Carotenoids

Description

The term carotenoid refers to a family of about 600 different plant pigments that function as antioxidants . The yellow, orange, and many of the red pigments in fruits, vegetables, and plant materials are usually carotenoids. In fall, when deciduous trees prepare for winter and stop their chlorophyll production, the green color of the leaves fade and the orange, yellow, and red colors of the carotenoids in the leaves are revealed before the leaves die and fall to the ground. Plants appear to produce carotenoids to protect their stems and leaves from the energy of the sun. Ultraviolet (UV) wavelengths can generate molecules called free radicals that can damage living cells. Free radicals are molecules, or fragments of molecules, that are unstable and highly reactive. Free radicals are produced as the result of a normal molecule losing or gaining an electron. In normal, stable molecules, electrons associate in pairs. However, radiation from the sun can result in the removal of an electron from a molecule and the formation of free radical. Carotenoids as antioxidants limit free radical damage by donating electrons to quench, or neutralize, the oxidant radicals.

In human nutrition , carotenoids, as antioxidants, serve to protect cells from the danger of free radicals that may be produced by the body during metabolism or by cigarette smoke, sunlight, radiation, pollutants, or even stress . Tens of thousands of free radicals are created in the body every second. When a free radical captures an electron from another molecule, a new free radical is created as the second molecule has a lone, unpaired electron. This new free radical seeks to capture another electron and become normal again. This continual process of forming free radicals becomes a chain reaction. Unless quenched, these free radicals can damage DNA, fats, and proteins. However, the body has a defense against these free radicals. With proper nourishment, the body can make sufficient quantities of antioxidant enzymes and substrates for those enzymes that can facilitate the quenching of free radical reactions by antioxidants. These enzymes include superoxide dismutase, catalase, and glutathione peroxidase. In addition to these enzymes produced by the body, antioxidant nutrients taken into the body through foods or through dietary supplements also can surrender electrons to the free radicals without adding to the chain reaction, thus terminating the free radical reactions. Antioxidant nutrients include vitamins A, C, and E, bioflavonoids , lipoic acid, and carotenoids.

Despite the large number of carotenoids in nature, only about 50 are present in foods that people in the United States eat, and only about 14 of those have been identified in blood, an indication of what is absorbed in the human body. All carotenoids are fat-soluble compounds, meaning that they can dissolve in fats and oils, but not in water . The carotenoid family consists of smaller families of pigments called carotenes and xanthophylls. Carotenes are hydrocarbons, containing only carbon and hydrogen atoms, while xanthophylls also contain oxygen. The carotenes have been studied more than the other carotenoids. The ones of most interest in human nutrition are beta-carotene, alpha-carotene, and lycopene . Important xanthophylls include lutein , astaxanthin, zeaxanthin, and cryptoxanthin.

As acceptance of the many health benefits of carotenoids increases and continues to be proven, the addition of five individual carotenoids (alpha-carotene, beta-carotene, lutein, lycopene, and beta-cryptoxanthin) were added to the National Cancer Institute's Diet History Questionnaire. The carotenoids appear to have many health benefits, but more research is required to confirm many of the health effects seen so far and to identify additional benefits.

As one of the most common carotenoids, beta-carotene is the most well-known and well-studied carotenoid. It is found in carrots, pumpkins, peaches, and sweet potatoes. Beta-carotene is the primary precursor to vitamin A . With the aid of dioxygenase enzymes, the human body can split one molecule of beta-carotene into two vitamin A molecules. Vitamin A has many vital functions in the human body, including being involved in: (1) the growth and repair of body tissues, (2) the formation of bones and teeth, (3) the resistance of the body to infection, and (4) the development of healthy eye tissues. Vitamin A deficiency symptoms include night blindness , dry eyes, dry, rough skin, impaired bone growth, and susceptibility to respiratory infections . Vitamin A, is a fat soluble vitamin, can be stored in the body long-term and can reach toxic levels over time if amounts above recommended levels (10,000 IU for adults and only 6,000 IU for pregnant women) are ingested. Too much vitamin A can cause headaches, vision problems, nausea, vomiting , an enlarged liver or spleen, birth defects, and even death at very high levels. Beta-carotene is a better source of vitamin A than vitamin A supplements because it is only converted to vitamin A on an as-needed basis; excess beta-carotene is stored in the body and unlike vitamin A, is not toxic when taken in amounts in excess of body needs. Beta-carotene also improves immune function, increases lung capacity, reduces DNA damage, may provide protection from the sun, and may lessen the risks of some types of cancer. However, for people who drink and smoke excessively, beta-carotene may increase their risk of lung cancer .

