clinical study (Age-Related Eye Disease Study, AREDS2) which was able to show that a combination product with lutein and zeaxanthin alone or in combination with EPA and DHA can reduce the incidence of AMD (22). A second analysis of this study revealed that administration of lutein and zeaxanthin in addition to the antioxidant micronutrients used in a precursor study (AREDS) was able to reduce the risk of developing advanced AMD by a further 10% (23). The first clinical study of this kind (AREDS) had already shown that daily consumption of a product with antioxidant micronutrients (500 mg vitamin C, 400 I.U. vita- min E, 15 mg beta-carotene, 80 mg zinc and 2 mg copper - to prevent a potential copper deficiency due to the high zinc content) could reduce the risk of developing age-related AMD by 25% after five years (24, 25).

A sufficient intake of antioxidant vitamins and trace elements could therefore protect the macular pigments against oxidative damage and hence against degradation. The LUTEGA and AREDS2 studies also provided evidence of an anti-inflammatory effect of EPA and DHA in the macula and the surrounding area. This could lead to a delay in the progression of existing AMD, as was already observed in the first AREDS study (26, 27). The antioxidant lycopene, too, is present in high concentrations in the retina. The potential benefits of increased lycopene intake on AMD are currently still under investigation (28).

Other eye conditions

An adequate supply of vitamin A (retinol) is essential to sight: vitamin A is required for the production of a visual pigment (rhodopsin) in the sensory cells of the retina. Rhodopsin triggers signal transmission to the optic nerve. In the condition night-blindness (nyctalopia), vision is poor at night or dusk. The reason for this impaired dark-adaptation, which can cause major problems when driving, for example, is a loss of function in the sensory cells in the retina (rods) which are responsible for seeing in low light conditions. Night-blindness can be inherited, but it can also be caused by vitamin A deficiency (29). Although the possibilities for treat- ment are limited, the condition can be prevented if an adequate intake of vitamin A (retinol) is ensured during pregnancy and lactation. Severe vitamin A deficiency can also cause dryness of the external eye (xerophthalmia), which can lead to complete blindness (30). Xerophthalmia is the most common cause of childhood blindness in developing countries. Targeted administration of dietary supplements containing vita- min A is being employed in developing and emerging nations in an attempt to prevent deficiency disorders.

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