Dietary zeaxanthin plays an essential role in protecting the retina from light damage, report researchers at the Schepens Eye Research Institute and Department of Ophthalmology, Harvard Medical School in the US this month.
Scientists have previously theorised that the two carotenoids found in high concentrations in the macula (the retinal region responsible for fine visual activities), zeaxanthin and lutein, protect the retina because of their ability to absorb harmful blue light and their status as powerful antioxidants.
But while some clinical studies have found evidence that people with higher dietary or serum levels of zeaxanthin and lutein had reduced risk for advanced stages of age-related macular degeneration (AMD), others have found no association. Now, the Schepens project researchers claim to have provided the first direct experimental evidence that carotenoids do protect the retina.
To test this protection, the team selected Japanese quail, because the retina resembles the human macula in having more cone photoreceptors than rods, and in highly selective accumulation of zeaxanthin and lutein from their diet. Rodents were not studied because their retinas have very few cones and do not accumulate carotenoids. The studies examined the effect of manipulating dietary carotenoids on light damage to retinas.
Kathleen Dorey, principal investigator, and her colleagues raised quail on diets that were normal, carotenoid-deficient, or carotenoid-deficient supplemented with high doses of zeaxanthin.
In the short-term study, reported in the November 2002 issue of Investigative Ophthalmology and Visual Science (IOVS), the team divided the carotenoid-deficient quail into two groups, and for one week preceding light damage, they fed one group zeaxanthin-supplemented diet. The study established that photoprotection was strongly correlated with the concentration of zeaxanthin in the retinas of the quail.
Retinas with low concentrations of zeaxanthin had suffered severe light damage, as evidenced by a very high number of apoptotic photoreceptor cells, while the group with high zeaxanthin concentrations had minimal damage. Apoptosis is programmed cell death, the final common pathway for photoreceptor death in retinal degeneration.
In the long-term study, reported in the November 2002 issue of Experimental Eye Research, groups of quail were raised for six months on carotenoid-deficient, normal or zeaxanthin-supplemented diets before exposure to brighter light. The results showed extensive damage to the retina in the carotenoid-deficient animals, as evidenced by large numbers of both dying photoreceptors and gaps or "ghosts" marking sites where photoreceptors had died. The group of quail with normal dietary levels of zeaxanthin showed significantly less retinal damage than did the zeaxanthin-deprived group, while the quail group receiving high levels of zeaxanthin had few ghosts in their retinas.
These experiments by Dr. Dorey's team showed protection of both rod and cone photoreceptors. The research further demonstrated that retinas were protected by both zeaxanthin and another antioxidant, vitamin E. Damage in these experiments was clearly reduced by zeaxanthin and tocopherol, but not lutein. Further experiments would be needed to determine whether elevated lutein would offer protection, said the team.
The results of the Schepens studies follow the Age Related Eye Disease Study (AREDS), sponsored by the National Eye Institute of the National Institutes of Health, which concluded that daily consumption of a formula containing high doses of dietary antioxidants are effective in slowing the progression of AMD in patients with advanced stages of the disease. It is noteworthy that zeaxanthin and lutein, two antioxidants selectively concentrated by the macula, were not commercially available when the AREDS began and were, therefore, not included in that study.
Dr Dorey said: "Our studies showed that light damage was strongly influenced by the amount of zeaxanthin in the retina, and that significantly greater retinal protection was provided at dietary levels higher than those normally occurring in the diet. Zeaxanthin has been extensively studied for safety and has been reviewed as a dietary ingredient by the FDA."
"We hope this work further stimulates interest in clinical trials, and believe that zeaxanthin has a potential to eventually complement other strategies to improve the treatment of this vision-robbing disease," she added.