What do carotenoids do?

What are carotenoids?

Life presents us with a kaleidoscope of colors. From the green, green grass of home to a forest's ruddy autumn hues, we are surrounded by living color. Living things obtain their colors, with few exceptions, from natural pigments. In addition to their role in coloration, natural pigments carry out a variety of important biological functions. Among the most common and most important natural pigments are the carotenoids.

Carotenoids are a class of natural fat-soluble pigments found principally in plants, algae, and photosynthetic bacteria, where they play a critical role in the photosynthetic process. They also occur in some non-photosynthetic bacteria, yeasts, and molds, where they may carry out a protective function against damage by light and oxygen. Although animals appear to be incapable of synthesizing carotenoids, many animals incorporate carotenoids from their diet. Within animals, carotenoids provide bright coloration, serve as antioxidants, and can be a source for vitamin A activity (Ong and Tee 1992; Britton et al. 1995).

Carotenoids are responsible for many of the red, orange, and yellow hues of plant leaves, fruits, and flowers, as well as the colors of some birds, insects, fish, and crustaceans. Some familiar examples of carotenoid coloration are the oranges of carrots and citrus fruits, the reds of peppers and tomatoes, and the pinks of flamingoes and salmon (Pfander 1992). Some 600 different carotenoids are known to occur naturally (Ong and Tee 1992), and new carotenoids continue to be identified (Mercadante 1999).

Carotenoids are defined by their chemical structure. The majority carotenoids are derived from a 40-carbon polyene chain, which could be considered the backbone of the molecule (Fig. 1). This chain may be terminated by cyclic end-groups (rings) and may be complemented with oxygen-containing functional groups. The hydrocarbon carotenoids are known as carotenes, while oxygenated derivatives of these hydrocarbons are known as xanthophylls. Beta-carotene, the principal carotenoid in carrots, is a familiar carotene, while lutein, the major yellow pigment of marigold petals, is a common xanthophyll (Fig. 1).

The structure of a carotenoid ultimately determines what potential biological function(s) that pigment may have. The distinctive pattern of alternating single and double bonds in the polyene backbone of carotenoids is what allows them to absorb excess energy from other molecules, while the nature of the specific end groups on carotenoids may influence their polarity. The former may account for the antioxidant properties of biological carotenoids, while the latter may explain the differences in the ways that individual carotenoids interact with biological membranes (Britton 1995).

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What do carotenoids do?

In human beings, carotenoids can serve several important functions. The most widely studied and well-understood nutritional role for carotenoids is their provitamin A activity. Deficiency of vitamin A is a major cause of premature death in developing nations, particularly among children. Vitamin A, which has many vital systemic functions in humans, can be produced within the body from certain carotenoids, notably beta-carotene (Britton et al. 1995). Dietary beta-carotene is obtained from a number of fruits and vegetables, such as carrots, spinach, peaches, apricots, and sweet potatoes (Mangels et al. 1993). Other provitamin A carotenoids include alpha-carotene (found in carrots, pumpkin, and red and yellow peppers) and cryptoxanthin (from oranges, tangerines, peaches, nectarines, and papayas).

Fig. 1. Structure of selected carotenoids

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References:

Britton, G. (1995). Structure and properties of carotenoids in relation to function. FASEB J., 9:1551-1558.

Britton, G., S. Liaaen-Jensen, and H. Pfander. (1995). Carotenoids today and challenges for the future. In: Britton, G., S. Liaaen-Jensen, and H. Pfander [eds], Carotenoids vol. 1A: Isolation and Analysis. Basel: Birkhäuser.

Di Mascio, P., Kaiser, S., and Sies, H. (1989) Lycopene as the most efficient biological carotenoid singlet oxygen quencher. Arch. Biochem. Biophys., 274:532-538.

Di Mascio, P., M. E. Murphy, and H. Sies. (1991) Antioxidant defense systems: the role of carotenoids, tocopherols, and thiols. Am. J. Clin. Nutr., 53:194S-200S.

Mangels, A.R., J.M. Holden, G.R. Beecher, M.R. Forman, and E. Lanza. (1993). Carotenoid content of fruits and vegetables: an evaluation of analytic data. J. Am. Diet. Assoc., 93:284-296.

Mathews-Roth, MM. (1990) Plasma concentration of carotenoids after large doses of beta-carotene. Am. J. Clin. Nutr., Sep 52:3, 500-1

Mercadante, A. (1999) New carotenoids: recent progress. Invited Lecture 2. Abstracts of the 12th International Carotenoid Symposium, Cairns, Australia, July 1999.

Nishino, H. (1998) Cancer prevention by carotenoids. Mutat. Res., 402:159-163.

Ong, A.S.H., and E.S. Tee. (1992) Natural sources of carotenoids from plants and oils. Meth. Enzymol., 213: 142-167.

Pfander, H. (1992) Carotenoids: an overview. Meth. Enzymol., 213: 3-13.

Snodderly, D.M. (1995) Evidence for protection against age-related macular degeneration by carotenoids and antioxidant vitamins. Am. J. Clin. Nutr., 62(suppl):1448S-1461S.