July 10 (foodconsumer.org) - By Ben Wasserman
Jul 11, 2006, 10:37

Eating fish or having a diet with higher levels of omega-3 fatty acids may help reduce risk of age-related macular degeneration (AMD) while smoking or have had a history of smoking may increase the risk of AMD, suggest two studies published in the July issue of the Archives of Ophthalmology.

AMD is the leading cause of age-related vision loss among people aged 60 or older. 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.

The studies found smokers were twice as likely as nonsmokers to get AMD. And elderly people who ate fish at least twice a week were nearly 50 percent less likely to have AMD than those who ate less than once a week.

One study led by Johanna Seddon, MD and collogues from Massachusetts Eye and Ear Infirmary and Department of Biostatistics at Harvard Medical School, involved 681 male twins at an average age 74.5 among whom 222 men suffered intermediate or late-stage AMD and 459 did not have any sign of the disease.

The twins were surveyed for a prior diagnosis of AMD and were subject to an eye examination, fundus photography, and food frequency and risk factor questionnaires including demographics, smoking, alcohol consumption and physical activity habits.

"Current smokers had a 1.9-fold increased risk of developing AMD, while past smokers had about a 1.7-fold increased risk," said Johanna M. Seddon, M.D., director of the Epidemiology Unit at the Massachusetts Eye and Ear Infirmary and an associate professor of ophthalmology at Harvard Medical School.

"We also found that increased intake of fish reduced the risk of AMD, particularly if they ate two more servings per week. Dietary omega-3 fatty intake was also inversely associated with AMD. This study of twins provides further evidence that cigarette smoking increases risk while fish consumption and omega-3 fatty acid intake reduce risk of AMD."

Those who had highest consumptions of fish (no less than two servings a week) were found 45 percent less likely to have AMD than those who ate less than one weekly serving. . The benefits of eating more omega-3 fatty acids were most apparent among those who ate less linoleic acid, an omega-6 fatty acid, 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," the Boston researchers conclude.

The effect of fish intake on age-related macular degeneration or AMD was also found in a separate study by Australian researchers Brian Chua, B.Sc., M.B.B.S., M.P.H., Westmead Millennium Institute and Vision Co-operative Research Centre, Syndney, Australia, and colleagues.

Brian Chua and colleagues studied 2,900 people aged 49 and older with an average age 63 to 65 years. Participants were surveyed for their dietary habits including food types and portion sizes consumed. Five years later, they were subject to eye examination for AMD. During the four-year follow-up, 158 subjects had developed early AMD and 26 late-stage AMD.

After considering other risk factors 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.

The study found those who reported eating no less than one serving of fish a week were 40 percent less likely to acquire early -stage AMD during the five-year study, compared to those who ate less than one serving of fish a month or did not eat fish.

Additionally, participants who ate fish frequently (at least three times a week) were less likely to have late-stage AMD, according to the study.

"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 (or AMD) provides support for this hypothesis."

It must be noted that both studies are observational although it is possible that the correlation between fish intake and the reduced risk of AMD is real. The results per se did not suggest that eating fish and or quitting smoking would definitely help maintain one's eyesight. Further case-control studies or trials are needed to confirm the benefit.

Seddon and colleagues suggest that the lower ratio of omega-6 and omega 3 fatty acids might be the reason why eating fish was associated with reduced risk of AMD as they found the greatest reduction in AMD risk was found in those who ate more fish, but less linoleic acid, an omega-6 fatty acid.

Fish or omega-3 fatty acids have been well known for a host of health benefits. While omega 6 is also often labeled as healthy oil, studies have found high intake of omega 6 fatty acids can be detrimental to more than eye vision.

Omega 3- fatty acids are commonly found in oily fish such as salmon fish, mackerel and tuna. Walnuts and flaxseed oil are also good foods rich in omega-3 fatty acids. Omega 6 fatty acids are commonly seen in corn oil, safflower and sunflower oil. Regardless of the potential benefit to AMD, these foods are beneficial to many other conditions.

Scientists believe that the ratio of omega 6 to omega 3 fatty acids in the American diet ranges from 11 to 36. The beneficial ratios should be below 4 or 3. Lower ratios have been found to render other health benefits. But no one knows for sure why eating fish is linked with reduced risk of age-related macular degeneration or AMD.