Alpha-carotene, another common carotenoid, is normally found in the same foods as beta-carotene. It is similar to beta-carotene in structure, with one of the ring structures being beta-ionone. However, the other ring is different, so one molecule of alpha-carotene yields only one molecule of vitamin A. Alpha-carotene has been found to have powerful anticancer properties in cell-culture studies.

Lycopene is often the most common carotenoid in the American diet because it is found in tomato products, including pizza and spaghetti sauce. It is also present in lesser amounts in watermelon, pink grapefruit, guava, and apricots. Lycopene does not produce vitamin A. However, lycopene in tomato juice and spaghetti and pizza sauces has been associated with a lower risk of prostate cancer in men. In late 2001, the first clinical trial showed that lycopene supplementation could even slow progression of prostate cancer growths.

Cooked tomato sauces were to found to be associated with greater health benefits, compared to uncooked tomatoes, because the lycopene in the cooked tomatoes was more easily absorbed. Also, since lycopene is fat-soluble, absorption increased when it was mixed with oil in the sauces. Uncooked tomatoes also demonstrated health benefits, though to a lesser degree, especially when they were used in a salad with a oil-based dressing or in a sandwich with fat-containing meat. Lycopene may help in the prevention of other cancers as well as protect against heart attacks. A study late in 2001 indicated that lycopene may also help patients with exercise-induced asthma . Research is continuing on the potential health benefits of lycopene.

Lutein, which is almost as common as beta-carotene in the American diet, and zeaxanthin are xanthophylls found in kale, spinach, broccoli, corn, alfalfa , and egg yolks. Both are components of the macula of the eye, a small area in the center of the retina responsible for detailed vision. These carotenoids may prevent and slow macular degeneration , a leading cause of blindness in the elderly. As antioxidants, they reduce the amount of free radical damage to the macula. Lutein may also help prevent the formation of cataracts , reduce the risk of heart disease , and protect against breast cancer .

Astaxanthin is a minor carotenoid that serves as a pigment in aquatic animals such salmon, trout, and Antarctic krill (small shrimp-like crustaceans that feed on algae and that serve as a food source for other sea animals such as whales). Astaxanthin is a strong antioxidant that appears to enhance the immune system and protect against cancer. It also may protect against UVA light, a wavelength of ultraviolet light that can cause sunburn and skin cancer .

Cryptoxanthin is a minor carotenoid found in peaches, papayas, tangerines, and oranges. Cryptoxanthin is second to beta-carotene in the amount of dietary carotene converted to vitamin A. Along with other carotenoids, it forms an antioxidant barrier in the human skin. It also appears to protect women from cervical cancer.

There are many other minor dietary carotenoids that most likely provide significant health benefits. A diet that includes many types of fruits and vegetables is important for supplying those nutrients and their associated health benefits.

General use

Although not classified as essential nutrients, carotenoids are important substances in human food sources, especially in fruits, vegetables, and plant greens, that provide many health benefits. In addition, some are precursors to vitamin A. They are primarily consumed through the diet; however the more common carotenoids are available as dietary supplements.

Preparations

Beta-carotene, lutein, and lycopene are sold as individual carotenoid supplements. Beta-carotene is available in two forms, natural and synthetic. The natural form is preferred to the synthetic, as the natural form appears to be a stronger antioxidant. Algae are an abundant source of beta-carotene and are used to produce supplements. Their presence in a supplement is usually identified on the label as Dunaliella salina or as some related type of algae. D. salina produces 10 to 100 times more beta-carotene than carrots. It grows in areas with strong sunlight, high temperatures, and salty water, environments where antioxidants are greatly needed for protection from free radicals. A dose for adults for beta-carotene may range up to 10–15 mg, or 25,000 IU, daily.

Lutein is prepared from marigold petals as either free lutein or lutein ester. Both forms are absorbed well by the body, though preliminary research has shown that lutein ester may be assimilated slightly better and be retained slightly longer than free lutein. For general health, 4–6 mg of lutein should be satisfactory. For those at risk for macular degeneration, 30–40 mg daily may be useful.

