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Vitamins are organic compounds found in plants and animals that are necessary in small quantities for life and health. Thirteen different vitamins have been identified as necessary for humans. The body can make small quantities of two of these vitamins, vitamins D and K. All other vitamins must be obtained either from food or from dietary supplements.
Each of the 13 vitamins has specific functions, and taken together vitamins play a role in almost every function in the body. They help convert food to energy, and are involved processes as diverse as blood clotting, vision, reproduction, and transmission of nerve impulses.
Humans need nine water-soluble vitamins. These vitamins dissolve in water and are not stored in the body for long periods. Most excess water-soluble vitamins are removed by the kidneys and leave the body in urine. Below is a list of the water-soluble vitamins and a very brief description of their importance to health. For details on how these vitamins function, see the specific entries for each vitamin. In general, B vitamins tend to be involved in reactions that convert nutrients to energy and reactions that synthesize new molecules. There are gaps in the numbering of the B-complex vitamins, because compounds originally named as vitamins, such as B4 (adenine), were renamed after further research showed that theydid not meet the definition of a vitamin.
Humans need four fat-soluble vitamins. Unlike water-soluble vitamins, fat-soluble vitamins can be stored in the body. High levels of these vitamins can cause health problems. Below is a list of the water-soluble vitamins and a very brief description of their importance to health. In general the fat-soluble vitamins
have antioxidant activity that helps protect cells from damage. For details on how these vitamins function, see the specific entries for each vitamin.
Vitamin supplements come as tablets, capsules, and elixirs (liquids). Supplements can contain a single vitamin, a group of related vitamins that work together in the body (e.g. B-complex vitamins), or a mixture of vitamins and minerals (e.g. vitamin D and calcium that work together to build bones). Vitamins are also added to foods that can then be labeled“fortified” or“enriched.” Many so-called functional foods, or nutra-ceuticals, have added vitamins, minerals, and herbs.
In the United States, the Food and Drug Administration (FDA) regulates dietary supplements under the 1994 Dietary Supplement Health and Education Act (DSHEA). Under DSHEA, supplements are subject to the same regulation as food, which is much less rigorous than the regulation of prescription or over-the-counter drugs. Vitamin manufacturers do not have to prove that their products are safe or effective before they can be sold to the public. By contrast, manufacturers of conventional prescription and over-the-counter drugs must prove both safety and effectiveness in extensive humans before their product can be marketed.
In 2007, ConsumerLab, an independent testing company in New York, evaluated 21 brands of multivitamins. They found that only 10 of these multivitamins contained all the vitamins and minerals in the quantities listed on the label. In addition, some brands contained contaminants, including lead. To get the most out of vitamin supplements, consumers should
The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for most vitamins and minerals. The DRIs consist of three sets of values. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97-98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.
Experts agree that vitamin supplements are not a substitute for nutrients from food. Most healthy people in developed countries who eat a varied diet high in fruits, vegetables, and whole grains get enough vitamins and do not need a vitamin supplement, although many take a daily multivitamin as“insurance.” However, some groups do tend to need either general supplementation with a multivitamin or supplementation with specific vitamins to prevent vitamin deficiency diseases. People in these groups should discuss their vitamin requirements with their healthcare provider. They include:
Both too little and too much of any of the 13 human vitamins may cause health consequences. See entries on specific vitamins for more detailed information about potential health concerns.
The interactions among various vitamins, enzymes, coenzymes, drugs, and herbal supplements are complex and incompletely understood. See entries on specific vitamins for more detailed information about their interactions.
Vitamins acquired by eating fruits and vegetables promote health. No complications are expected from vitamins in food. Vitamin supplements may cause hypervitaminosis or interact with other supplements, prescription drugs, over-the-counter drugs, and herbal supplements in ways that cause undesirable side effects. See entries on specific vitamins for more detailed information about potential complications.
Parents should encourage their children to eat a healthy and varied diet high in fruits, vegetables, and whole grains to meet their vitamin needs.
Most vitamin poisonings and deaths occur in children under age 6 as the result of accidental intake of excessive vitamin supplements. Parents should treat vitamin supplements as they would any drug and store them out of the reach of children.
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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.
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Guyton JR, Bays HE.“Safety considerations with niacin therapy.” Am J Cardiol. (March 19, 2007):S22-31.
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. <http://caonline.amcancersoc.org/cgi/content/full/56/5/254>
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>
Harvard School of Public Health.“Vitamins.” Harvard University, November 10, 2006. <http://www.hsph.harvard.edu/nutritionsource/vitamins.html/>
Mayo Clinic Staff.“Dietary Supplements: Using Vitamin and Mineral Supplements Wisely.” MayoClinic.com, June 5, 2006. <http://www.mayoclinic.com/health/>
supplements/NU00198/ Medline Plus.“Medline Encyclopedia: Vitamins.” U. S. National Library of Medicine, October 27, 2004. <http://www.nlm.nih/gov/medlineplus/ency/article/002399.htm/>
Tish Davidson, A.M.