Submitted to: Eggs and Health: Risk-Benefit Considerations
Publication Type: Book / Chapter
Publication Acceptance Date: December 10, 2006
Publication Date: N/A
Technical Abstract: A growing body of evidence suggests that dietary xanthophylls may contribute to the protection against several age-related diseases, including cataract, AMD, some forms of cancer, and heart disease. Lutein and zeaxanthin are found principally in green leafy vegetables and in chicken egg yolks. While the lutein/zeaxanthin content of a yolk is modest compared to a source like spinach, their greater bioavailability from eggs makes them an important food source for these carotenoids. Studies in humans as well as in animal models show that ingestion of these xanthophylls from foods and/or supplements increase their concentrations in blood and especially in the macula, eye lens, and skin. Although epidemiologic studies have produced mixed results regarding the association between lutein and/or zeaxanthin and AMD, cataract, and cancers of the skin and other tissues, the totality of available evidence from in vitro and in vivo studies remains promising for a protective role of xanthophylls in human health. It is important to appreciate that some of the inconsistencies in the epidemiological data may be due to problems inherent in using the xanthophyll values in common nutrient databases, the difficulty in determining associations with health outcomes when the range of intake within a cohort is limited, and the high degree of co-linearity among plasma levels of carotenoids. Of course, observational studies always present the potential for confounding from unknown dietary or other lifestyle factors not considered in the assessment instrument or for which the xanthophylls may merely serve as markers. Nonetheless, a protective role of lutein and zeaxanthin in the eye is quite plausible because of mechanisms supporting distinct actions in eye health, including binding selectively to retinal transport proteins and concentrating in this tissue, possessing potent antioxidant capacity, filtering blue light, and stabilizing membranes. When completed, the AREDS2 randomized clinical trial should provide a better understanding of the preventive and therapeutic role of xanthophylls in AMD and age-related cataract as well as its interactions with n-3 fatty acids and multivitamins. Indeed, interventions testing combinations of xanthophylls together with other carotenoids and related antioxidant nutrients may reveal the benefit of lutein/zeaxanthin is provided more through a synergistic role than via simple and independent mechanisms of action. As the evidence for a role of lutein and zeaxanthin in health promotion continues to evolve from basic research as well as from human studies directed to intermediary biomarkers and clinical outcomes as well as to the impact of nutrigenomics, it is worth noting that recommendations to consume foods rich in xanthophylls are consistent with all current dietary guidelines.