Carotenoids and the Retina
Countless studies demonstrate an association between consumption of carotenoids and lowered risk of cancer and cardiovascular disease. Carotenoids, especially lutein and zeaxanthin, have also been found to help preserve eye health. Lutein is a pigment found in dark, green, leafy vegetables (eg, spinach, kale, broccoli, and collard greens). Zeaxanthin is found in fruits and vegetables with yellow hues (eg, corn, peaches, persimmons, and mangoes). Because lutein and zeaxanthin are structurally very similar, found in many of the same foods, and both present in the retina, they are often lumped together when discussed or studied. Lutein and zeaxanthin have been found to positively affect macular pigment density and help prevent age-related macular degeneration (AMD).
Although there are several hundred carotenoids in fruits and vegetables, only lutein and zeaxanthin are found in the retina (Schalch 1992; Yeum 1999). 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 (Siems 1999). Authors suggested 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. Using retinas from human donor eyes, investigators demonstrated that the concentration of lutein and zeaxanthin was 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 (Rapp 2000). The fact that lutein and zeaxanthin are particularly concentrated in these parts of the eye suggests they may act as a shield or filter to help absorb harmful UVB light and dangerous free-radical molecules, both of which threaten retinal tissue (Moeller 2000; Bernstein 2001).