Lutein ( ) (from Latin lutea meaning "yellow") is one of over 600 known naturally occurring carotenoids. Found in green leafy vegetables such as spinach and kale, lutein is employed by organisms as an antioxidant and for blue light absorption. Lutein is present in the plant as fatty-acid ester, with one or two fatty acids bound to the two hydroxyl-groups. Saponification of lutein esters yields lutein in approximately a 2:1 weight-to-weight conversion. Lutein is also found in egg yolks, animal fats and the corpus luteum.

Lutein is a lipophilic molecule and is generally insoluble in water. The presence of the long chromophore of conjugated double bonds (polyene chain) provides the distinctive light-absorbing properties. The polyene chain is susceptible to oxidative degradation by light or heat and is chemically unstable in acids.

The principal natural stereoisomer of lutein is (3R,3'R,6'R)-beta, epsilon-Carotene-3,3'-diol.

As a pigment

This xanthophyll, like its sister compound zeaxanthin, has primarily been used as a natural colorant due to its orange-red color. Lutein absorbs blue light and therefore appears yellow at low concentrations and orange-red at high concentrations.

Lutein was traditionally used in chicken feed to provide the yellow color of broiler chicken skin. Polled consumers viewed yellow chicken skin more favorably than white chicken skin. Such lutein fortification also results in a darker yellow egg yolk. Today the coloring of the egg yolk has become the primary reason for feed fortification. Lutein is not used as a colorant in other foods due to its limited stability, especially in the presence of other dyes.

Role in human eyes

Lutein was found to be present in a concentrated area of the macula, a small area of the retina responsible for central vision. The hypothesis for the natural concentration is that lutein helps protect from oxidative stress and high-energy light. Various research studies have shown that a direct relationship exists between lutein intake and pigmentation in the eye. Johnson, E.J., et al., Relation among serum and tissue concentrations of lutein and zeaxanthin and macular pigment density. ''Am J Clin Nutr. 2000 Jun; 71(6): 1555-62. {{PMID]] was a small study, in which the authors concluded that more study was needed.

Lutein may also play a role in Haidinger's brush, an entoptic phenomenon that allows humans to detect polarized light.

Lutein is a natural part of human diet when fruits and vegetables are consumed. For individuals lacking sufficient lutein intake, lutein-fortified foods are available, or in the case of elderly people with a poorly absorbing digestive system, a sublingual spray is available. As early as 1996, lutein has been incorporated into dietary supplements. While no recommended daily allowance currently exists for lutein as for other nutrients, positive effects have been seen at dietary intake levels of 6 mg/day. The only definitive side effect of excess lutein consumption is bronzing of the skin (carotenodermia).