Lycopene supplements are prepared from tomatoes. A typical daily dose is 4 mg, which is the amount in one large ripe tomato. Zeaxanthin is not available as a supplement. However, the body can convert some lutein to zeaxanthin. Also lutein supplements usually contain some zeaxanthin.

Mixed carotenoid supplements are available, with different formulations. For example, one typical formula contains mostly beta-carotene, with smaller amounts of lutein, zeaxanthin, and cryptoxanthin. Another type contains less beta-carotene but a higher percentage of alpha-carotene. Mixed carotenes may also be included in some multi-vitamin and multi-oxidant supplements. Labels of supplements should be read carefully to determine the types of carotenoids present and their dosages.

A person consuming the typical American diet obtains only about 1.5 mg of carotenoids per day. The Recommended Dietary Allowance (RDA), as established by the United States National Research Council for the purpose of evaluating diets , for vitamin A is 1,000 RE (retinol equivalents), or 6 mg of beta-carotene. The USRDA, established by the United States Food and Drug Administration as a consumer convenience for labeling purposes, is 5,000 IU of vitamin A, or 3 mg of beta-carotene. The United States Department of Agriculture and the National Cancer Institute have suggested that perhaps 5–6 mg of carotenoids should be a dietary target. A study completed in 2001 found that the carotenoids present in fruit increased as fruit ripened and that greater concentrations can be found in the peels than in the pulp.

To enhance dietary health benefits, it may be useful to supplement a diet high in fruits and vegetables with an additional 10–15 mg of carotenoids per day. Those with poor diets may consider supplementation with 25 mg of supplementation per day. Since it is not possible to put every beneficial carotenoid in a supplement, the best way to obtain a wide variety of carotenoids is to eat a diet containing an assortment of carotenoid-containing foods.

Research has not yet answered the question of whether a person requires additional vitamin A if he is taking beta-carotene supplements. Vitamin A is only available in foods of animal origin, so vegetarians should consider using vitamin A supplements. Persons with diseases such as diabetes may not be as efficient in converting beta-carotene into vitamin A, so they need to get some from their diet or from supplements.

Precautions

A study conducted to investigate the effects of vitamin E and beta-carotene on the incidence of lung cancer and other cancers in male smokers indicated that, in the subjects who were heavy smokers and also were heavy drinkers, beta-carotene supplements may result in increased adverse health effects, including a slight increase in cancer. Another study of smokers indicated that high supplemental doses of beta-carotene and vitamin A increased the risk of lung cancer (though in former smokers, beta-carotene and vitamin A decreased the chances of developing lung cancer). Additional studies are being conducted to further investigate the effects of beta-carotene supplementation. However, it would be prudent for those who drink heavily or smoke to avoid the use of beta-carotene supplements and vitamin A.

Side effects

A person taking high doses of beta-carotene and other carotenoids may develop a yellowish color on his feet and hands. There is no indication that this is a toxic effect. If the color is undesirable, the individual should stop taking the supplements for a month or so, and then resume them at a lower dose.

Interactions

Carotenoids seem to work best together in a complementary and synergistic manner to provide antioxidant and other health benefits; they also seem to work well with other antioxidants. Therefore the use of a mixed carotenoid supplement in combination with a multi-antioxidant formula, along with a diet rich in a variety of fruits and vegetables, is most desirable.

Carotenoid supplements are readily assimilated by the body, but to optimize absorption, they should be taken with the highest fat-content meal of the day.

Research has not yet determined how the consumption of one type of carotenoid as a supplement may affect the absorption of other carotenoids. One study showed that beta-carotene reduced the absorption of canthaxanthin, another showed that beta-carotene reduced the levels of lutein in the body, while other studies have shown that beta-carotene actually increased levels of other carotenoids in the body. This is an area that researchers are continuing to investigate to gain a better understanding of potential interactions.

Resources

BOOKS

Challem, Jack. All About Carotenoids. Garden City Park, NY: Avery Publishing Group, 1999.

Passwater, Richard A. Beta-Carotene and Other Carotenoids. New Canaan, CT: Keats Publishing, Inc., 1999.

PERIODICALS

Bauer, Jeff. "A Tomato Antioxidant May Relieve Asthma." RN (October 2001):21.

