Carotenoids are naturally occurring organic pigments (absorb blue light) in plants.
The two classes of carotenoids are the oxegen based xanthophylls and the unoxidized (oxygen free) carotenes.
Over 600 carotenoids have been identified, although only 60 or so are found in food and around 20 in the modern diet. The xanthophylls carotenoids include lutein, zeaxanthin and more. The carotene carotenoids include alpha-carotene, beta-carotene, lycopene and more.
Carotenoids are the main source of pigments in fruit and vegetables in a class of natural fat-soluble pigments found in algae, and photosynthetic bacteria, where they play a critical role in the photosynthetic process.
Carotenoids occur in some non-photosynthetic bacteria, yeasts, and molds, where they may carry out a protective function against damage by light and oxygen. People appear to be incapable of synthesizing carotenoids, we must incorporate them from their diet. Within the body, carotenoids provide bright coloration, serve as antioxidants, and can be a source for vitamin A activity.
The most abundant carotenoids in the North American diet are beta-carotene, alpha-carotene, gamma-carotene, lycopene, lutein, beta-crpytoxanthin, zeaxanthin, and astaxanthin.
Carotenoids are a potential anti-cancer and anti-aging compound. Carotenoids are powerful antioxidants, protecting the cells of the body from damage caused by free radicals. Carotenoids, and specifically beta-carotene, are believed to enhance the function of the immune system.
Cryptoxanthin appears to play an important role in health - one study found women with cervical cancer to have very low blood levels of this carotenoid. Best food sources are oranges, papaya, peaches and tangerine.
Lycopene is a red coloured compound, found in tomatoes. It acts as an antioxidant in the body helping to protect the body's cells from damage by excess free radicals. Probably the most effective scavenger of singlet oxygen free radical. Research suggests that a high consumption of tomatoes and tomato products - the main source of lycopene - may lower risk of prostate and stomach cancer as well as the risk of heart attack. The best food sources of lycopene are tomatoes, watermelon and pink grapefruit.
Most recent interest has focused on antioxidant, anticancer, and immune-enhancing properties of carotenoids. Research is also continuing on food carotenoids as sources of dietary provitamin A.
Carotenoids participate in female reproduction. The exact function of carotenoids in female reproduction has not yet been identified, it is known that the corpus luteum has the highest concentration of beta-carotene of any organ in the body, suggesting that this nutrient plays an important role in reproductive processes.
Until late in the 20th Century, the functions of carotenoids were discussed only in terms of their potential vitamin A activity. Approximately 50 carotenoids of the known 600, are called "provitamin A" compounds because the body can convert them into retinol, an active form of vitamin A.
Carotenoids are defined by their chemical structure. The majority carotenoids are derived from a 40-carbon polyene chain, which could be considered the backbone of the molecule. This chain may be terminated by cyclic end-groups (rings) and may be complemented with oxygen-containing functional groups. The hydrocarbon carotenoids are known as carotenes, while oxygenated derivatives of these hydrocarbons are known as xanthophylls. Beta-carotene, the principal carotenoid in carrots, is a familiar carotene, while lutein, the major yellow pigment of marigold petals, is a common xanthophyll.
The structure of a carotenoid determines what biological function(s) the pigment may have. The distinctive pattern of alternating single and double bonds in the polyene backbone of carotenoids is what allows them to absorb excess energy from other molecules, while the nature of the specific end groups on carotenoids may influence their polarity. The former may account for the antioxidant properties of biological carotenoids, the latter may explain the differences in the ways that individual carotenoids interact with biological membranes.
Beta-carotene supplements reduce blood levels of lutein, suggesting that carotenoids may compete with each other for absorption.
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