Carotenoids are almost everywhere: in bacteria, plankton, fungi, fish, reptiles, insects, birds, mammals, flowers, leaves and fruits.  More than 600 different carotenoids have been identified so far.  The most familiar is beta-carotene, the orange-red pigment in carrots that was discovered almost 200 years ago, and which gave the name to the whole class of these compounds.  Synthesized around the globe by plants, fungi and some bacteria at the rate of some 3 tons every second, carotenoids show great diversity in structure, function and color.

Most carotenoids are polyunsaturated hydrocarbons, containing 40 carbon atoms, and comprising two terminal ring systems (see below: a molecule of ß-carotene).  Carotenoids that are composed entirely of carbon and hydrogen are known as carotenes, while those that contain oxygen are termed xanthophylls.

No life without carotenoids 
Carotenoids, together with the green pigment chlorophyll, are the basis of all life on earth.  They play a key role in photosynthesis, which enables some organisms to convert carbon dioxide and water into organic material using energy from light.  Carotenoids are also needed to protect the organic molecules produced from being destroyed by oxidation.  Since animals, including humans, cannot make carotenoids from simpler molecules, they are totally dependent on the amounts supplied in their food.  In this respect, carotenoids are like vitamins; if intakes are inadequate, health and survival may be compromised.

Antioxidant role

The primary role of carotenoids are a potent antioxidants.  Through research, we are beginning to have a better understanding that carotenoids play a very impoartant role in animal health.  The role is mostly through their role as excellent antioxidants.  For example, cold-water fishes, like salmon, have a high level of polyunsaturated fat in their membranes, and protection of lipid tissue from peroxidation seems to be a metabolic function for astaxanthin.  Astaxanthin has been shown to be one hundred times more effective than vitamin E as an antioxidant.

Radiation, highly reactive chemicals from the environment or produced in the body, and infective agents such as bacteria and fungi can damage cells and tissues, causing disease and premature death.  Carotenoids absorb and reflect radiation, stop unwanted chemical reactions, increase resistance to infection, and promote wound healing.
Carotenoids are a source of vitamin A

Some carotenoids are converted in the body to vitamin A, which is essential for vision, growth, reproduction, proper development of the skin and mucous membranes of the body, and resistance to infection.  For herbivores and vegans, these carotenoids are, in fact, the only natural source of vitamin A.  Plants do not make vitamin A only carotenoids.

Carotenoids maintain body function

Carotenoids are involved in other physiological functions as well as body protection.  They are important for the proper functioning of cell membranes, proteins and enzymes, for example.  For many animal species, like salmon, carotenoids play a key role in sexual development and reproduction.  They might also contribute to maintaining water balance, cell respiration and appetite stimulation.

Carotenoids are key to communication

Without carotenoids, seeing would not be possible.  Smell and taste would also be impaired.  Both animals and plants use the colors of carotenoids as signals and triggers in communication.  Plants use color to lure insects and birds, and so ensure reproduction and the spread of their seeds.  The distinctive colors of some fruits, insects and animals are a warning that they are not edible.  Some creatures protect themselves by adopting colors that blend into the surroundings.  Many change their colors to indicate that they are ready to mate or to defend their territory.

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Information provided by Roche S.A.