Beta carotene is one of the most important naturally occurring antioxidants. It is a fat-soluble pigment found in plants (notably carrots and many colorful vegetables and fruits) and in the sea alga Dunaleilla salina and D. bardawil. Naturalbeta carotene supplements are derived primarily from D. salina. Beta carotene is one of the major dietary carotenoids and one of the most biologically active of approximately 800 carotenes and more than 1,000 carotenoids present in food. It is responsible for the orange or yellow colors of many fruits and vegetables. In the human body, beta carotene is found in lipids and in fat tissues. Sometimes beta carotene is called provitamin A because it is more easily converted to vitamin A (retinol) in the liver than other carotenoids. Beta carotene is considered to be a conditionally essential nutrient because it becomes essential when vitamin A intake is low.
Beta carotene consists of a chain of 40 carbon atoms, with conjugated double bonds and a ring structure at each end of the chain. Depending on the positions of the molecular groups attached to the carbon chain, naturally occurring beta carotene may be:
All-trans beta-carotene
9-cis beta-carotene
13-cis beta-carotene, in smaller amounts
Synthetic beta carotene is primarily all-trans.
In plants and alga, beta carotene and other carotenoids attract light for photosynthesis and provide protection from toxic forms of oxygen. Beta carotene is a powerful antioxidant because it destroys toxic free radicals, including singlet oxygen—an oxygen atom that is missing an electron and is very damaging to human tissue if not taken up quickly and "deactivated."
Vitamin A precursor
Vitamin A is obtained in the diet from animal products or is made in the liver from beta carotene and other carotenoids. Vitamin A is essential for:
In sub-Saharan Africa about three million children under the age of five suffer from an eye disorder, caused by vitamin-A deficiency, that can lead to blindness and death. Although red palm oil, a traditional African food, contains high provitamin A, its substitution by imported cooking oils has reduced this dietary source in many homes. Many vegetables and fruits also contain provitamin A, but are not always consumed in adequate amounts. Vitamin A deficiency is the leading cause of blindness worldwide.
In the 1920s vitamin-A deficiency was linked to stomach cancer and to precancerous conditions in the epithelial (lining) cells of the throat and lungs. In 1977 vitamin A supplementation was shown to inhibit certain cancers and to reduce the growth of certain tumors in atrisk animals.
Dietary beta carotene
Daily values (DVs) are determined from the RDA. They are based on a 2,000-calorie diet and usually are expressed as a percentage of an RDA. The IUs and DVs for beta carotene, per serving, in common foods are listed in table 1.
Carrots and sweet potatoes that are more orange contain more beta carotene. New carrot cultivars that contain more beta carotene have been developed and high-beta-carotene sweet potatoes are being introduced into sub-Saharan Africa to treat vitamin-A deficiency.
According to the Institute of Medicine, a daily intake of 3–6 mg of beta carotene will keep the blood level within the range associated with a lower risk for chronic diseases. The recommended daily diet of five or more servings of fruits and vegetables provides 3–6 mg of beta carotene (if carrots, sweet potatoes, papaya, apricots or other very high carotenoid food is used, the RDA can be met in a single serving). In contrast, the average American diet contains 1.3–2.9 mg daily. Vegetarians may have twice as much beta carotene in their blood as compared to non-vegetarians, because they generally consume a lot more greens and fruits.
Beta carotene in food is found within an oil or a matrix of sugars and proteins; and, therefore, the absorption of beta carotene by the body varies greatly. The elderly, and those with bad digestion and liver trouble may be at risk for poor absorption from an adequate beta carotene diet.
Animal sources of vitamin A are more easily absorbed than plant sources of beta carotene, particularly if the vegetables and fruits are eaten raw or whole. Although beta carotene can be converted to vitamin A in the body, it has its own unique physiological functions and beta carotene and vitamin A are not totally identical in the health benefits they deliver. So it is good to eat sources of both. While supplementation is helpful to those who have trouble absorbing adequate beta carotene, getting all or some beta carotene through food sources rather than supplements alone is by far the best. This is substantiated by research showing there are many beneficial carotenoids in foods, and that they may also work together synergistically to optimize health.
Supplemental beta carotene
Beta carotene supplements are inexpensive and readily available over-the-counter. They are available as pills, powders, and oils and they vary greatly in potency. Some supplements contain a mixture of carotenoids. There is a major problem with shelf life stability for beta carotene, as it "oxidizes" quickly when in pure form. When buying a supplement of it, shelf life stability or the presence of such stabalizers as vitamin E can guarantee biological activity of the capsule.