Carlson, Mike, et al. "Eat the Peel (Smart Nutrition)." Men's Fitness (October 2001):20.

"Clinical Intervention Trial Finds Benefit of Lycopene." Cancer Weekly (November 27, 2001) :38.

Dixon, Lori Beth, Amy F. Subar, and Frances E. Thompson. "Addition of Five Carotenoids to the National Cancer Institute Diet History Questionnaire Database." The Journal of Nutrition (November, 2001) 3133S.

Judith Sims

Teresa G. Odle

Carotenoids

Definition

Purpose

Description

Precautions

Interactions

Complications

Parental concerns

Resources

Definition

Carotenoids are fat-soluble plant pigments, some of which are important to human health. The most common carotenoids in the diet of North Americans are alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, zeaxanthin, and lycopene.

Purpose

The role carotenoids play in human health is not well understood. Carotenoids are antioxidants that react with free radicals. Molecules called free radicals form during normal cell metabolism and with exposure to ultraviolet light or toxins such as cigarette smoke. Free radicals cause damage by reacting with fats and proteins in cell membranes and genetic material. This process is called oxidation. Antioxidants are compounds that attach themselves to free radicals so that it is impossible for the free radical to react with, or oxidize, other molecules. In this way, antioxidants may protect cells from damage. Although carotenoids have antioxidant activity in the laboratory, it is not clear how much they function as antioxidants in the body. Claims that carotenoids can protect against cancer and cardiovascular disease are primarily based on their antioxidant properties.

One subgroup of carotenoids that includes alpha-carotene, beta-carotene, and beta-cryptoxanthine is converted into vitamin A (retinol) by the body. Vitamin A affects is important for maintaining good vision, a healthy immune system, and strong bones. Vitamin A also helps turn on and off certain genes (gene expression) during cell division and differentiation. The degree to which this group of provitamin A carotenoids is converted into vitamin A appears to depend on whether or not the body is getting enough vitamin A in other forms. Only 10% of all carotenoids can be converted into vitamin A.

Description

Carotenoids are highly colored red, orange, and yellow pigments found in many vegetables. A German scientist isolated the first carotenoid in 1826 from carrots and named it beta-carotene. Since then, more than 600 carotenoids have been identified in plants, algae, fungi, and bacteria. Carotenoids must be dissolved in a small amount of fat to be absorbed from the intestine. Dietary supplements of carotenoids contain oil, which makes them more readily available to the body than carotenoids in food. Carotenoids in.

Carotenoids .

(Illustration by GGS Information Services/Thomson Gale.)

vegetables are best absorbed if they are cooked in oil or eaten in a meal that contains at least some fat. (A very tiny amount of fat is adequate)

The United States Institute of Medicine (IOM) of the National Academy of Sciences develops values called Dietary Reference Intakes (DRIs) for vitamins and minerals . The DRIs define the amount of a nutrient a person needs toconsume daily and the largest daily amount from food or dietary supplements that can be taken without harm. The IOM has not developed any DRIs for carotenoids because not enough scientific information is available and because no diseases have been identified as being caused by inadequate intake of carotenoids. The IOM, the American Cancer Society, and the American Heart Association all recommend that people get all their antioxidants, including carotenoids, from a diet high in fruits, vegetables, and whole grains rather than from dietary supplements.

Health claims for carotenoids

Many health claims for carotenoids are based on laboratory and animal studies. Results from human studies are often inconsistent and confusing. One difficulty in evaluating these studies comes from the variety of ways in which they are conducted. When increased carotenoid intake comes from eating foods high in carotenoids, it is hard to separate the effects of the carotenoids from the effects of other vitamins and minerals in the food. When a dietary supplement is given to increase the level of a specific carotenoid, the outcomes often differ from those that occur in a diet of carotenoid-rich vegetables. In addition, the fact that some carotenoids are converted into vitamin A blurs the line between their effects and that of vitamin A from other sources. More controlled research needs to be done on these compounds. Many clinical trials are underway to determine safety and effectiveness of different carotenoids, both alone and in combination with other drugs and supplements.

