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Carotenoids and macular pigment
Scientific evidence is mounting to demonstrate the power of dietary antioxidants in maintaining eye health and warding off age-related macular degeneration. The most recent study to raid the headlines was the Age-Related Eye Disease Study (AREDS), which was carried out by the National Eye Institute. The large, multicenter study showed that a daily intake of 500 mg of vitamin C, 15 mg of beta carotene, 400 IU of vitamin E and 80 mg of zinc reduced the risk of developing advanced disease in those with intermediate damage by about 25%.
Other research has more specifically set its sights on examining the protective role of the carotenoids, lutein and zeaxanthin, against macular degeneration. Why? With several hundred carotenoids to be found, consider that only lutein and zeaxanthin are found in the retina.[22,23] Compared to other antioxidant concentrations found in the eye, German researchers found that lutein and zeaxanthin did not break down nearly as fast as lycopene and beta-carotene when exposed to free radical or UV light induced oxidative stress. The authors suggest that perhaps the slow degradation of lutein and zeaxanthin may explain the strong presence of these carotenoids in the retina. Also, the quick breakdown of lycopene and beta-carotene may suggest why these carotenoids are lacking in the same retinal tissues.
Researchers have also found that lutein and zeaxanthin are more highly concentrated in the center of the macula. There, the amounts of lutein and zeaxanthin are much greater than their concentrations in the peripheral region. At the Baylor College of Medicine, in Houston, scientific investigators demonstrated, using retinas from human donor eyes, that the concentration of lutein and zeaxanthin were 70% higher in rod outer segment (ROS) membranes where the concentration of long-chain polyunsaturated fatty acids, and susceptibility to oxidation is highest, than in residual membranes. The fact that lutein and zeaxanthin are particularly concentrated in these parts of the eye suggests that they may act as a shield or filter that helps to absorb harmful UVB light and dangerous free-radical molecules, both of which threaten the retinal tissue.[26,27]
Moreover, while macular pigment density decreases with age, and the risk of AMD increases-a coincidence that cannot be overlooked-researchers have also found that older folks with higher lutein and zeaxanthin concentrations in their macula tend not to develop the disease. Researchers at Arizona State University suggested that increasing macular pigment through dietary intake of lutein and zeaxanthin may retard age-related declines in visual function, and that high macular pigment density was associated with the retention of youthful visual sensitivity. After measuring the macular pigment density and visual sensitivity of 27 older subjects (aged 60 to 84 years) and 10 younger ones (aged 24 to 36 years), results showed that older subjects with high levels of lutein and zeaxanthin had visual sensitivity comparable to younger subjects. Conversely, older subjects with low lutein and zeaxanthin in their macula had lower visual sensitivity.
Weighing all the evidence to date, it stands to reason that increasing or maintaining levels of the carotenoids that make up the pigment, namely lutein and zeaxanthin, would support the protective role of macular pigment. While larger and longer trials will bear out what research now seems to suggest, increasing our lutein and zeaxanthin intake through diet seems to be a safe bet. However, nutritional scientists have not yet pinned down ideal amounts to recommend for lutein and zeaxanthin supplements. Bernstein explains that we currently hear much more about lutein, "since it is much more common in our diet, and commercial supplements of lutein have been available for a much longer time. Zeaxanthin supplements have been approved for human use only recently." For the time being, though, Bernstsein suggests consuming a diet high in fruits and vegetables. He adds that daily supplementation with at least 4 milligrams of lutein per day may be beneficial, but further studies are needed.
1. VandenLangenberg GM, et al. Associations between antioxidant and zinc intake and the 5-year incidence of early age-related maculopathy in the Beaver Dam Eye Study. Am J Epidemiol 1998;148:204-214.