Supplemental intake of beta carotene probably should not exceed 3–15 mg per day. Common preparation of supplemental beta carotene include:
30- or 60-mg capsules
5,000-, 10,000-, or 25,000-IU capsules
10,000- or 25,000-IU tablets
A typical dosage of beta carotene for treating cancer is 75,000–150,000 IU daily. Absorption of beta carotene in nutritional supplements can be 70% or more. There is no established maximum daily intake for beta carotene.
Some common beta carotene nutritional supplements include:
Manufacturers often supplement food with beta carotene. One study showed that bakery products enriched with beta carotene increased beta carotene levels in the blood.
Measuring beta carotene
A recommended dietary allowance (RDA) for beta carotene has not been established and most foods are not labeled as to vitamin A content. There are two incompatible systems for quantifying beta carotene. IUs are used most often for nutritional labeling:
1 IU equals 0.6 µg of all-trans beta carotene
3.33 IU of all-trans beta carotene, 2 µg, is equal to 1 µg of all-transretinol (vitamin A)
5,000 IU equals 3 mg of beta carotene, the RDA for vitamin A
1 IU equals 1.2 µg of other provitamin A carotenoids
The second system uses retinol equivalents (RE):
1 RE equals 1 µg of all-trans retinol
1 RE equals 6 µg of all-trans beta carotene
1 RE equals 12 µg of other provitamin A carotenoids
Precautions
Antioxidants such as beta carotene often work together with other antioxidants and an excess or deficiency of one can inhibit the other. The Food and Nutrition Board of the Institute of Medicine does not recommend beta carotene supplementation except in cases of vitamin A deficiency.
Pregnant and nursing mothers should limit their intake of supplemental beta carotene to 6 mg per day or less.
Side effects
Even long-term high-dosage use of supplemental beta carotene appears to be non-toxic. Daily doses of 30 mg or more over a long period may cause carotenosis (carotenodermia), a yellowing of the skin, which is harmless and reversible. In contrast, very high daily doses of vitamin A are very dangerous and damage the liver and other organs, as well as provoke hair loss).
Interactions
Drugs and other substances that may interfere with beta-carotene absorption include:
The absorption of luteine, another carotenoidantioxidant, may be reduced if taken in conjunction with beta carotene.
BOOKS
American Institute for Cancer Research. Nutrition and Cancer Prevention: New Insights into the Role of Phytochemicals. New York: Kluwer Academic/Plenum Publishers, 2001.
Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington: National Academy Press, 2000.
PDR for Nutritional Supplements. Montvale, NJ: Thomson PDR, 2001.
PERIODICALS
Baron, J. A., et al. "Neoplastic and Antineoplastic Effects of Beta-Carotene on Colorectal Adenoma Recurrence: Results of a Randomized Trial." Journal of the National Cancer Institute 95 (2003): 717–22.
Bendich, Adrianne. "From 1989 to 2001: What Have We Learned about the 'Biological Actions of Beta-carotene?'" Journal of Nutrition 134, no. 1 (January 2004): 125S–130S.
Heinrich, U., et al. "Supplementation with Beta-carotene or a Similar Amount of Mixed Carotenoids Protects Humans from UV-induced Erythema." Alternative Medicine Review 8, no. 2 (May 2003): 202–203.
"Incidence and Mortality Following Alpha-tocopherol and Beta-carotene Supplementation: A Postintervention Follow-up."Journal of the American Medical Association 290, no. 4 (July 23, 2003): 476–85.
Quilez, Joan, et al. "Bakery Products Enriched with Phytosterol esters, [Alpha]-Tocopherol and [Beta]-Carotene Decrease Plasma LDL-Cholesterol and Maintain Plasma [Beta]-Carotene Concentrations in Normocholesterolemic Men and Women." Journal of Nutrition 133, no. 10 (October 2003): 3103.
Russell, R. M., et al. "The Enigma of Beta-Carotene in Carcinogenesis: What Can Be Learned From Animal Studies." Journal of Nutrition 134, no. 1 (2004): 262S–268S.
"What's Up Doc? Getting to the Root of Carrots." Environmental Nutrition 26, no. 9 (September 2003): 8.
"The Word: Sweet Potato." New Scientist 179, no. 2402 (July 5, 2003): 51.
Beta Carotene. Mayo Clinic. May 10, 2002 [cited May 4, 2004]. <http://www.mayoclinic.com/invoke.cfm?objectid=DFE6CA61-7BE5-45E0-86BFD9009E27BAE2>.
Task Force Finds Little Evidence to Support Use of Vitamin Supplements to Prevent Cancer or Heart Disease. Agency for Healthcare Research and Quality. June 30, 2003 [cited May 4, 2004]. <http://www.ahrq.gov/news/press/pr2003/vitapr.htm>.