BETA-CAROTENE . Beta-carotene is a yellow-orange provitamin A carotenoid. Good sources of beta-carotene include carrots, sweet potatoes, winter squash, pumpkin, spinach, kale, and broccoli. When vitamin A stores are low, the body can convert beta-carotene into vitamin A to prevent symptoms of vitamin A deficiency. It takes 12 mcg of beta-carotene to make 1 mcg of retinol, the active form of vitamin A. Therefore, vitamin A deficiency is usually more effectively treated by eating more foods high in vitamin A and/or taking a vitamin A supplement than by increasing beta-carotene intake.

The only use for beta-carotene dietary supplements proven in well-controlled clinical trials is to treat a rare genetic disorder called erythropoietic protoporphyria. This disorder causes the skin to be painfully sensitive to sunlight and causes the development of gallstones and problems with liver function. Symptoms are relieved by giving beta-carotene supplements under the supervision of a physician.

A diet high in vegetables rich in beta-carotene appears to reduce the risk of developing certain cancers. However, in a large study of 29,000 men, when a beta-carotene dietary supplement was taken by men who smoked, they developed lung cancer at a rate 18% higher and died at a rate 8% higher than men who did not take the supplement. Another study that gave men dietary supplements of beta-carotene and vitamin A was stopped when researchers found the men receiving the beta-carotene had a 46% greater chance of dying from lung cancer than those who did not take it. The official position of the IOM is that “beta-carotene supplements are not advisable for the general population.”

ALPHA-CAROTENE . Alpha-carotene is the lesser-known cousin of beta-carotene. It also is a provitamin A carotenoid, but it takes 24 mcg of alpha-carotene to make 1 mcg of retinol. Good sources of alpha-carotene include pumpkin, carrots, winter squash, collard greens, raw tomatoes, tangerines, and peas. Less research has been done on alpha-carotene than beta-carotene, but it is not recommended as a dietary supplement.

BETA-CRYPTOXANTHIN . Beta-cryptoxanthin is also a provitamin A carotenoid. It takes 24 mcg of beta-cryptoxanthin to make 1 mcg of retinol. Good sources of beta-cryptoxanthin include pumpkin, red bell peppers, papaya, tangerines, nectarines, oranges and orange juice, carrots, yellow corn, and watermelon.

LUTEIN AND ZEAXANTHIN Lutein and zeaxanthin do not have vitamin A activity. They are the only carotenoids found in the human eye. It has been proposed, but not proven, that they may help slow the development of cataracts. Cataracts are changes in the lens of the eye that result in clouding and vision loss. These carotenoids are also found in the retina. They absorb light in the blue wavelength range. It is theorized that they can help slow or prevent age-related breakdown of the retina (age-related macular degeneration), a common cause of vision loss in the elderly. Good sources of lutein and zeaxanthin include spinach, kale, turnip, collard, and mustard greens, summer squash, peas, broccoli, Brussels sprouts, and yellow corn.

LYCOPENE Lycopene is the carotenoid that gives tomatoes, watermelons, and guavas their reddish color. In the American diet, almost all dietary lycopene comes from tomato products.

The relationship between dietary intake of lycopene and the risk of men developing prostate cancer is of great interest to researchers. One large study of 58,000 Dutch men found no relationship between the two. However, an analysis of 21 studies examining the relationship between lycopene intake and prostate cancer found that men with the highest dietary intake.

KEY TERMS

Antioxidant— A molecule that prevents oxidation. In the body antioxidants attach to other molecules called free radicals and prevent the free radicals from causing damage to cell walls, DNA, and other parts of the cell.

Cell differentiation— The process by which stem cells develop into different types of specialized cells such as skin, heart, muscle, and blood cells.

Dietary supplement— A product, such gas avitamin, mineral, herb, amino acid, or enzyme, intended to be consumed in addition to an individual’s diet with the expectation that it will improve health.

Provitamin— A substance the body can convert into a vitamin.

Retina— The layer of light-sensitive cells on the back of the eyeball that function in converting light into nerve impulses.

of lycopene were less likely to develop prostate cancer. The reduction appeared real, but modest—11–19%.

Precautions

The relationship between lung cancer and beta-carotene strongly suggests that all carotenoids should be obtained through diet and not through dietary supplements. There is also no information on the safety of carotenoid dietary supplements in children or women who are either pregnant or breastfeeding .

