Omega-3 fat exclusive to seafood may reduce risk of cataracts, macular degeneration
by Craig Weatherby
Walter Willet, M.D., Ph.D., M.P.H., is a professor of Epidemiology and Nutrition at Harvard University’s School of Public Health. Dr. Willett ranks among the leading nutrition researchers in the U.S.
Much of his team’s research has focused on the effects of diet on the health of participants in the Nurses' Health Study II (NHS II): one of the largest prospective investigations into the risk factors for major chronic diseases in women, established by Dr. Willett in 1989.
Cataracts and AMD: side effects of aging
A cataract is a clouding of the normally clear lens of the eye, usually related to aging, and no drugs or dietary supplements have been shown to prevent or cure them. Most people develop some lens clouding after age 60. Glasses that screen out ultraviolet (UV) rays can help slow the progression of cataracts. Symptoms include difficulty seeing at night, seeing halos around lights, and sensitivity to glare. Surgical removal—the only sure cure—becomes necessary only when you cannot see well enough with glasses.
Age-related macular degeneration (AMD) is one of the leading causes of blindness and vision impairment in older Americans. Symptoms include blurred, distorted, dim, or absent central vision (The macula is the part of the retina that allows the eye to see fine details at the center of the field of vision). In 90 percent of cases, people with AMD have the "dry" form, and studies show that its progression can be slowed by taking high levels of zinc and antioxidants. The "wet" form constitutes only 10 percent of cases, but it accounts for 90 percent of all severe vision loss from AMD. Treatments include laser and photodynamic therapy.
Over the past four years several large analytical studies based on the NHS II data—each exploring connections between various dietary fats and eye health—were published by Dr. Willet’s team. These analyses yielded intriguing results concerning seafood and vision protection.
Since 1991, the 116,000-plus female registered nurses (RNs) participating in NHS II have completed questionnaires about their diets and health condition at four-year intervals.
Together with the huge number of participants, the nurses' extraordinarily high questionnaire-response rate—upwards of 90 percent—makes NHS II a uniquely valuable nutrition-research resource.
Does diet affect vision?
Much of what you read about prevention of cataracts (cloudy lenses) and age-related macular degeneration (AMD) focuses on dietary antioxidants; especially vitamin C and the xanthophyll-type carotenoids found in spinach and other greens. (Read about cataracts and AMD in our sidebar.)
We know that oxidation plays a role in the formation of cataracts, and some forms of AMD. However, the mixed results of research leave us in the dark as to whether the dietary anti-oxidants studied—primarily, vitamins C and E and carotenoids—can help prevent these conditions. While anti-oxidants may slow the progression of the most common form of AMD, it is not clear that can do the same for cataracts.
Selecting eye-friendly fats: focus falls on fish
In recent years, Dr. Willet’s team undertook three studies of dietary fat intake and rates of eye disease, based on responses to NHS II questionnaires. To gather data on eye health and fat intake, more than 71,000 nurses in the NHS II group were followed for up to 16 years between 1984 and 2000.
The two most recent Willett studies, whose results were published earlier this year, looked at the links between dietary fat and rates of cataract surgery and the age-related eye conditions known as lens opacities.
In the cataract study, fish was shown to exert a protective effect: the 20 percent of nurses who consumed the most fish—hence the greatest amount of long-chain omega-3 fatty acids (EPA and DHA from fish)—had a 12 percent lower risk of cataract surgery, compared with the 20 percent of nurses who consumed the least fish. The authors characterized their findings this way: “… higher intake of long-chain omega-3 fatty acid (eicosapentaenoic acid and docosahexaenoic acid) and consumption of fish may modestly reduce the risk of cataract.”
In the study on lens opacities, it appeared that high intake of omega-6 EFAs and short-chain omega-3s (the kind found in plant foods such as nuts and seeds) was associated with higher risk. The authors reported no association, positive or negative, between lens opacities and intake of the long-chain omega-3s in fish.
The third Willett study, which examined links between various kinds of fat and rates of AMD, produced findings that give fish a boost. They found that risk of AMD grew with fat intake, with omega-6 EFAs and short-chain omega-3s (the kind found in plant foods) seeming chiefly responsible for increasing the risk of AMD. In contrast, the long-chain omega-3s in fish seemed to reduce the risk.
As Dr. Willett’s team reported, “Total fat intake was positively associated with risk of AMD, which may have been due to intakes of individual fatty acids, such as linolenic acid, rather than to total fat intakes per se. A high intake of fish may reduce the risk of AMD.”
As in other areas of preventive health, it seems that the long-chain omega-3s in fish may help protect our vision as we age.
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