Research highlights risk factors for age-related vision loss
Chicago – Eating fish frequently may be associated with decreased chances of developing age-related macular degeneration, while smoking nearly doubles the risk for this common cause of vision loss and hormone therapy appears to have no effect, according to three articles in the July issue of Archives of Ophthalmology, one of the JAMA/Archives journals.
Age-related macular degeneration (AMD) occurs when the macula, the area at the back of the retina that produces the sharpest vision, begins to deteriorate. The condition affects approximately 30 percent of Americans age 75 years and older, with 6 to 8 percent developing advanced cases, according to background information in one of the articles. It is the most prevalent cause of vision loss and blindness in the elderly population. Researchers have hypothesized that many of the risk factors for cardiovascular disease, including atherosclerosis or blocked arteries, may also contribute to the development of AMD, possibly by affecting blood flow to the eye.
In the first study, Johanna M. Seddon, M.D., Sc.M., of the Massachusetts Eye and Ear Infirmary, Boston, and colleagues studied genetic and environmental risk factors for AMD in 681 elderly male twins. The men underwent an examination by an ophthalmologist, filled out a food questionnaire and participated in a telephone interview to assess other risk factors, including demographics, smoking, alcohol consumption and physical activity habits. AMD was diagnosed using photographs of the inner eye.
Of the 681 men, 222 (average age 75.9 years) had intermediate or late-stage AMD and 459 (average age 74.5 years) had no AMD or were in the very early stages. Those who currently smoked had a 1.9-fold increased risk of AMD and those who had smoked in the past had a 1.7-fold increased risk. Those who ate more fish and more omega-3 fatty acids (found in salmon and other fish), were less likely to have AMD. The greatest reduction in risk was seen among individuals who ate two or more servings of fish per week. The benefits of eating more omega-3 fatty acids were most apparent among those who consumed less linoleic acid, an omega-6 fatty acid, suggesting that the proper balance of fats is key, the authors write.
"About a third of the risk of AMD in this twin study cohort could be attributable to cigarette smoking, and about a fifth of the cases were estimated as preventable with higher fish and omega-3 fatty acid dietary intake," they conclude. "Age-related macular degeneration is a common eye disease in older persons, smoking is a common avoidable behavior and dietary habits are modifiable; therefore, a proportion of visual impairment and blindness due to AMD could be prevented with attention to healthy lifestyles." (Arch Ophthalmol. 2006;124:995-1001. Available pre-embargo to the media at www.jamamedia.org.)
Eating Fish Protects Against Macular Degeneration
In a second study, Brian Chua, B.Sc., M.B.B.S., M.P.H., Westmead Millennium Institute and Vision Co-operative Research Centre, Syndney, Australia, and colleagues examined the association between dietary fat intake and AMD risk in 2,895 Australians age 49 years or older, beginning in 1992-1994. At the beginning of the study and again five years later, participants had a comprehensive eye exam that included photographs of the retina. They also filled out a questionnaire with data about food types and portion sizes consumed, including specific information about margarines, butters, oils and supplements.
Of the 2,335 participants who participated in the five-year follow-up, 158 had developed early AMD and 26 late-stage AMD. After adjusting for other factors that may influence risk, including smoking, age, sex and vitamin C intake, those in the group with the highest intake of polyunsaturated fat had a 50 percent reduced chance of developing early AMD compared with those who ate the least. Those who ate fish once a week had reduced risk of early AMD by 40 percent compared with those who ate fish less than once per month, and those who ate fish three or more times per week also had reduced risk for late-stage AMD. Contrary to previous studies showing an increased risk for AMD with higher unsaturated fat intake, no link was found between AMD and consumption of butter, margarine or nuts, which all contain high levels of unsaturated fats.
"To explain our findings, we suggest that insufficient essential fatty acid intake could result in abnormal retinal metabolism and cell renewal," the authors write. "Studies have shown cardioprotective benefits of monounsaturated fatty acids in the Mediterranean diet and that diets high in n[omega]-3 fatty acids, particularly docosahexaenoic acid, derived largely from fish, may protect against retinal oxidation and degeneration. Our finding that at least weekly fish consumption was protective against incident early age-related maculopathy provides support for this hypothesis." (Arch Ophthalmol. 2006;124:981-986. Available pre-embargo to the media at www.jamamedia.org.)
Hormone Therapy Does Not Affect Age-Related Vision Loss
Postmenopausal hormone therapy does not appear to increase or decrease the overall risk of AMD among women, although combination hormones may slightly reduce the chances of developing certain risk factors or types of the condition, according to a third report in the same issue.
Mary N. Haan, M.P.H., Dr.P.H., University of Michigan, Ann Arbor, and colleagues studied 4,262 women age 65 years and older who were part of the Women's Health Initiative Sight Exam Study, part of the larger Women's Health Initiative clinical trial of hormone therapy. Of those, 1,627 were in the estrogen-only group of the study, with 48.1 percent taking hormones and 51.9 percent taking placebo. The other 2,635 women were in the combination hormone trial; 52.3 percent of those participants were taking estrogen plus progestin pills and 47.7 percent received placebo. Participants underwent eye assessments and retinal photography at the beginning of the study, between April 2000 and June 2002.
After an average of five years of follow-up, 21 percent of the women had developed AMD. Neither combination nor estrogen-alone therapy was found to be associated with developing AMD. Among women in the combination trial only, active hormone therapy was associated with a slightly reduced risk of developing soft drusen--deposits in the eye that may precede AMD--and also lower odds of having neovascular AMD, a less common form of the condition in which blood vessels grow underneath the retina, impairing vision.
"We conclude that treatment with hormones does not influence the occurrence of early AMD," the authors conclude. "As an exception, a possible protective effect was found for soft drusen or neovascular AMD in relation to combined equine estrogens plus progestin." (Arch Ophthalmol. 2006;124:988-992. Available pre-embargo to the media at www.jamamedia.org.)
Editor's Note:This study was funded by a grant from the National Institutes of Health, Bethesda, Md.; the Foundation Fighting Blindness Inc., Owings Mills, Md.; DSM Inc., Parsippany, N.J.; the Retirement Research Foundation, Chicago; the Massachusetts Lions Eye Research Fund Inc., Northboro; and the Epidemiology Unit Research Fund, Massachusetts Eye and Ear Infirmary, Boston.
The WHI program is supported by the National Heart, Lung and Blood Institute, U.S. Department of Health and Human Services (this study is an ancillary study to the WHI CT and Observational Study); and support for the evaluation of macular degeneration in women recruited into the WHISE Study was provided by Wyeth Ayerst Laboratories. Dr. Musch has been a consultant to Iridex, Inc., and MacuSight, Inc.
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