Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some types of fungus some bacteria and at least one species of aphid. Carotenoids are generally not manufactured by species in the animal kingdom, although one species of aphid is known to have acquired the genes for synthesis of the carotenoid torulene from fungi, by the known phenomenon of horizontal gene transfer.
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). 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 and zeaxanthin) apparently act directly to absorb damaging blue and near-ultraviolet light, in order to protect the macula lutea.
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 Vietnam Gac 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).
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 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.
In photosynthetic organisms, specifically flora, 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-photosynthesizing organisms, specifically humans, carotenoids have been linked to oxidation-preventing mechanisms.
Carotenoids have many physiological functions. Given their structure (above), 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. Not all carotenoids are helpful, e.g. etretinate is a teratogen.
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 presence of chlorophyll. However, 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.
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.