Interactions

Interactions of specific carotenoids with drugs, herbs, and dietary supplements have not been well studied. In general, cholesterol-lowering drugs, orli-stat (Xenical or Alli), and mineral oil reduce the absorption of carotenoids from the intestine, but it is not known whether this has an effect on health.

Complications

There are no identified complications from carotenoid deficiency.

Beta-carotene supplements of 30 mg per day or more or excessive consumption of carrots and other beta-carotene rich food can cause the skin to become yellow, a condition called carotenodermia. Caroteno-dermia is not associated with any health problems and disappears when beta-carotene intake is reduced.

Lycopene supplements or excessive intake of tomatoes and tomato products can cause the skin to turn orange, a condition called lycopenodermia. This condition disappears when lycopene intake is reduced.

No recommendations have been set about the maximum daily intake of carotenoids from diet, but dietary supplements of carotenoids are not recommended by the IOM, the American Heart Association or the American Cancer Society.

Parental concerns

Parents should encourage their children to eat a healthy and varied diet high in fruits, vegetables, and whole grains. There is no need to give children dietary supplements of carotenoids. The safety of these supplements in children has not been studied.

Resources

BOOKS

Challem, Jack, and Marie Moneysmith. Basic Health Publications User’s Guide to Carotenoids & Flavonoids: Learn How to Harness the Health Benefits of Natural Plant Antioxidants North Bergen, NJ: Basic Health Publications, 2005.

Gaby, Alan R., ed. A-Z Guide to Drug-Herb-Vitamin Interactions Revised and Expanded 2nd Edition: Improve Your Health and Avoid Side Effects When Using Common Medications and Natural Supplements Together New York: Three Rivers Press, 2006.

Krinsky, Norman, Susan T. Mayne, Helmut Sies, eds. Carotenoids in Health and Disease New York: Marcel Dekker, 2004.

Lieberman, Shari, and Nancy Bruning. The Real Vitamin and Mineral Book: The Definitive Guide to Designing Your Personal Supplement Program 4th ed. New York: Avery, 2007.

Pressman, Alan H., and Sheila Buff. The Complete Idiot’s Guide to Vitamins and Minerals 3rd ed. Indianapolis, IN: Alpha Books, 2007.

Rucker, Robert B., ed. Handbook of Vitamins Boca Raton, FL: Taylor & Francis, 2007.

PERIODICALS

Kushi, Lawrence H., Tim Byers, Colleen Doyle, et al.

“American Cancer Society Guidelines on Nutrition and Physical Activity for Cancer Prevention.” CA: Cancer Journal for Clinicians. 56(2006): 254-281. [cited May 2, 2007]. <http://caonline.amcancersoc.org/cgi/content/full/56/5/254>.

Voutilainen, Sari, Tarja Nurmi, Jaakko Mursu, et al.

“Carotenoids and Cardiovascular Health.” American Journal of Clinical Nutrition83, no.6 (June 2006): 1265-71. [cited May 2, 2007]. <http://www.ajcn.org/cgi/content/full/83/6/1265>.

ORGANIZATIONS

American Cancer Society. 1599 Clifton Road NE, Atlanta GA 30329-4251. Telephone: 800 ACS-2345. Website: <http://www.cancer.org>

American Heart Association. 7272 Greenville Avenue, Dallas, TX 75231. Telephone: (800) 242-8721. Website: <http://www.americanheart.org>

American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, Illinois 60606-6995. Telephone: (800) 877-1600. Website: <http://www.eatright.org>

Linus Pauling Institute. Oregon State University, 571 Weniger Hall, Corvallis, OR 97331-6512. Telephone: (541) 717-5075. Fax: (541) 737-5077. Website: <http://lpi.oregonstate.edu>

Office of Dietary Supplements, National Institutes of Health. 6100 Executive Blvd., Room 3B01, MSC 7517, Bethesda, MD 20892-7517 Telephone: (301)435-2920. Fax: (301)480-1845. Website: <http://dietary-supplements.info.nih.gov>

OTHER

Medline Plus. “Beta-carotene.” U. S. National Library of Medicine, August 1, 2006. [cited May 2, 2007]. <http://www.nlm.nih.gov/medlineplus/druginfo/natural/patient-betacarotene.html>.

Office of Dietary Supplements. “Dietary Supplement Fact Sheet: Vitamin A and Carotenoids.” National Institutes of Health, April 23, 2006. [cited May 2, 2007]. <http://ods.od.nih.gov/factsheets/vitaminA.asp>.

