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Reprinted from Medical Sciences Bulletin , February 1995, published by Pharmaceutical
Information Associates, Ltd.
Age-related macular degeneration (AMD) is the most common cause of blindness in people
aged 65 and over. The disease affects the macula densa, the small central part of the retina responsible for the most acute vision (necessary for activities such as reading, writing,
and driving). The macula degenerates when capillaries proliferate under the retinal pigment epithelium, sometimes growing into the subretinal space. These capillaries leak, which can
detach the retinal pigment epithelium, and they may bleed, which can result in scar formation. Early signs of macular degeneration include decrease in visual acuity, distortion of the
shape of objects, and paracentral scotoma. Sometimes only one eye is affected and the patient is unaware of visual changes until the other eye is covered during an eye exam. AMD
is a widespread condition in the elderly, and the total number of afflicted patients is increasing as the population ages. Today there are 30 million Americans over age 65, more than
the total population of Canada, and by the year 2050 there may be as many as 70 million people in this age group. According to Framingham data, macular degeneration may affect as many
as 20% of people age 65 and over, compared with less than 2% of people aged 52 to 64. In the Beaver Dam Eye Study, almost 37% of subjects aged 75 or older had AMD. This means that
today, some 6 to 11 million elderly people are condemned to progressive loss of freedom as their eyesight deteriorates. There are very few treatment options available for AMD.
Argon-laser photocoagulation is used to destroy the capillaries, but neovascularization often recurs. There are anecdotal reports that oral zinc is useful, but clinical data are
limited. Several drugs are in early stages of investigation for the disease, including interferons, angiogenesis inhibitors, and nerve growth factors. (D'Amico DJ. N Engl J Med. 1994;
331: 95-106.)
Since AMD is so prevalent and therapy is so limited, several researchers are focusing on prevention. The outer retina is rich in polyunsaturated fatty acids, hich
are easily oxidized by free-radicals and singlet oxygen produced in the course of normal metabolism and also through the effects of light. Because nutritional factors may play a role
in AMD, Seddon et al. decided to correlate the disease with dietary antioxidant intake in subjects participating in the NIH Eye Disease Case-Control Study. Enrolled were 356 case
subjects with AMD (56% female; average age 71) and 520 control subjects with other eye diseases (55% female; average age 68). Since AMD is uncommon among nonwhites, only six were
enrolled.
Each subject completed a food frequency questionnaire (with 60 food items) and answered questions about the use of multivitamin supplements containing vitamins A, C,
and E, and selenium, iron, zinc, and calcium. After adjusting for energy intake and confounding factors--age, sex, clinical center, education, current and former smoking status,
alcohol intake, body mass index, systolic blood pressure, and self-reported physical activity--the investigators found that higher intakes of carotenoids were associated with a
reduced risk of exudative neovascular AMD. There was a statistically significant and apparently linear trend for a reduction in risk for AMD with increasing amounts of carotenoids in
the diet. The risk was reduced by an estimated 43% in subjects whose dietary intake of carotenoid-rich foods was in the highest quintile compared with those in the lowest quintile
(odds ratio 0.57). The odds ratio in cigarette smokers was about the same (0.56).
Of the carotenoids evaluated, lutein and zeaxanthin were the most strongly associated with
reduced risk of AMD. These two carotenoids are obtained primarily from dark green, leafy vegetables such as spinach and collard greens (which were on the food list) and kale, mustard
greens, and turnip greens (not on the list). Eating spinach and collard greens five or more times a week markedly reduced the risk of AMD (odds ratio 0.14). The intake of retinol
(preformed vitamin A) supplements was not related to AMD. Vitamin C obtained from food reduced the risk somewhat, but this was not statistically significant. Vitamin E did not offer a
protective effect; in fact, said the investigators, "overall results were in the opposite direction."
Lutein and zeaxanthin may protect the eye through antioxidant
mechanisms, or they may have another effect. According to Seddon et al., these two yellow pigments are the dominant pigments in the macula. By contrast, beta carotene and lycophene
are virtually absent in the macula. Lutein and zeaxanthin enter the eye from the plasma and selectively accumulate in the retina, where they filter out visible blue light. The lens
and cornea filter out ultraviolet light, but visible blue light reaches the retina, where it can cause photodamage that contributes, over time, to degeneration of the macula. Thus,
lutein and zeaxanthin may protect against photodamage of the retina, they may protect against peroxidation of fatty acids in the photoreceptor membrane, and they may protect the blood
vessels that supply the macular region. Whatever the mechanism, increasing the consumption of dark green, leafy vegetables appears to be a good idea in general for protecting against
macular degeneration. (Seddon JM et al. JAMA. 1994; 272:1413-1420.) |
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