Laura A. Stokowski
November 27, 2012
The Impact of Vision Loss
Vision loss on a grand scale would have a far-reaching impact on society. Yet, if we continue in the direction we are headed now, we could be facing a future in which a large segment of the population suffers vision loss from diseases such as age-related macular degeneration (AMD), cataracts, glaucoma, and diabetic retinopathy. Vision loss, in turn, is a key reason for loss of independence in the elderly. Certainly, improved methods of diagnosing and treating age-related eye disease can obviate some of the vision loss that would otherwise cripple millions of older adults. Prevention, however, is not only cheaper but likely to be far more effective in averting such a tragedy.
Prevention Is the Key
How can healthcare providers motivate patients to take steps to prevent age-related eye disease? A recent survey[1] found that baby boomers (born between 1945 and 1964) fear vision loss almost as much as they fear heart disease or cancer. Still, almost half do not receive an annual eye exam, and few are knowledgeable about how dietary factors influence eye health.[2] Furthermore, supplement use to promote eye health is low, even in patients already diagnosed with age-related eye disease.[2] Eye care providers, primary care providers, and specialists in diabetes can point to a burgeoning body of research to convince their patients to increase their dietary intake of key nutrients.
Prevention Is the Key
How can healthcare providers motivate patients to take steps to prevent age-related eye disease? A recent survey[1] found that baby boomers (born between 1945 and 1964) fear vision loss almost as much as they fear heart disease or cancer. Still, almost half do not receive an annual eye exam, and few are knowledgeable about how dietary factors influence eye health.[2] Furthermore, supplement use to promote eye health is low, even in patients already diagnosed with age-related eye disease.[2] Eye care providers, primary care providers, and specialists in diabetes can point to a burgeoning body of research to convince their patients to increase their dietary intake of key nutrients.
Lutein
Lutein and zeaxanthin are xanthophyll carotenoids that have antioxidant and light-screening mechanisms. They are deposited in the macula, where they boost macular pigment optical density that is lost with age, reducing photo-oxidation in the central retina.[4] Lutein’s purported benefits are believed to be related to its effects on immune responses and inflammation.[5] The CARMA study failed to provide definitive evidence for the protective effects of carotenoids on AMD.[6] Epidemiologic studies also suggest that dietary intake of lutein and zeaxanthin is inversely related to the risk for cataract.[7,8] The human body does not synthesize lutein, so it must be obtained from dietary sources. Green leafy vegetables are the major dietary sources of lutein.[9]
Zeaxanthin
Like lutein, zeaxanthin is contained within the retina. Whereas lutein predominates in the peripheral retina, zeaxanthin is the dominant component in the central macula. Zeaxanthin may be a better photoprotector than lutein, but its protective role might have been obscured because investigations usually studied both carotenoids together.[10] The major dietary sources of zeaxanthin are corn, spinach, collard greens, lettuce, and tangerines. A recent study showed that the effect on macular pigment level did not differ whether lutein and zeaxanthin intake came from plant sources or dietary supplements.[11]
Zinc
Zinc is an effective antioxidant and anti-inflammatory mineral, and it was included in the AREDS study.[3] Participants were randomly assigned to receive oral supplements of high-dose antioxidants, zinc, antioxidants plus zinc, or placebo. The only statistically significant reduction in rates of at least moderate visual acuity loss occurred in persons assigned to receive antioxidants plus zinc. Patients with AMD or cataract had lower survival, but survival was higher in patients taking zinc.[12] High doses of zinc can lead to copper deficiency anemia, so copper has been added to AREDS formula supplements. Good dietary sources of zinc include oysters, crab, toasted wheat germ, veal liver, low-fat roast beef, mutton, pumpkin seeds, dark chocolate, and peanuts.
Omega-3 Fatty Acids
An important functional role for the essential fatty acid docosahexaenoic acid (DHA) is suggested by its high concentration in the retina. Prospective data from a large cohort of female health professionals without AMD at baseline indicate that regular consumption of omega-3 fatty acids and fish is associated with a significantly lower risk for AMD and may be of benefit in primary prevention of AMD.[13] The AREDS2 study is evaluating the effect of omega-3 fatty acids on progression to advanced AMD. Dietary omega-3 fatty acids have also been shown to control the symptoms of dry eye syndrome.[14] Omega fatty acids can be acquired through higher consumption of fatty fish (salmon, tuna, mackerel, sardines) or through omega-3 fatty acid supplements in the form of oil or capsules.
Vitamin E
It has been suggested that vitamin E, like other antioxidants, might have a role in preventing, slowing progression of, or improving macular degeneration. However, the evidence is inconclusive. Studies have not shown a benefit of vitamin E supplementation in reducing the development or progression of either AMD[15] or cataracts.[16] However, a combination of vitamin E and other nutrients (a 400-IU/day intake of vitamin E plus beta-carotene, vitamin C, and zinc) slows the progression of advanced AMD. Vitamin E is more difficult to obtain from food sources alone because it is found in very small quantities in foods. Dietary sources of vitamin E include fortified cereals, wheat germ, sunflower seeds, and vegetable oils.
Vitamin C
Vitamin C (500 mg) was also included in the AREDS formulation that reduced the risk for progression to advanced AMD. An antioxidant found in fruits and vegetables, vitamin C may have a role in lowering risk for cataracts and, when taken in combination with other essential nutrients, can slow the progression of AMD and visual acuity loss. Independent effects of vitamin C on eye health are inconclusive. In a large-scale, randomized trial of physicians, daily use of 500 mg of vitamin C for 8 years had no appreciable beneficial or harmful effect on risk for incident diagnosis of AMD.[17]
Flavonoids
Natural flavonoids are believed to have a role in the prevention of cataracts. Flavonoids efficiently influence the multiple key molecular mechanisms involved in cataract formation and lens opacification, including oxidative stress, lens calpain proteases, epithelial cell signaling, nonenzymatic glycation, and the polyol pathway.[18] The flavonol quercetin is the most widely consumed flavonoid in the human diet, found in apples, tea, onions, nuts, and berries.
Beta-carotene
Beta-carotene is a carotenoid that is active in the prevention of free radical formation. To determine the effects of a carotenoid on the development of AMD, beta-carotene was included in the original AREDS formula because lutein and zeaxanthin were not yet commercially available. However, beta-carotene is not present in the retina, so any beneficial effects of the combination of nutrients are unlikely to be related to beta-carotene. In a large population of healthy men, beta carotene supplementation had no beneficial or harmful effect on the incidence of AMD.[19] The safety of beta-carotene supplementation, especially in smokers, has been questioned. Some evidence points to a paradoxic increase in the incidence of lung cancer in smokers who take beta-carotene supplements.[20]
What Should Providers Recommend?
The bottom line? The evidence is insufficient to recommend nutritional supplementation for primary prevention of AMD.[21] Strong evidence supports the use of antioxidants and zinc in patients with certain forms of intermediate and advanced AMD. Growing evidence for adverse effects associated with the AREDS formula vitamins (lung cancer in smokers taking beta-carotene, heart failure in people with vascular disease or diabetes from vitamin E, and urinary tract infections from zinc) compels providers to consider these risks and explain them to patients who are considering supplementation.[22] Observational studies suggest benefit from higher dietary intake of macular xanthophylls (lutein and zeaxanthin) and omega-3 fatty acids. These are currently being evaluated prospectively in AREDS2.