Preventing Macular Degeneration
This section is compiled by Frank M. Painter, D.C.
Send all comments or additions to: Frankp@chiro.org
From The January 1998 Issue of Nutrition Science News
by Michelle BadashCan the leading cause of blindness in people over 65 be averted by antioxidants? What about zinc, lutein and vitamin C?
It's been said that the eyes are the first to go. As we get older, one experience we commonly share with others in our age group is deteriorating vision. "Macular degeneration," an eye disease affecting the central part of the retina, [1] has recently been the focus of a lot of research--possibly because so many of us are now approaching the time of life when our eyes cease to see as well as they used to.
In the United States, macular degeneration is the leading cause of severe loss of vision, even legal blindness, in people over 65. It affects about 6 percent of Americans between the ages of 65 and 75, and accounts for 14 percent of all new cases of blindness, with 16,000 cases reported annually. [2] Prevalence is slightly higher among women, while it is rarely found in people of color. Although its causes have not yet been determined, some scientists speculate that frequent exposure to sunlight and smoking may contribute to the disease's development.
What Is Macular Degeneration?
Macular degeneration occurs when the cells of the macula, which is the central portion of the retina, become damaged and stop functioning. The retina is a thin layer of light-sensitive tissue that stretches across the back of the eye. It functions like a screen onto which all visual images are projected. The role of the macula is to view complex images. It allows us to focus on objects directly in front of us; enables us to see fine detail during activities such as reading, writing, sewing and driving; and determines our ability to distinguish color.
There are several types of macular degeneration. One is called juvenile macular dystrophy. It's a fairly rare, inherited form of the disease that may occur in children or young adults. Another less common form, called pigment epithelial detachment, is caused by a blister formation under the retina. In general, however, the two most common forms of macular degeneration are termed "wet" and "dry," and occur primarily after age 60.
Dry: The most prevalent, and less serious, form of macular degeneration (accounting for approximately 85 percent of all cases) is "dry," characterized by a gradual loss of central vision. The process may occur over a period of several years and can affect one or both eyes. The deterioration may be related to general atrophy or, in certain cases, caused by deposits of drusen, a yellowish substance that accumulates under the retina.
Typically, the symptoms of this form of macular degeneration include blurriness when reading, difficulty in seeing objects that are far away, or shape distortion (such as a tree appearing bent or crooked). Some people may also notice a dark spot in the center of an object, or find that colors and sizes vary in each eye.
Wet: The "wet" form of macular degeneration is caused by abnormal blood vessel growth behind the retina that may leak or bleed and cause the retina to separate from the eye. These abnormal blood vessels are called subretinal neovascular membranes, and their proliferation results in a rapid loss of vision. Occasionally, the dry form of macular degeneration may develop into the wet form.
Although both wet and dry forms drastically diminish visual acuity, neither result in complete vision loss. Furthermore, peripheral vision is not affected.
At this time, there is no treatment for the dry form of macular degeneration. If the wet form is detected in its early stages, laser surgeries are recommended in certain cases; however, it is estimated that within one year up to 50 percent of the laser lesions may experience regrowth of new blood vessels. [3] Laser treatment seals the abnormal blood vessels to prevent further bleeding and growth--it does not restore vision. In addition to laser treatment, there are some experimental forms of surgery that actually remove the abnormal blood vessels. However, these procedures are still considered quite risky.
Given the fact that macular degeneration causes significant vision loss in thousands of people each year, and that there are few treatment options, a pertinent question is: Can supplements help prevent the progression of this debilitating condition?
For some time, researchers have been investigating the possibility that antioxidants may be able to combat--or prevent--macular degeneration. Why? The answer lies in the fascinating physiology of the eye.
How The Eye Works
The eye functions much like a camera. A lens system at the front of the eye collects and focuses light rays. The iris (the colored part of the eye) acts as an aperture, and the retina may be compared to film that captures the images. When light rays reach the retina, they are converted into electrical nerve signals, then sent through the optic nerve to the brain.
To perform these functions, a host of nutrients are needed to nourish the eye. For example, antioxidants such as vitamins C and E, beta-carotene and lutein, as well as zinc, selenium and copper, are all found in the macula. In addition to providing nourishment, these antioxidants protect against free radicals, which inevitably form as a result of all the activity generated by the eye. David Newsome, M.D., of the Tulane University School of Medicine in New Orleans, is the author of several studies on macular degeneration. He notes, "Where the oxygen-containing environment is especially rich and the metabolic rate is high, as in the macula, oxidative free radicals are generated plentifully." [4]
In addition to the wear and tear of chemical activity, the constant processing of light, particularly blue and ultraviolet, can cause photo damage to the eye. One interesting theory is that melanin, which is present in the skin and also contributes to the color of the eye, helps absorb wavelengths of light and acts as an antioxidant by protecting against free radical damage. Consequently, people with green or blue eyes and light skin are more likely to experience macular degeneration than people with dark eyes or skin. Newsome observes that melanin may be the protective factor. [5]
The bottom line is that researchers believe antioxidants may help stave off some of the harmful oxidative processes that result from the eye's many chemical reactions. This theory is based on an understanding of how antioxidants function in other parts of the body, as well as in the eye.
