Carotenoid - Physiological Effects

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.ki 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.