Tish Davidson, A.M.

Carotenoids

The carotenoids are red, orange, and yellow pigments synthesized by all green plants and some microbes. They have an essential function in photosynthesis and in attracting the attention of animals. Several of these pigments also have an important nutritional function for animals and some of the familiar colors of animals are derived from plant or microbial carotenoids they consume.

Structure and Occurrence

Carotenoids are synthesized in the plastids of a plant cell and typically contain forty carbon atoms derived from eight subunits of the five-carbon compound, isoprene. Larger and smaller carotenoids do occur. Two categories of carotenoids occur in nature. These are the carotenes that contain only carbon and hydrogen, and the xanthophylls (also termed oxycarotenoids) that contain carbon, hydrogen, and oxygen atoms. Each carotenoid has its own distinctive color. Their chemical structure makes carotenoids very insoluble in water, but they are fat-soluble. Therefore they are usually associated with cell membranes and lipids, the primary water-insoluble component of cells. Some plant carotenoids occur as crystals in a protein matrix, and in some animals carotenoids occur with proteins. These animal carotenoproteins can be a very different color than their component carotenoids. For example, the carotenoprotein responsible for the distinctive blue color of some live lobsters breaks into a bright red carotenoid and colorless protein upon heating. Over six hundred carotenoids occur in plants, animals, and microbes. Since only plants and some microbes can synthesize carotenoids, those carotenoids in animals all come from their dietary sources. Typically several different carotenoids occur in plant tissue containing this class of pigments. For example, orange carrots contain at least six different carotenes that account for their color. All green leaves contain beta-carotene and three xanthophylls, lutein, neoxanthin, and violaxanthin. Most leaves also contain alpha-carotene and several other xanthophylls as well.

Both chlorophylls and carotenoids occur in all green leaves, but chlorophylls mask the carotenoids to the human eye. When the chlorophylls break down as leaves senesce (mature), the yellow and orange carotenoids persist and the leaves turn yellow.

Role in Plants

In the process of photosynthesis, potentially harmful oxidizing compounds are generated. The carotenoids occur in photosynthetic tissues along with chlorophyll to protect them from photooxidative damage. In fact, this protection is essential for photosynthesis. The photoprotective role of carotenoids is demonstrated in plant mutants that cannot synthesize essential leaf carotenoids. These mutants are lethal in nature since without carotenoids, chlorophylls degrade, their leaves are white in color, and photosynthesis cannot occur. Carotenoids also assist chlorophylls in harvesting light. Carotenoids absorb wavelengths of blue light that chlorophylls do not. The energy that carotenoids harvest in the blue range of the spectrum and transfer to chlorophyll contributes significantly to photosynthesis. The growth and development of plants is often stimulated by light, and carotenoids have sometimes been implicated as the photoreceptors of light to trigger these responses.

Outside of photosynthesis, plant carotenoids also serve as one of the pigments, along with anthocyanins and betalains, that provide color to flowers, ripening fruit, and other plant parts. Familiar examples of carotenoids having this role are found in sunflowers, marigolds, bananas, peaches, oranges, tomatoes, peppers, melons, and yellow corn. Two root crops, carrots and sweet potatoes, also acquire their color from carotenoids. These colors attract insects, birds, and bats for pollinating flowers, and they attract a wide range of animals to aid in the dispersal of seeds and fruits.

Role in Animals

Beyond their role in attracting animals, carotenoids are important nutrients and colorants for animals that consume them. Vitamin A is an essential nutrient for humans and animals, and all vitamin A ultimately comes from dietary carotenoids. A small subset of all carotenoids, including betacarotene, can be converted to vitamin A by animal metabolic systems. Animals cannot synthesize these provitamin A carotenoids or vitamin A itself from scratch, and humans around the world obtain about two-thirds of their vitamin A in this provitamin form from plants they eat. The rest is consumed as vitamin A from meat, eggs, and dairy products that ultimately came from carotenoids in the diets of these animal sources.

