The structural isomers lutein and zeaxanthin are non-provitamin A carotenoids that are also measurable in human blood and tissues. Lutein and zea-xanthin have been identified as the xanthophylls that constitute the macular pigment of the human retina. The relative concentration of lutein to zeaxanthin in the macula is distinctive. Zeaxanthin is more centralized and lutein predominates towards the outer area of the macula. A putative xanthophyll-binding protein has also been described, which may explain the high variability among people to accumulate these carotenoids into eye tissues. Increased lutein intake from both food sources and supplements is positively correlated with increased macular pigment density, which is theorized to lower risk for macular degeneration. AMD is the leading cause of irreversible blindness in the elderly in developed countries. AMD adversely affects the central field of vision and the ability to see fine detail. Some, but not all, population studies suggest lower rates of AMD among people with higher levels of lutein and zea-xanthin in the diet or blood. Possible mechanisms of action for these carotenoids include antioxidant protection of the retinal tissue and the macular pigment filtering of damaging blue light.
Free radical damage is also linked to the development of cataracts. Cataracts remain the leading cause of visual disability in the US and about one-half of the 30-50 million cases of blindness throughout the world. Although cataracts are treatable, blindness occurs because individuals have either chosen not to correct the disease or do not have access to the appropriate medical treatment. Several epidemiological studies have shown inverse associations between the risk of cataracts and carotenoid intake. However, these studies also present inconsistencies with regard to the different carotenoids and their association with cataract risk. Lutein and zea-xanthin are found in the lens and are thought to protect cells in the eye against oxidative damage, and consequently prevent formation of cataracts. However, to date, there is no evidence that any carotenoid supplement can protect against cataract development. Eating plenty of fruits and vegetables, good sources of many antioxidants including carot-enoids, is a preventative measure for many diseases.
Because lutein and zeaxanthin may be involved in disease prevention, much needs to be learned regarding human consumption of these carotenoids. One complicating factor that requires better understanding is the bioavailability of lutein from food sources and supplements. The food matrix is an important factor influencing lutein bioavailability and the amount and type of food processing generally influences the bioavailability of all carotenoids. For example, the processing of spinach does not affect bioavailability of lutein, but it does enhance that of fi-carotene. Such studies have been conducted with lutein supplements and/or foods containing lutein fed to human subjects. In humans, lutein from vegetables seems to be more bioavailable than fi-carotene; however, this may be partially explained by bioconversion of fi-carotene to vitamin A. Competition between carotenoids, such as lutein and fi-carotene, for incorporation into chylomicra has been noted in humans consuming vegetables and supplements. The amount of fat consumed with the lutein source also affects bioavailability, as higher fat increases the bioavail-ability of lipid-soluble carotenoids. Decreased plasma lutein concentrations are noted when alcohol is consumed, but the mechanism is poorly defined.
Lutein may also protect against some forms of cancer and enhance immune function. Lutein may work in concert with other carotenoids such as fi-carotene to lower cancer risk due to their antimutagenic and antitumor properties. Because of these potential health benefits, lutein supplements are sold commercially and incorporated into some multivitamins. However, the amount provided in multivitamins (about 10-20% of the level in an average diet) is likely to be too low for a biological influence. Levels of lutein available as a single supplement vary widely and neither benefit nor safety of lutein supplements has been adequately studied. Major dietary sources of both lutein and zeaxanthin in the diet include corn, green leafy vegetables, and eggs. Lutein tends to be the predominant isomer in foods. Lutein supplements are often derived from marigold flowers.
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