Zeaxanthin
Zeaxanthin is one of the most common carotenoids in nature, and is used in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika (made from bell peppers), corn, saffron, goji (wolfberries), and many other plants and microbes their characteristic color. The name (pronounced "zee-uh-zan'-thin") is derived from "Zea mays" (common yellow maize corn, in which zeaxanthin provides the primary yellow pigment), plus "xanthos", the Greek word for "yellow" (see xanthophyll). 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 (an excited form of 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. Lutein 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 (3R,3′S) stereoisomer is called meso-zeaxanthin. The  principal natural form of zeaxanthin is (3R,3′R)-zeaxanthin. The macula mainly contains the (3R,3′R)- and meso-zeaxanthin forms, but it also contains much smaller amounts of the third (3S,3′S) 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 (heterologous) 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 (-OH) to carbon 3 and 3′ of the beta-carotene molecule. Zeaxanthin biosynthesis therefore proceeds from beta-carotene to zeaxanthin (a di-hydroxylated product) via beta-cryptoxanthin (the mono hydroxylated intermediate). Although functionally identical, several distinct beta-carotene hydroxylase proteins are known. Due to the nature of zeaxanthin, relative to astaxanthin (a carotenoid of significant commercial value) 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 (AMD), most notably the Age-Related Eye Disease Study (AREDS2). 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. 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 (10 mg) and zeaxanthin (2 mg) 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 "dietary supplement", such  as "Helps maintain eye health", as long as the FDA disclaimer statement ("These statements have not been evaluated...") 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. Natural occurrence. Zeaxanthin is the pigment that gives paprika, corn, saffron, wolfberries (goji), and many other plants their characteristic colors of red, orange or yellow. Spirulina is also a rich source and can serve as a dietary supplement. Zeaxanthin breaks down to form picrocrocin and safranal, which are responsible for the taste and aroma of  saffron. Foods containing the highest amounts of lutein and zeaxanthin are dark green leaf vegetables, such as kale, spinach, turnip greens, collard greens, romaine lettuce, watercress, Swiss chard and mustard greens; in green plants, the zeaxanthin may still be present, but is secondary to the prominent green of chlorophyll. Safety. An acceptable daily intake level for zeaxanthin was proposed as 0.75 mg/kg of body weight/day, or 53 mg/day for a 70 kg adult. In humans, an intake of 20 mg/day for up to six months had no adverse effects. As of 2016, neither the U.S. Food  and Drug Administration nor the European Food Safety Authority had set a Tolerable Upper Intake Level (UL) for lutein or zeaxanthin.