Research is uncovering yet another reason to eat with health in mind—our eyes. Going beyond the vitamin A in carrots that help us to see well in the dark, other nutrients may protect vision, particularly for aging eyes, including vitamins C and E, beta-carotene, two related carotenoids (lutein and zeaxanthin), zinc, and even certain types of fats.
Age-Related Eye Diseases
Age brings about changes that can lead to two common sight-robbing disorders, cataracts and age-related macular degeneration (AMD).
A cataract is a cloudy area in a part of the eye lens or the entire eye lens that keeps light from passing through the lens. As a cataract develops, it can cause blurred vision, sensitivity to light, increased nearsightedness, and distorted images, eventually causing partial or full loss of eyesight. Cataracts are also associated with diabetes, other systemic diseases, alcoholism, premature birth or birth defects, heredity, smoking, eye injuries, exposure to ultraviolet (UV) rays, and certain medications.
Macular degeneration involves damage to the macula; an area of the retina in the back of the eye responsible for the sharp central vision needed to read, drive, and perform other daily activities. Although the causes of macular degeneration are not known, risk factors include family history, smoking, high blood pressure, high blood cholesterol, and exposure to sunlight.
Visionary Research
Research is shedding rays of hope for individuals suffering from age-related eye disease, and for those at risk of eye disease. Emerging evidence suggests that risk of certain eye changes associated with aging may be reduced by dietary components.
Antioxidant Vitamins and Zinc
The Age-Related Eye Disease Study (AREDS), launched by the National Institutes of Health's National Eye Institute, is an ongoing study aimed at evaluating various combinations of high-dose antioxidant vitamins (beta-carotene and vitamins C and E) and zinc supplements on eye health. One published analysis of a 6-year period in the double-blind study examined these effects on nearly 3,600 people between the ages of 55 and 80. Subjects included men and women with varying macular status ranging from no evidence of AMD in either eye to relatively severe disease in one eye.
Study results related to AMD were impressive, although somewhat limiting. Only individuals with the intermediate and advanced stages of AMD appeared to benefit. Nevertheless, the benefit was such that the study's authors urged high risk individuals with no contraindications such as smoking, to consider taking daily supplements similar to those used in the study: 500 mg vitamin C, 400 International Units vitamin E, 15 mg beta carotene, 80 mg zinc, and 2 mg copper (copper needs are increased with high doses of zinc). The effectiveness and safety of routine use of this regimen by individuals with early AMD or persons at risk for developing AMD remain unclear. It is also important to note that this is a single study and further research is needed.
The high-dose formulations used in the AREDs study had no significant effect on the development or progression of age-related cataracts. Nevertheless, other studies have found that antioxidants have favorable effects, especially vitamin C in larger amounts and for a longer duration (>10 years).
Lutein and Zeaxanthin
The yellow-colored carotenoids, lutein and zeaxanthin, are found in a variety of vegetables, including leafy greens, broccoli, zucchini, corn, peas, and brussels sprouts to name a few. Highly concentrated deposits of lutein and zeaxanthin are also present in the macula. Here they are referred to as macular pigment.
Research has shown that the macular pigment density, or the amount of lutein and zeaxanthin in the macula, appears to be associated with AMD. Dr. Richard Bone, a professor of biophysics at Florida International University, has been studying macular pigments for more than 20 years. In a postmortem study of the eyes of people who had AMD and those who did not, Bone found that those with the highest concentrations of lutein and zeaxanthin had an 82 percent lower incidence of AMD.
Bone and colleagues also studied how lutein and zeaxanthin in the diet affect macular pigment density. "These results suggest an association between dietary intake of lutein and zeaxanthin and macular pigment density," said Bone. "But we don't know yet how much lutein and zeaxanthin are needed to raise the macular pigment density to a protective level." Currently Bone is conducting a dose-response study to determine the appropriate doses.
Dietary Fats
Several studies have hinted at an association between the amount and type of dietary fats consumed and the risk for AMD. Findings from the ongoing Beaver Dam Eye Study suggest that higher intakes of saturated fats and cholesterol may confer an increased risk for AMD.
The high level of polyunsaturated fatty acids in the retina supports the possibility that certain fats may have a protective effect against the development of AMD. Although some results have linked higher intakes of omega-3 fatty acids and fish with a decreased risk for advanced AMD, the majority of population studies have failed to establish a clear connection.
More recently, a case-control population study led by Dr. Johanna Seddon, an eye expert at Harvard University, found that diets high in vegetable (mono- and polyunsaturated) fats were associated with a higher risk for advanced AMD. Although polyunsaturated fats are considered protective against cardiovascular disease, the study's authors suggest that consumption of high levels of unsaturated fats may increase the susceptibility of the macula to oxidative damage. However, an optimal balance of high levels of omega-3 fatty acids (found in fatty fish such as salmon and mackerel) and lower levels of linoleic acid (an omega-6 fatty acid found in various vegetable fats) in the diet appeared to offer a protective effect.
Seeing is Believing
Although some of the results of studies related to the maintenance of eye health through diet are compelling for specific populations, it is too early in the study of nutrition and eye disease to draw general conclusions, advises Julie Mares-Perlman, PhD, associate professor in the Department of Ophthalmology and Visual Sciences at the University of Wisconsin-Madison.
"There are holes in the evidence that need further study," contends Mares-Perlman. As with any research, one or two studies showing a tendency are not enough. "What we do know is that a large body of evidence supports the fact that diet is important in maintaining eye health. How specific nutrients-from food or supplements-affect the types and stages of eye disease is yet to be defined," she said.
Focus on Food
Although nutrients from supplements may help slow the progression of AMD—mainly for people with intermediate and advanced stages of AMD—data on the benefits of supplementation of these nutrients is still preliminary. A strong body of evidence, however, points to foods that can be consumed to reduce the risk of both AMD and cataracts.
In a study that looked at the intakes of carotenoids and antioxidants from food, people who ate the most antioxidant-rich dark, leafy greens, particularly those rich in lutein and zeaxanthin, had about a 40 percent lower risk of macular degeneration than those who ate the least amount of these vegetables.
The project examined food records and cataract development in women between 50 and 70 years old. The results confirm that antioxidant nutrients, particularly vitamin C, at daily intakes of about three times the Daily Value for vitamin C (60 mg/day), reduced the odds for the development of cataracts by nearly half. With higher intakes, easily attained by five to nine daily servings of fruits and vegetables, the odds were reduced even more.
Resources
National Eye Institute
(National Institutes of Health)
www.nei.nih.gov
American Academy of Ophthalmology
www.eyenet.org
American Society of Cataract and Refractive Surgery
www.ascrs.org
Prevent Blindness America
www.preventblindness.org
