Carotenoids are organic pigments that are found in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some fungi. Carotenoids can be produced from fats and other basic organic metabolic building blocks by all these organisms. Carotenoids generally cannot be manufactured by species in the animal kingdom so animals obtain carotenoids in their diets, and may employ them in various ways in metabolism.
There are over 600 known carotenoids; they are split into two classes, xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons, and contain no oxygen). All carotenoids are tetraterpenoids, meaning that they are produced from 8 isoprene molecules and contain 40 carbon atoms. Carotenoids in general absorb blue light. They serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they protect chlorophyll from photodamage. In humans, four carotenoids (beta-carotene, alpha-carotene, gamma-carotene, and beta-cryptoxanthin) have vitamin A activity (meaning they can be converted to retinal), and these and other carotenoids can also act as antioxidants. In the eye, certain other carotenoids (lutein, astaxanthin and zeaxanthin) apparently act directly to absorb damaging blue and near-ultraviolet light, in order to protect the macula of the retina, the part of the eye with the sharpest vision.
People consuming diets rich in carotenoids from natural foods, such as fruits and vegetables, are healthier and have lower mortality from a number of chronic illnesses. However, a recent meta-analysis of 68 reliable antioxidant supplementation experiments involving a total of 232,606 individuals concluded that consuming additional β-carotene from supplements is unlikely to be beneficial and may actually be harmful, although this conclusion may be due to the inclusion of studies involving smokers. With the notable exception of the gac fruit and crude palm oil, most carotenoid-rich fruits and vegetables are low in lipids. Since dietary lipids have been hypothesized to be an important factor for carotenoid bioavailability, a 2005 study investigated whether addition of avocado fruit or oil, as lipid sources, would enhance carotenoid absorption in humans. The study found that the addition of both avocado fruit and oil significantly enhanced the subjects' absorption of all carotenoids tested (α-carotene, β-carotene, lycopene, and lutein).
Carotenoids belong to the category of tetraterpenoids (i.e., they contain 40 carbon atoms, being built from four terpene units each containing 10 carbon atoms). Structurally, carotenoids take the form of a polyene hydrocarbon chain which is sometimes terminated by rings, and may or may not have additional oxygen atoms attached.
- Carotenoids with molecules containing oxygen, such as lutein and zeaxanthin, are known as xanthophylls.
- The unoxygenated (oxygen free) carotenoids such as α-carotene, β-carotene and lycopene are known as carotenes. Carotenes typically contain only carbon and hydrogen (i.e., are hydrocarbons), and are in the subclass of unsaturated hydrocarbons.
Probably the most well-known carotenoid is the one that gives this second group its name, carotene, found in carrots (also apricots) and are responsible for their bright orange colour. Crude palm oil, however, is the richest source of carotenoids in nature in terms of retinol (provitamin A) equivalent. Vietnamese Gac fruit contains the highest known concentration of the carotenoid lycopene.
Their colour, ranging from pale yellow through bright orange to deep red, is directly linked to their structure. Xanthophylls are often yellow, hence their class name. The double carbon-carbon bonds interact with each other in a process called conjugation, which allows electrons in the molecule to move freely across these areas of the molecule. As the number of conjugated double bonds increases, electrons associated with conjugated systems have more room to move, and require less energy to change states. This causes the range of energies of light absorbed by the molecule to decrease. As more frequencies of light are absorbed from the short end of the visible spectrum, the compounds acquire an increasingly red appearance.
Physiological effects 
In oxygenic photosynthetic organisms, specifically flora and cyanobacteria, the carotenoid β-carotene plays a vital role in the photosynthetic reaction centre where, due to quantum mechanical reasons arising from the symmetry of the molecule, it provides a mechanism for photoprotection against auto-oxidation. They also participate in the energy-transfer process. In non-photosynthesizing organisms, such as humans, carotenoids have been linked to oxidation-preventing mechanisms.
Carotenoids have many physiological functions. Given their structure, carotenoids are efficient free-radical scavengers, and they enhance the vertebrate immune system. There are several dozen carotenoids in foods people consume, and most carotenoids have antioxidant activity. Epidemiological studies have shown that people with high β-carotene intake and high plasma levels of β-carotene have a significantly reduced risk of lung cancer. However, studies of supplementation with large doses of β-carotene in smokers have shown an increase in cancer risk (possibly because excessive β-carotene results in breakdown products that reduce plasma vitamin A and worsen the lung cell proliferation induced by smoke). Similar results have been found in other animals.
Humans and animals are mostly incapable of synthesizing carotenoids, and must obtain them through their diet. The notable exception is the red pea aphid, which has the genes necessary for synthesizing carotenoids, thought to have been acquired from fungi via horizontal gene transfer. Carotenoids are a common and often ornamental feature in animals. For example, the pink colour of flamingos and salmon, and the red colouring of cooked lobsters are due to carotenoids. It has been proposed that carotenoids are used in ornamental traits (for extreme examples see puffin birds) because, given their physiological and chemical properties, they can be used as honest indicators of individual health, and hence they can be used by animals when selecting potential mates.