It has been observed in several studies that people who have macular degeneration tend to have low levels of certain types of antioxidants in their blood. However, the research gets fuzzy when it tries to pinpoint which antioxidant is most--or least--beneficial. One study noted that low levels of lycopene (an antioxidant found in tomatoes) was most strongly linked with the development of macular degeneration. [6]
Carotenoid Protection
Another study focused on the positive role that carotenoids seemed to play in preventing macular degeneration--particularly lutein and zeaxanthin, both found exclusively in dietary sources such as dark green leafy vegetables (e.g., kale and spinach). [7 ]Researchers are intrigued by these two carotenoids because they are important to macula function. The foveal part of the retina (a depression in the center of the macula) has a yellow pigmentation that is composed primarily of lutein and zeaxanthin. Given that these carotenoids are present in large quantities, researchers suspect that their role is significant, although the mechanism for their functioning has not yet been determined.
It is also unclear whether a cause-and-effect relationship exists between the dietary intake of carotenoids and their absorption in the body. Interesting clues suggest that men and women may actually use these carotenoids differently. One recent study found that men showed a 38 percent higher macular pigment density than women, despite matching carotenoid blood levels. [8] More research is needed to determine the functions of lutein and zeaxanthin and their potential role in preventing macular degeneration.
Another nutrient that has garnered interest as a potentially beneficial infection fighter is the trace mineral zinc. While results concerning antioxidants are still sketchy, research results from zinc studies are slightly more conclusive.
Zinc is highly concentrated in the retina and tissues surrounding the macula. More than 100 different enzymes rely on zinc to function, and it is instrumental in many chemical reactions in the retina. [9] As with antioxidants, zinc also helps to protect cell membranes from free radical damage.
Several studies have found that zinc does seem to provide some protective benefits against certain types of macular degeneration, most notably the early stages of dry macular degeneration. However, zinc does not appear to affect the wet form of macular degeneration. According to a recent study of nine patients who had the wet form of macular degeneration in one eye, no improvement occurred when they were given zinc supplements. [10]
Since macular degeneration generally occurs in the later stages of life, it's important to consider how dietary factors could contribute to its development. It is a well-known fact that many elderly people eat diets that are low in antioxidants. Furthermore, food sources that are rich in zinc, such as meat and seafood, may also be eaten in reduced amounts for many reasons: They're relatively expensive; they're more difficult to store; they're subject to spoilage; and they can be hard to digest. Even if they do eat zinc-rich protein sources, many elderly people experience reduced output of stomach acid (achlorhydria). Consequently, their bodies' ability to absorb sufficient levels of zinc, as well as antioxidants, may be compromised.
Although current research does not provide strong conclusive evidence to support the potential role of antioxidants and zinc in preventing macular degeneration, many physicians and ophthalmologists do recommend supplementing with these nutrients if it is not contraindicated by other health conditions. In the case of zinc, however, patients must be instructed not to take more than the recommended levels (50 mg/day) to avoid toxicity and possible interference with absorption of other trace minerals such as copper. Zinc is also not advised for patients who are taking blood-thinning medications such as coumadin®.
Last year, a study heralded as a "breakthrough discovery" in macular degeneration research was published in Science. Study authors discovered a cluster of genetic mutations believed to cause nearly one-sixth of all macular degeneration cases. [11] These mutations affect a particular gene that if mutated causes cells to produce a protein that helps destroy the macula.
This is one of the first concrete findings in macular degeneration research. Further studies are needed to explore how nutritional interventions and other methods, such as photodynamic therapy, may be combined to treat the disease.
Sidebars:
Preventing Macular Degeneration
References:
1. "Consumer's Guide to Macular Degeneration." http://www.eyecare.org/consumer/disease/md.html
2. Ibid.
3. "Wet Age-Related Macular Degeneration." http://www.iex.net/eol/retinal/wet_amd.html
4. Newsome, D. "The role of antioxidants in macular degeneration: An update." Ophthalmic Practice, 12(4): 169-71, 1994.
5. Ibid.
6. Mares-Perlman, J.A., et al. "Serum antioxidants and age-related macular degeneration in a population-based case-control study." Archives of Ophthalmology, 113(12, 15): 18-23, December 1995.
7. Seddon, J.M., et al. JAMA, 272: 1413-20, 1994.
8. Hammond, B.R., et al. "Sex differences in macular pigment optical density: Relation to plasma carotenoid concentrations and dietary patterns." Vision Research, 36(13): 2001-12, July 1996.
9. "Nutrition and Macular Degeneration." http://www. eyenet.org//public/faqs/nutrition_faq.html
10. Stur, M., et al. "Oral zinc and the second eye in age-related macular degeneration." Invest Ophthalmol Vis Sci, 37(7, 12): 25-35, June 1996.
11. Allikmets, R., et.al. "Mutation of the stargardt disease gene (ABCR) in age-related macular degeneration." Science, 277(5333): 1805, Sept. 19, 1997.Michelle Badash has 12 years of experience working at a nutrition research center in Boston. She has written and edited a newsletter about nutrition research and is also a free-lance writer.
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