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.
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.
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.
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.
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.
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.
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[Article by: Judith Sims; Teresa G. Odle]