Eating fish protects against macular degeneration

Eating fish frequently may be associated with decreased chances of developing age-related macular degeneration ( AMD ), 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 published in the Archives of Ophthalmology.

Age-related macular degeneration 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.
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 age-related macular degeneration , possibly by affecting blood flow to the eye.

In the first study, Johanna M. Seddon, of the Massachusetts Eye and Ear Infirmary, Boston, and colleagues studied genetic and environmental risk factors for age-related macular degeneration 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.
Age-related macular degeneration was diagnosed using photographs of the inner eye.

Of the 681 men, 222 ( average age 75.9 years ) had intermediate or late-stage age-related macular degeneration and 459 ( average age 74.5 years ) had no age-related macular degeneration or were in the very early stages. Those who currently smoked had a 1.9-fold increased risk of age-related macular degeneration 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 age-related macular degeneration. 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 age-related macular degeneration 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 age-related macular degeneration could be prevented with attention to healthy lifestyles."

In a second study, Brian Chua, Westmead Millennium Institute and Vision Co-operative Research Centre, Syndney, Australia, and colleagues examined the association between dietary fat intake and age-related macular degeneration 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 age-related macular degeneration and 26 late-stage age-related macular degeneration. 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 age-related macular degeneration compared with those who ate the least. Those who ate fish once a week had reduced risk of early age-related macular degeneration 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 age-related macular degeneration. Contrary to previous studies showing an increased risk for age-related macular degeneration with higher unsaturated fat intake, no link was found between age-related macular degeneration 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."

Postmenopausal hormone therapy does not appear to increase or decrease the overall risk of age-related macular degeneration 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, 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 age-related macular degeneration. Neither combination nor estrogen-alone therapy was found to be associated with developing age-related macular degeneration . 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 age-related macular degeneration – and also lower odds of having neovascular age-related macular degeneration, 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 age-related macular degeneration, " the authors conclude. " As an exception, a possible protective effect was found for soft drusen or neovascular age-related macular degeneration in relation to combined equine estrogens plus progestin."

Source: Archives of Ophthalmology, 2006


Link: Xapedia - Medical Encyclopedia