Lutein and Zeaxanthin
Carotenoids are a family of compounds abundantly found in fruits, vegetables and green plants. Of the more than 600 carotenoids found in nature, only about 20 are found in human plasma and tissues. Of these, only lutein and zeaxanthin are specifically located in the macula and lens of the eye. Lutein and zeaxanthin, until recently, had been reported together because of their similar chemical structures. They do, however, differ in the placement of one key double bond and in their stereochemistry, which give lutein and zeaxanthin distinct properties from one another.
The risk of chronic diseases, such as heart disease, cancer and age-related eye diseases, increases as the average person ages. The anticipated rise in the incidence of these chronic diseases as the population ages has led to increased research on preventive approaches, including nutrition. Recent studies have found that diets rich in lutein and zeaxanthin may play a role in reducing the risk of serious eye diseases such as age-related macular degeneration (AMD) and cataracts.
Lutein and zeaxanthin are the only carotenoids generally found in the eye despite the fact that higher concentrations of beta-carotene and lycopene are found in the blood. This phenomenon is especially striking for zeaxanthin considering its relatively minor presence in plasma. The biological function of lutein and zeaxanthin in the eye is not fully understood, although two functions have been proposed: their role as antioxidants and the absorption of damaging near-to-UV blue light, the most energetic portion of visible light.
n antioxidant is a molecule that protects against damaging reactive oxygen species (ROS). Singlet oxygen and peroxyl radicals are two ROS that arise from normal reactions in the body as well as from exposure to cigarette smoke, air pollutants, radiation, certain drugs and environmental toxins. ROS can react with DNA, protein, or lipids and impair their physiological function. It is believed that these reactions with ROS can initiate diseases such as cancer, cardiovascular disease and age-related macular degeneration. Carotenoids inactivate singlet oxygen by physical or chemical quenching. The efficiency of quenching depends upon molecular properties of the carotenoid, such as the number of double bonds, the type of end groups or other chemical properties.
Age-related macular degeneration (AMD) is the leading cause of legal blindness among the elderly in developed countries and a leading cause of vision impairment in general. Some surveys estimate that approximately 10 million Americans show early signs of this disease and 450,000 or more may already have significant vision loss from late-stage AMD. The risk of AMD increases with age and women may be at a higher risk than men. AMD is an incurable condition. Both the severity and irreversibility of AMD have prompted a search for ways to prevent or slow its progression.
The amount of lutein and zeaxanthin in the eye is referred to as macular pigment density. People with the highest risk of AMD, that is the elderly, women, smokers and people with light iris colors, also tend to have low macular pigment density. Macular pigment density can be increased by dietary means. Such evidence provides an indirect link between dietary intake of lutein and zeaxanthin, macular pigment density and AMD risk.