In the macula lutea of the human eye, certain carotenoids are actively concentrated to the point that they cause a yellow coloring, and this may help to protect the retina from blue and actinic light, in the same way that carotenoids protect the photosystems of plants. Carotenoids are also actively concentrated in the corpus luteum of the ovaries, where they impart the characteristic color, and may act as general antioxidants.
The most common carotenoids include lycopene and the vitamin A precursor β-carotene. In plants, the xanthophyll lutein is the most abundant carotenoid and its role in preventing age-related eye disease is currently under investigation. Lutein and the other carotenoid pigments found in mature leaves are often not obvious because of the masking presence of chlorophyll. When chlorophyll is not present, as in young foliage and also dying deciduous foliage (such as autumn leaves), the yellows, reds, and oranges of the carotenoids are predominant. For the same reason, carotenoid colours often predominate in ripe fruit (e.g., oranges, tomatoes, bananas), after being unmasked by the disappearance of chlorophyll.
However, the reds, the purples, and their blended combinations that decorate autumn foliage usually come from another group of pigments in the cells called anthocyanins. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.
Aroma chemicals 
Products of carotenoid degradation such as ionones, damascones and damascenones are also important fragrance chemicals that are used extensively in the perfumes and fragrance industry. Both β-damascenone and β-ionone although low in concentration in rose distillates are the key odour-contributing compounds in flowers. In fact, the sweet floral smells present in black tea, aged tobacco, grape, and many fruits are due to the aromatic compounds resulting from carotenoid breakdown.
Some carotenoids are produced by bacteria to protect themselves from oxidative immune attack. The golden pigment that gives some strains of S. aureus their name (aureusis = golden) is a carotenoid called staphyloxanthin. This carotenoid is a virulence factor with an antioxidant action that helps the microbe evade death by reactive oxygen species used by the host immune system.
Question of synthesis in the corpus luteum 
Following a 1968 report that beta-carotene was synthesized in laboratory conditions in slices of corpus luteum from cows, an organ known to concentrates beta-carotene (hence its color and name), attempts have been made to replicate these findings, but have not succeeded. The idea is not presently accepted by the scientific community. Rather, the mammalian corpus luteum, like the macula lutea in the retina of the mammalian eye, merely concentrates carotenoids from the diet.
Artificial synthesis 
Naturally occurring carotenoids 
- Cryptomonaxanthin (3R,3'R)-7,8,7',8'-Tetradehydro-β,β-carotene-3,3'-diol
- Crustaxanthin β,-Carotene-3,4,3',4'-tetrol
- Gazaniaxanthin (3R)-5'-cis-β,γ-Caroten-3-ol
- OH-Chlorobactene 1',2'-Dihydro-f,γ-caroten-1'-ol
- Loroxanthin β,ε-Carotene-3,19,3'-triol
- Lutein (3R,3′R,6′R)-β,ε-carotene-3,3′-diol
- Lycoxanthin γ,γ-Caroten-16-ol
- Rhodopin 1,2-Dihydro-γ,γ-caroten-l-ol
- Rhodopinol aka Warmingol 13-cis-1,2-Dihydro-γ,γ-carotene-1,20-diol
- Saproxanthin 3',4'-Didehydro-1',2'-dihydro-β,γ-carotene-3,1'-diol
- Diadinoxanthin 5,6-Epoxy-7',8'-didehydro-5,6-dihydro—carotene-3,3-diol
