Zeaxanthin is one of the most common carotenoid alcohols found in nature. It is important in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika, corn, saffron, wolfberries, and many other plants and microbes their characteristic color. The name is derived from Zea mays, plus xanthos, the Greek word for "yellow". Xanthophylls such as zeaxanthin are found in highest quantity in the leaves of most green plants, where they act to modulate light energy and perhaps serve as a non-photochemical quenching agent to deal with triplet chlorophyll which is overproduced at high light levels during photosynthesis. Animals derive zeaxanthin from a plant diet. Zeaxanthin is one of the two primary xanthophyll carotenoids contained within the retina of the eye. Zeaxanthin supplements are typically taken on the supposition of supporting eye health. Although there are no reported side effects from taking zeaxanthin supplements, the actual health effects of zeaxanthin and lutein are not proven, and, as of 2018, there is no regulatory approval in the European Union or the United States for health claims about products that contain zeaxanthin. As a food additive, zeaxanthin is a food dye with E number E161h.
Isomers and macular uptake
and zeaxanthin have identical chemical formulas and are isomers, but they are not stereoisomers. The only difference between them is in the location of the double bond in one of the end rings. This difference gives lutein three chiral centers whereas zeaxanthin has two. Because of symmetry, the stereoisomers of zeaxanthin are identical. Therefore, zeaxanthin has only three stereoisomeric forms. The stereoisomer is called meso-zeaxanthin. The principal natural form of zeaxanthin is -zeaxanthin. The macula mainly contains the - and meso-zeaxanthin forms, but it also contains much smaller amounts of the third form. Evidence exists that a specific zeaxanthin-binding protein recruits circulating zeaxanthin and lutein for uptake within the macula. Due to the commercial value of carotenoids, their biosynthesis has been studied extensively in both natural products and non-natural systems such as the bacteria Escherichia coli and yeast Saccharomyces cerevisiae. Zeaxanthin biosynthesis proceeds from beta-carotene via the action of a single protein, known as a beta-carotene hydroxylase, that is able to add a hydroxyl group to carbon 3 and 3′ of the beta-carotene molecule. Zeaxanthin biosynthesis therefore proceeds from beta-carotene to zeaxanthin via beta-cryptoxanthin. Although functionally identical, several distinct beta-carotene hydroxylase proteins are known. Due to the nature of zeaxanthin, relative to astaxanthin beta-carotene hydroxylase proteins have been studied extensively.
Relationship with diseases of the eye
Several observational studies have provided preliminary evidence for high dietary intake of foods including lutein and zeaxanthin with lower incidence of age-related macular degeneration, most notably the Age-Related Eye Disease Study. Because foods high in one of these carotenoids tend to be high in the other, research does not separate effects of one from the other.
Three subsequent meta-analyses of dietary lutein and zeaxanthin concluded that these carotenoids lower the risk of progression from early stage AMD to late stage AMD.
A 2017 Cochrane review of 19 studies from several countries, however, concluded that dietary supplements containing zeaxanthin and lutein have little to no influence on the progression of AMD. In general, there remains insufficient evidence to assess the effectiveness of dietary or supplemental zeaxanthin or lutein in treatment or prevention of early AMD.
As for cataracts, two meta-analyses confirm a correlation between high serum concentrations of lutein and zeaxanthin and a decrease in the risk of nuclear cataract, but not cortical or subcapsular cataract. The reports did not separate a zeaxanthin effect from a lutein effect. The AREDS2 trial enrolled subjects at risk for progression to advanced age-related macular degeneration. Overall, the group getting lutein and zeaxanthin did not reduce the need for cataract surgery. Any benefit is more likely to be apparent in subpopulations of individuals exposed to high oxidative stress, such as heavy smokers, alcoholics or those with low dietary intake of carotenoid-rich foods. In 2005, the US Food and Drug Administration rejected a Qualified Health Claims application by Xangold, citing insufficient evidence supporting the use of a lutein- and zeaxanthin-containing supplement in prevention of AMD. Dietary supplement companies in the U.S. are allowed to sell lutein and lutein plus zeaxanthin products using Structure:Function language, such as "Helps maintain eye health", as long as the FDA disclaimer statement is on the label. In Europe, as recently as 2014, the European Food Safety Authority reviewed and rejected claims that lutein or lutein plus zeaxanthin improved vision.
