• Description
• Functions

Description

Carotenoids are organic pigments naturally occurring in plants and some other photosynthetic organisms like algae, some types of fungus and some bacteria.

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 are known as xanthophylls, the un-oxidized carotenoids are known as carotenes.

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. The pink colour of flamingos and salmon, and the red colouring of lobsters are caused by carotenoids.

Functions

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.

Animals are incapable of synthesizing carotenoids, and must obtain them through their diet.

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. 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. But studies of supplementation with large doses of beta-carotene in smokers have shown an increase in cancer risk (possibly because excessive beta-carotene results in breakdown products that reduce plasma Vitamin A and worsen the lung cell proliferation induced by smoke).

Carotenoids are absorbed from the intestine with the aid of dietary fat and incorporated into chylomicrons for transport in the serum. The different structural features possessed by carotenoids account for selective distribution in organ tissue, biological activity and pro-vitamin A potency, or in vivo conversion to vitamin A. Due to the hydrophobic character, carotenoids are associated with lipid portions of human tissues, cells, and membranes. In general, 80-85% of carotenoids are distributed in adipose tissue, with smaller amounts found in the liver, muscle, adrenal glands and reproductive organs. Approximately 1% circulate in the serum on high and low density lipoproteins. The major serum carotenoids are b-carotene, a-carotene, lutein, zeaxanthin, lycopene and cryptoxanthin. Smaller amounts of polyenes such as phytoene and phytofluene are also present.

As adipose tissue is the largest body pool for carotenoids, serum concentrations are fairly constant and slow to change during periods of low intake. The estimated half-life was estimated to be 11-14 days for lycopene, a-carotene, b-carotene, lutein and zeaxanthin. Evidence for the existence of more than one body pool has been published.

Serum levels reflect lifestyle choices and dietary habits within and between cultures.26 Variations can be attributed to different intakes, unequal abilities to absorb certain carotenoids, and different rates of metabolism and tissue uptake. Decreased serum levels occur with alcohol consumption, the use of oral contraceptives, smoking , and prolonged exposure to UV light.