- Luteoxanthin 5,6: 5',8'-Diepoxy-5,6,5',8'-tetrahydro-β,β-carotene-3,3'-diol
- Zeaxanthin furanoxide 5,8-Epoxy-5,8-dihydro-β,β-carotene-3,3'-diol
- Neochrome 5',8'-Epoxy-6,7-didehydro-5,6,5',8'-tetrahydro-β,β-carotene-3,5,3'-triol
- Vaucheriaxanthin 5',6'-Epoxy-6,7-didehydro-5,6,5',6'-tetrahydro-β,β-carotene-3,5,19,3'-tetrol
- Acids and acid esters
- Canthaxanthin aka Aphanicin, Chlorellaxanthin β,β-Carotene-4,4'-dione
- Capsanthin (3R,3'S,5'R)-3,3'-Dihydroxy-β,κ-caroten-6'-one
- Capsorubin (3S,5R,3'S,5'R)-3,3'-Dihydroxy-κ,κ-carotene-6,6'-dione
- Cryptocapsin (3'R,5'R)-3'-Hydroxy-β,κ-caroten-6'-one
- 2,2'-Diketospirilloxanthin 1,1'-Dimethoxy-3,4,3',4'-tetradehydro-1,2,1',2'-tetrahydro-γ,γ-carotene-2,2'-dione
- Flexixanthin 3,1'-Dihydroxy-3',4'-didehydro-1',2'-dihydro-β,γ-caroten-4-one
- 3-OH-Canthaxanthin aka Adonirubin aka Phoenicoxanthin 3-Hydroxy-β,β-carotene-4,4'-dione
- Hydroxyspheriodenone 1'-Hydroxy-1-methoxy-3,4-didehydro-1,2,1',2',7',8'-hexahydro-γ,γ-caroten-2-one
- Okenone 1'-Methoxy-1',2'-dihydro-c,γ-caroten-4'-one
- Pectenolone 3,3'-Dihydroxy-7',8'-didehydro-β,β-caroten-4-one
- Phoeniconone aka Dehydroadonirubin 3-Hydroxy-2,3-didehydro-β,β-carotene-4,4'-dione
- Phoenicopterone β,ε-caroten-4-one
- Rubixanthone 3-Hydroxy-β,γ-caroten-4'-one
- Siphonaxanthin 3,19,3'-Trihydroxy-7,8-dihydro-β,ε-caroten-8-one
- Esters of alcohols
- Astacein 3,3'-Bispalmitoyloxy-2,3,2',3'-tetradehydro-β,β-carotene-4,4'-dione or 3,3'-dihydroxy-2,3,2',3'-tetradehydro-β,β-carotene-4,4'-dione dipalmitate
- Fucoxanthin 3'-Acetoxy-5,6-epoxy-3,5'-dihydroxy-6',7'-didehydro-5,6,7,8,5',6'-hexahydro-β,β-caroten-8-one
- Isofucoxanthin 3'-Acetoxy-3,5,5'-trihydroxy-6',7'-didehydro-5,8,5',6'-tetrahydro-β,β-caroten-8-one
- Zeaxanthin (3R,3'R)-3,3'-Bispalmitoyloxy-β,β-carotene or (3R,3'R)-β,β-carotene-3,3'-diol
- Siphonein 3,3'-Dihydroxy-19-lauroyloxy-7,8-dihydro-β,ε-caroten-8-one or 3,19,3'-trihydroxy-7,8-dihydro-β,ε-caroten-8-one 19-laurate
- β-Apo-2'-carotenal 3',4'-Didehydro-2'-apo-b-caroten-2'-al
- Apo-6'-lycopenal 6'-Apo-y-caroten-6'-al
- Azafrinaldehyde 5,6-Dihydroxy-5,6-dihydro-10'-apo-β-caroten-10'-al
- Bixin 6'-Methyl hydrogen 9'-cis-6,6'-diapocarotene-6,6'-dioate
- Citranaxanthin 5',6'-Dihydro-5'-apo-β-caroten-6'-one or 5',6'-dihydro-5'-apo-18'-nor-β-caroten-6'-one or 6'-methyl-6'-apo-β-caroten-6'-one
- Crocetin 8,8'-Diapo-8,8'-carotenedioic acid
- Crocetinsemialdehyde 8'-Oxo-8,8'-diapo-8-carotenoic acid
- Crocin Digentiobiosyl 8,8'-diapo-8,8'-carotenedioate
- Hopkinsiaxanthin 3-Hydroxy-7,8-didehydro-7',8'-dihydro-7'-apo-b-carotene-4,8'-dione or 3-hydroxy-8'-methyl-7,8-didehydro-8'-apo-b-carotene-4,8'-dione
- Methyl apo-6'-lycopenoate Methyl 6'-apo-y-caroten-6'-oate
- Paracentrone 3,5-Dihydroxy-6,7-didehydro-5,6,7',8'-tetrahydro-7'-apo-b-caroten-8'-one or 3,5-dihydroxy-8'-methyl-6,7-didehydro-5,6-dihydro-8'-apo-b-caroten-8'-one
- Sintaxanthin 7',8'-Dihydro-7'-apo-b-caroten-8'-one or 8'-methyl-8'-apo-b-caroten-8'-one
- Nor- and seco-carotenoids
- Actinioerythrin 3,3'-Bisacyloxy-2,2'-dinor-b,b-carotene-4,4'-dione
- β-Carotenone 5,6:5',6'-Diseco-b,b-carotene-5,6,5',6'-tetrone
- Peridinin 3'-Acetoxy-5,6-epoxy-3,5'-dihydroxy-6',7'-didehydro-5,6,5',6'-tetrahydro-12',13',20'-trinor-b,b-caroten-19,11-olide
- Pyrrhoxanthininol 5,6-epoxy-3,3'-dihydroxy-7',8'-didehydro-5,6-dihydro-12',13',20'-trinor-b,b-caroten-19,11-olide
- Semi-α-carotenone 5,6-Seco-b,e-carotene-5,6-dione
- Semi-β-carotenone 5,6-seco-b,b-carotene-5,6-dione or 5',6'-seco-b,b-carotene-5',6'-dione