Animal products and tissues containing vitamin A, which has no color, often contain carotenoids as well. This is the case in egg yolk, butterfat, and the pink or red flesh color of certain fish, such as salmon and trout. Dietary carotenoids are readily visible in the striking and familiar colors of flamingoes, canaries, lobsters, prawn, goldfish, and ladybugs. Some of the coloration in human skin is also due to the carotenoids consumed. Beyond their important role as a source of vitamin A for humans, dietary carotenoids, including those that are not provitamin A carotenoids, have been implicated as protecting against certain forms of cancer and cardiovascular disease. Two carotenoids that appear to impart health benefits are lycopene and lutein. Lycopene is the red carotene that accounts for the typical color of tomatoes and watermelon. Lutein is a yellow xanthophyll common in all green leaves and responsible for much of the color of milk, butter, and egg yolks. The antioxidant properties that make carotenoids essential for photosynthesis may provide a similar type of protection in preventing human disease.

Commercial Importance

The color of food is an important variable contributing to its selection for consumption. Carotenoids in plant extracts such as red palm oil, saffron, annatto, and paprika have been used as food colors through much of history. More recently, industrially produced (synthetic) carotenoids have also been used as food color. Poultry, fish, and mammalian food animal diets also are frequently supplemented with natural or synthetic carotenoids to not only provide a dietary vitamin A source, but primarily to color meat and animal products and make them more appealing for consumers. Medicines and cosmetic products are often colored with carotenoids to enhance their appeal.

see also Pigments; Plastids.

Philipp W. Simon

Bibliography

Bauernfeind, J. Christopher, ed. Carotenoids as Colorants and Vitamin A Precursors. New York: Academic Press, 1981.

Britton, George, Synnove Liaaen-Jensen, and Hanspeter Pfander, eds. Carotenoids, Vol. 1. Basel: Birkhauser Verlag, 1995.

Packer, Lester, ed. Carotenoids, Methods in Enzymology, Vols. 213 and 214. New York:Academic Press, 1993.

Genetic engineering of rice may allow high expression of carotenoids. However, the normally white rice becomes yellow-orange in the process, which may not be appealing to consumers.

Carotenoids

Carotenoids are a group of red and yellow fat-soluble compounds that pigment different types of plants, such as flowers, citrus fruits, tomatoes, and carrots, as well as animals, such as salmon, flamingos, and goldfish. The ingestion of carotenoids is essential to human health, not only because some convert into Vitamin A, but also because they have antioxidant effects, which may combat such diverse problems as cancer and macular degeneration . Carotenoids also help prevent heart disease by inhibiting lowdensity lipoprotein (LDL) cholesterol (the "bad" cholesterol) from sticking to artery walls and creating plaques.

Up to one-third of the Vitamin A consumed by humans comes from the conversion of alpha-carotene and beta-carotene, the two most active of the over 600 carotenoids that have been identified. These two compounds combat early cancers, regulate the immune system , and maintain the integrity of the skin, lungs, liver, and urinary tract, among other organs. Food sources include eggs, liver, milk, spinach, and mangos.

Lycopene is a carotenoid that offers protection to the prostate and the intestines . It has also been associated with a decreased risk of lung cancer. Found in tomatoes, it remains intact despite the processing involved in making ketchup and tomato paste. The carotenoids lutein and zeaxanthin seem to aid in the prevention of cataracts and macular degeneration, and can be found in spinach and collard greens.

see also Antioxidants; Beta-Carotene; Vitamins, Fat-Soluble.

Chandak Ghosh

Bibliography

Margen, Sheldon, and Editors of U.C. Berkeley Wellness Letter (2002). Wellness Foods A to Z: An Indispensable Guide for Health-Conscious Food Lovers. New York: Rebus.

Internet Resources

National Institutes of Health Clinical Center. "Facts about Dietary Supplements: Vitamin A and Carotenoids." Available from <http://www.cc.nih.gov>

WebMD Health. "What Are Vitamins and Carotenoids and What Are the Adverse Effects of Deficiencies and Overdose?" Available from <http://www.my.webmd.com>

carotenoid Any of a group of yellow, orange, red, or brown pigments chemically related to terpenes. Carotenoids are responsible for the characteristic colour of many plant organs, such as ripe tomatoes, carrots, and autumn leaves. They also occur in certain algae and other photosynthesizing organisms (such as phototrophic bacteria), in which they function as accessory pigments in the light-dependent reaction of photosynthesis (see photosynthetic pigments). See carotene; xanthophyll.