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Carotenoids are characterized by a large (35-40 carbon atoms) polyene chain, sometimes terminated by rings. Carotenoids where some of the double bonds have been oxidized such as lutein and zeaxanthin, are known as xanthophylls; the un-oxidized carotenoids such as alpha-carotene, beta-carotene and lycopene are known as carotenes. Probably the most well-known carotenoid is the one that gives this group its name, carotene, found in carrots and responsible for their bright orange colour. Carotenoids are a highly colored (red, orange, and yellow) group of fat-soluble plant pigments. All organisms, whether bacteria or plants, rely on the sun for energy contain carotenoids.

Their color, ranging from pale yellow, through bright orange, to deep red, is directly linked to their structure: The double carbon-carbon bonds interact with each other in a process called conjugation. As the number of double bonds increases, the wavelength of the absorbed light increases, giving the compound an increasingly red appearance.

Carotenoids are fat-soluble compounds formerly called lipochromes. Their nutraceutical significance is attributed to their well documented antioxidant properties. Their antioxidant effects enable these compounds to play a crucial role in protecting organisms against damage during photosynthesis, the process of converting sunlight into chemical energy. In photosynthetic organisms, carotenoids play a vital role in the photosynthetic reaction centre. They either participate in the energy-transfer process, or protect the reaction center from auto-oxidation. In non-photosynthetic organisms, carotenoids have been linked to oxidation-preventing mechanisms.

Health benefits of carotenoids

Carotenoids are converted to vitamin A mainly in the intestine and liver. About 10% of dietary carotenoids are converted to vitamin A in the body and contribute 25% of our total vitamin A. Of the 600 carotenoids that have been identified, about 30 to 50 are believed to have vitamin A activity. Carotenoids the body converts to vitamin A are referred to as "provitamin A" carotenoids. The most well known of this group are beta-carotene and alpha-carotene. Many carotenoids are antioxidants that protect cells against free radicals by neutralizing them before they cause oxidative damage. As antioxidants, carotenoids protect cells from damage by unstable oxygen molecules called free radicals. Carotenoids may inhibit the growth of cancer cells. Epidemiological studies have shown that people with high beta-carotene intake and high plasma levels of beta-carotene have a significantly reduced risk of lung cancer. Lycopene and beta-carotene, taken along with vitamins C and E, may help protect the body against the effects of chemotherapy or radiation.

All carotenoids help to prevent heart disease and heart attack by inhibiting the formation of harmful LDL cholesterol. Because they are very soluble in fat and very insoluble in water, carotenoids circulate in lipoproteins along with cholesterol and other fats. Carotenoids are able to absorb light in the visible range of the spectrum. By filtering out the sun's harmful ultraviolet (UV) rays and by keeping free radicals from damaging the retina, carotenoid may help to protect against macular degeneration. Reducing the amount of blue light that reaches the structures of the eye that are critical to vision may protect them from light-induced oxidative damage.

Major carotenoids

Beta-carotene - Beta-carotene is one of the orange dyes found in most green leaves, and in carrots. Carotene is a terpene, an orange photosynthetic pigment, important for photosynthesis. Carotenes are hydrocarbons and most are found in yellow, orange, and red vegetables. Beta-carotene is made up of eight isoprene units, which are cyclised at each end. Beta-carotene is the most potent precursor to vitamin A. Beta-carotene is considered a conditionally essential nutrient. Beta-carotene becomes an essential nutrient when the dietary intake of retinol (vitamin A) is inadequate.

Lycopene - Lycopene is a carotenoid responsible for the red color of the tomato, watermelon and pink grapefruit. It has a unique long chain molecular structure containing 13 double bonds, more than any other carotenoid. This configuration is responsible for lycopene's special ability to neutralize free radicals. It protects against breast, skin and prostate cancer. It is a very potent anti-oxidant, detoxifier, and helps inhibit the production of cholesterol. Lycopene is more than twice as powerful as beta-carotene at quenching free radicals.

Lutein and zeaxanthin - Lutein is a carotenoid pigment found in many green vegetables, and is usually derived from marigold flower petals. Lutein and zeaxanthin belong to the xanthophyll family of carotenoids and are the two major components of the macular pigment of the retina. kale and broccoli. Zeaxanthin is a stereoisomer of lutein, differing only in the location of a double bond in one of the hydroxyl groups. Lutein is found in high concentrations in the eye, and has been shown to protect against blindness caused by macular degeneration. Lutein and zeaxanthin, which are naturally present in the macula of the human retina, filter out potentially phototoxic blue light and near-ultraviolet radiation from the macula.

Dietary sources of carotenoids

Carotenoids are naturally present in edible leaves, flowers, and fruits, and are readily obtained from flowers (i.e. marigold), berries, and root tissue (i.e. carrots). In general, the greater the intensity of color, the higher the level of carotenoids. Hydrocarbon carotenes, such as β-carotene and lycopene, are typically present in an uncombined free form, which is entrapped within chloroplast bodies within plant cells. Xanthophylls, such as lutein, are abundant in a number of yellow or orange fruits and vegetables such as peaches, mango, papaya, prunes, acorn squash, and oranges. Food sources of lutein and zeaxanthin, include corn, egg yolks and green vegetables and fruits, such as broccoli, green beans, green peas, brussel sprouts, cabbage, kale, collard greens, spinach, lettuce, kiwi and honeydew. Lutein and zeaxanthin are also found in nettles, algae and the petals of many yellow flowers. Lycopene is found in raw (unprocessed) tomatoes is contained within a matrix. The richest sources of beta-carotene are yellow, orange, and green leafy fruits and vegetables (such as carrots, spinach, lettuce, tomatoes, sweet potatoes, broccoli, cantaloupe, and winter squash). Generally, the free forms of carotenoids are present in the chlorplasts of green plants such as alfalfa, spinach, kale and leafy green plant materials. The free form of the carotinoids provides better adsorption when consumed in foods or as a supplement.