Why do we have plant pigments in our eyes?
Professor Julie Mares’ (PhD) research team has asked this question for the past twenty years. In fact, the question goes back over two hundred years, since a yellow spot was first noticed in post mortem specimens of human eyes.
In recent decades, researchers have confirmed that these yellow pigments might protect the retina from blue light damage, an idea first proposed by Nobel laureate George Wald in 1945, which is especially important as we age and the harmful effects of excess blue light accumulate.
The biochemical structure of these pigments – lutein, zeaxanthin and mesozeaxanthin – was identified about 30 years ago. They are structural isomers, having the same molecular formula but different shapes. Humans can absorb these carotenoid plant pigments and others, like beta-carotene, from their diet. However, of the eight most abundant carotenoids in human blood, only lutein and its isomers are taken up into the neural retina. They are the predominant carotenoids in vision processing areas in the brain, acting to lower the exposure of eye tissues to blue light. They are also strong antioxidants, helpful in reducing cell damage and inflammation.
Why is this important?
There is a potential link between these carotenoids and the development of age-related macular degeneration (AMD) – the leading cause of severe and often progressive vision loss in people over 60 years of age. Professor Mares’ team and that of coprincipal investigator Dr. Barbara Blodi are conducting the first long-term population study to determine whether women with high densities of these plant pigments in the eye (commonly referred to as macular pigment or MP) are less likely to develop age-related macular degeneration over fifteen years; or, if they already have AMD, are less likely to undergo progression of the disease. This Carotenoids in Age-Related Eye Disease Study (CAREDS) has tracked women who completed baseline study visits in 2001-2004 (CAREDS1), and are currently – 15 years later – being invited back for follow-up study visits (CAREDS2). CAREDS2 will also, in Dr. Yao Liu’s lab, be the first to study the relationship of macular pigment to the development of glaucoma over time.
In CAREDS1, researchers observed that levels of macular pigment vary widely (>10-fold) across individuals. CAREDS2 will help determine the levels of macular pigment needed to preserve vision with age and to protect against more generalized aging of the neural retina. It will also quantify the amount of lutein intake from foods and supplements – data required to set guidelines for adequate intakes, which are currently lacking. Lutein and zeaxanthin supplements now on the market vary widely, from 5 mg – the amount we get from eating a diet rich in fruits and vegetables – to more than 10 times that amount, approximately 60 mg. Levels that are effective and safe over the long term are unclear.
Results from CAREDS1 also suggested that even among women with a genetic predisposition for developing AMD, the odds of developing the disease might be decreased three-fold by following a healthy lifestyle (not smoking, eating enough fruits and vegetables, and getting enough physical activity.) CAREDS2 will determine whether these and other factors found to contribute to higher (thus beneficial) levels of macular pigments in the first study, continue to maintain higher pigment levels as the women age. The bottom line goal: to test a variety of preventive measures, including those which would increase macular pigment, in order to lower the risk of AMD.