Omega-3s re-affirmed as likely guardians against age-related macular degeneration; clinical trials needed to confirm indications from epidemiological studies
by Craig Weatherby
A growing body of evidence links the omega-3 fatty acids in fish to reduced rates of age-related macular degeneration (AMD).
AMD is the leading cause of blindness in Americans aged 55 and older, and of the 30,000,000 people over age 65 in the US in 1990, almost one in three showed signs of AMD.
What is AMD?
AMD comes in two forms: early or “dry” stages, and subsequent “wet” stages. The wet forms are named for the under-retina overgrowth of blood vessels that characterize this type of AMD. Although it afflicts less than 10 percent of patients, wet AMD causes 85 percent of severe AMD-related vision loss.
The majority of wet AMD cases get little help from the leading therapy, called laser photocoagulation.
In recent years, this procedure’s efficacy has been greatly enhanced by injecting patients with a drug called verteporfin before the laser treatment is applied. (While results vary, a 78-year-old relation of this writer underwent the drug-laser AMD treatment recently with very good and durable results.)
The number of Americans over age 65 will double by the year 2030, so researchers have sought to find foods that might help prevent the vision-crippling condition.
Recent research results—such as from Australia’s “Blue Mountains” eye study—indicate that diets rich in fish and/or their characteristic omega-3s (EPA and DHA) may help prevent AMD.
(Other research findings suggest that omega-3s may also help prevent or improve dry eye syndrome, cataracts, and lens opacities.)
Earlier this month, Australian scientists published their analysis of pertinent population studies. They concluded that diets high in omega-3s and/or fatty fish may reduce the risk of early- and late-stage AMD by about one-third.
In addition to leafy green vegetables—which are rich in eye-protecting, carotene-class antioxidants such as zeaxanthin and lutein—fish is emerging as a possible partner in preventing AMD (See “Omega-3s from Fish Affirmed as Potential Eye-Protectors”).
Australian analysis affirms vision-guarding potential of fish fats
The ability of fish or omega-3 supplements to deter or treat AMD has not yet been tested in controlled clinical trials.
Accordingly, lead author Elaine Chong and her colleagues searched medical databases for relevant epidemiological studies, in which researchers looked for statistically significant links between various foods in people’s diets and the rates of AMD.
They found nine epidemiological studies that met their quality criteria: three “prospective cohort” studies, three “cross-sectional” studies, and three “case-control” studies.
The Aussies’ examination of these combined studies—called a meta-analysis—produced positive results (Chong EW et al. 2008):
- People who consumed lots of omega-3 DHA were 30 percent less likely to develop early AMD.
- People with high dietary intakes of total omega-3s (EPA+DHA) were 38 percent less likely to develop advanced AMD.
- People who consumed fish at least twice a week were less likely to develop early or late AMD, with risk reductions of 24 and 33 percent, respectively.
While noting a lack of confirming evidence from clinical trials, the results of their meta-analysis led the Australian team to this unsurprising conclusion:
“…this meta-analysis suggests that consumption of fish and foods rich in omega-3 fatty acids may be associated with a lower risk of AMD…”
Surprisingly, people who reported higher intake of alpha-linolenic acid (ALA)—the short-chain omega-3 found in flax oil and in leafy green plant foods—were 49 percent more likely to have developed early AMD.
The body only converts about five percent of dietary ALA to the long-chain, fish-style omega-3s (EPA and DHA), which are essential to basic cell functioning and to regulation of the inflammatory responses of humans’ immune systems.
Positive findings supported by plausible biological explanation
It takes more than statistical associations between a certain food (or dietary supplement) and incidence rates of a given disease to make a persuasive case that intake levels of the former are really responsible for reducing the latter.
But as lead author Elaine Chong said, “Our findings are supported by a strong underlying biological rationale.”
The long chain omega-3s found in human cell membranes and in fish fat—especially DHA (docosahexaenoic acid)—play important roles in the layer of nerve cells in the retina.
And, as the Aussie team wrote, “…a diet rich in omega-3 fatty acids and fish, as a proxy for long-chain omega-3 fatty acid intake, has therefore been hypothesized as a means to prevent AMD.”
In particular, the retina contains high levels of omega-3 DHA, which plays an essential role in the membranes of nerve cells in the retina and brain. (This is one reason that DHA is considered essential to adequate child development.)
As the Australians explained, there are at least two specific reasons why diets rich in omega-3s might help protect against AMD or slow its progress:
- The outer photoreceptor-cell segments of the retina are constantly shed in the normal visual cycle, and a deficiency of dietary DHA could promote AMD.
- Omega-3s exert anti-inflammatory influences that could protect against age-related declines in the vascular and neural retina.
As we noted in our last report on this issue, the “omega-imbalance” that characterizes the standard American diet—which is extremely high in omega-6s and very low in omega-3s—appears to promote AMD.
A data analysis conducted under the auspices of the National Eye Institute indicated that omega-3s protect against AMD, and that excessive intake of omega-6 fatty acids promotes increased risk of AMD (Seddon JM, Cote J, Rosner B 2003).
And as we reported last spring, a study indicates that vitamin D—which is uniquely abundant in wild Salmon—exerts preventive effects on AMD (See “Vitamin D Adds Eye Health to Roster of Recent Accolades”).
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