- Triphasiaxanthin 3-Hydroxysemi-b-carotenone 3'-Hydroxy-5,6-seco-b,b-carotene-5,6-dione or 3-hydroxy-5',6'-seco-b,b-carotene-5',6'-dione
- Retro-carotenoids and retro-apo-carotenoids
- Eschscholtzxanthin 4',5'-Didehydro-4,5'-retro-b,b-carotene-3,3'-diol
- Eschscholtzxanthone 3'-Hydroxy-4',5'-didehydro-4,5'-retro-b,b-caroten-3-one
- Rhodoxanthin 4',5'-Didehydro-4,5'-retro-b,b-carotene-3,3'-dione
- Tangeraxanthin 3-Hydroxy-5'-methyl-4,5'-retro-5'-apo-b-caroten-5'-one or 3-hydroxy-4,5'-retro-5'-apo-b-caroten-5'-one
- Higher carotenoids
- Nonaprenoxanthin 2-(4-Hydroxy-3-methyl-2-butenyl)-7',8',11',12'-tetrahydro-e,y-carotene
- Decaprenoxanthin 2,2'-Bis(4-hydroxy-3-methyl-2-butenyl)-e,e-carotene
- C.p. 450 2-[4-Hydroxy-3-(hydroxymethyl)-2-butenyl]-2'-(3-methyl-2-butenyl)-b,b-carotene
- C.p. 473 2'-(4-Hydroxy-3-methyl-2-butenyl)-2-(3-methyl-2-butenyl)-3',4'-didehydro-l',2'-dihydro-b,y-caroten-1'-ol
- Bacterioruberin 2,2'-Bis(3-hydroxy-3-methylbutyl)-3,4,3',4'-tetradehydro-1,2,1',2'-tetrahydro-y,y-carotene-1,1'-dio
See also 
- Armstrong GA, Hearst JE (1996). "Carotenoids 2: Genetics and molecular biology of carotenoid pigment biosynthesis". FASEB J. 10 (2): 228–37. PMID 8641556.
- Kidd, Parris (December 2011). "Astaxanthin, Cell Membrane Nutrient with Diverse Clinical Benefits and Anti-Aging Potential". Alternative Medicine Review 16 (4): 335–364.
- A. T. Diplock1, J.-L. Charleux, G. Crozier-Willi, F. J. Kok, C. Rice-Evans, M. Roberfroid, W. Stahl, J. Vina-Ribes. Functional food science and defence against reactive oxidative species, British Journal of Nutrition 1998, 80, Suppl. 1, S77–S112
- Bjelakovic G, et al.; Nikolova, D; Gluud, LL; Simonetti, RG; Gluud, C (2007). "Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: systematic review and meta-analysis". JAMA 297 (8): 842–57. doi:10.1001/jama.297.8.842. PMID 17327526.
- It is known that taking β-carotene supplements is harmful for smokers, and the meta-analysis of Bjelakovic et al. was influenced by inclusion of these studies. See the letter to JAMA by Philip Taylor and Sanford Dawsey and the reply by the authors of the original paper.
- Unlu N, et al.; Bohn, T; Clinton, SK; Schwartz, SJ (1 March 2005). "Carotenoid Absorption from Salad and Salsa by Humans Is Enhanced by the Addition of Avocado or Avocado Oil". Human Nutrition and Metabolism 135 (3): 431–6. PMID 15735074.
- Choo Yuen May Palm oil carotenoids
- β-Carotene and other carotenoids as antioxidants. From U.S. National Library of Medicine. November, 2008.
- Alija AJ, Bresgen N, Sommerburg O, Siems W, Eckl PM (2004). "Cytotoxic and genotoxic effects of β-carotene breakdown products on primary rat hepatocytes". Carcinogenesis 25 (5): 827–31. doi:10.1093/carcin/bgh056. PMID 14688018.
- Davies, Kevin M. (2004). Plant pigments and their manipulation. Wiley-Blackwell. p. 6. ISBN 1-4051-1737-0.
- Liu GY, Essex A, Buchanan JT, et al. (2005). "Staphylococcus aureus golden pigment impairs neutrophil killing and promotes virulence through its antioxidant activity". J. Exp. Med. 202 (2): 209–15. doi:10.1084/jem.20050846. PMC 2213009. PMID 16009720.
- Brian H. Davies Carotenoid metabolism as a preparation for function. Pure & Applied Chemistry, Vol. 63, No. 1, pp. 131-140, 1991. available online. Accessed April 30, 2010.
- Patent Pending: US Application Number 11/817,120
- "Biosynthesis of carotenoids".
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