Natural products beneficial to AMD

"It is clearly important to try to prevent AMD. There are a number of non-modifiable risk factors such as age, heredity, gender, pigmentation, race, and iris colour," Dr Bernstein said. "However, if we are going to try to prevent AMD, we should address the modifiable risk factors, such as smoking, cardiovascular disease, alcohol consumption, light exposure, and nutrition."

Nutrition is important for preventing AMD, because the retina and retinal pigment epithelium have highly unsaturated lipids that are susceptible to oxidative stress, according to Dr Bernstein. The lipids are in a region of the retina that is subjected to oxidative stress because of the high oxygen load and exposure to light.

Nutrition research

"Epidemiology studies performed over the years have shown that low intakes of a number of nutrients are linked to increased risk of AMD," Dr Bernstein said. "These nutrients include antioxidant vitamins and minerals, polyunsaturated fats, lutein, and zeaxanthin."

The results of the original Age-Related Eye Disease Study (AREDS) drive current clinical practice. AREDS showed that there was a significant decrease in the progression to severe AMD when patients took high doses of zinc, vitamin C, vitamin E and betacarotene. Cataract progression did not decrease significantly.

Now, AREDS II is evaluating a 'next-generation' formulation that includes the carotenoids lutein (10 mg daily) and zeaxanthin (2 mg daily) and other nutrients such as fish oil (EPA and DHA, 1,000 mg daily), along with reduced doses of zinc and beta-carotene, to see whether the effect of this combination is even more beneficial against AMD.

More than 4,000 patients currently are being followed for 5 years in the AREDS II multicentre trial at about 85 sites around the United States. Final results will not be available until 2013 or 2014.

The Moran Eye Centre has an ancillary study within AREDS II that is imaging the concentrations and distributions of lutein and zeaxanthin in the macula using autofluorescence at every study visit to assess response to the nutritional interventions, according to Dr Bernstein.

Lutein and zeaxanthin are concentrated specifically in the macula and are derived exclusively from the diet.

"They act as antioxidants and light-screening compounds in the eye," he said. "The Eye Disease Case-Control Study showed that there is an inverse correlation between serum carotenoid levels and exudative AMD."

In a subsequent ancillary study, subjects who consumed the highest levels of lutein and zeaxanthin from sources such as spinach and collard greens had a 43% lower risk of AMD, Dr Bernstein said.

"Our research has shown that there is a decrease in the levels of macular pigment in patients at risk of developing AMD," he said. "About thirty percent lower levels of macular pigment are found in patients with AMD or those who are at high risk of AMD. These levels can be modified by the diet."

The location of the carotenoids in the retina is ideal to screen blue light. Animals raised on carotenoid-free diets appear to be more susceptible to light damage in their retinas, according to Dr Bernstein.

"Fish oil constituents, EPA and DHA, are major precursors for the phospholipids of photoreceptor membranes, and multiple epidemiologic studies have indicated that they are likely to protect the retina against AMD," Dr Bernstein said. "Moreover, studies on dominant Stargardt's disease at the Moran Eye Centre have linked high consumption of fish oils with decreased severity of the disease."

Herbal supplements

Various herbal compounds have been promoted as being protective against AMD, specifically bilberry, eyebright, ginkgo biloba, and red wine (resveratrol). No prospective studies of these compounds have been conducted to support the various claims, however, he said.

On the other hand, goji berries long have been promoted as "good for the eyes" in Chinese herbal medicine, and we now know that these orange berries are a very rich source of zeaxanthin, he said.

"We have to encourage our patients to consume a healthy diet rich in green, red, and orange fruits and vegetables; to eat fish regularly; and to decrease excessive fat," Dr Bernstein concluded. "Those at high risk for visual loss from AMD should consider taking an AREDS supplement. There is growing evidence that supplements containing lutein, zeaxanthin, and fish oils also are important in protecting against AMD. Other supplements clearly require more research."