Normally, you should be able to get all the nutrients you need from a balanced diet. However, taking supplements can provide additional nutrients when your diet is lacking or when certain health conditions cause you to develop an insufficiency or deficiency. In most cases, multiple-vitamin supplements provide all the basic micronutrients (vitamins and minerals) your body needs. These multiple-vitamins are generally safe because they contain only small amounts of the each nutrient.
Individual nutrients can also be sold as dietary supplements, but usually in larger amounts than what's found in a typical multiple-vitamin. They may be used to treat a simple deficiency, such as an iron deficiency, but sometimes they're used therapeutically to treat specific health conditions or risk factors. For example, large doses of niacin may be used to raise good cholesterol (but may cause an uncomfortable niacin flush) , and folic acid (the synthetic form of folate) has been used to reduce the risk of a birth defect called spina bifida.
Scientific research supports some of the benefits of using many dietary supplements for certain health conditions, but in many more cases, the effectiveness has not been backed up by the research evidence (for example - selenium supplements). The National Institutes of Health Office of Dietary Supplements has dietary supplement factsheets that assess the evidence for (and against) the therapeutic use of a large number of dietary supplements.
In the United States, dietary supplements are not regulated as strictly as drugs; manufacturers do not have to prove that their use is either safe or effective. Standardization of supplements is optional, although they are prohibited from selling unsafe products. The Food and Drug Administration (FDA) maintains a list of tainted products that are sold as dietary supplements. The worst offenders are usually weight loss aids, "natural" sexual enhancement pills, and supplements targeted at bodybuilders.
Dietary supplement manufacturers do have to follow some rules regarding labeling and the claims that can be made about the supplements. The claim can be made that a dietary supplement addresses a nutritional deficiency, supports health, or reduces the risk for a specific health problem when there is enough evidence to support that claim. Supplement labels must also use this statement:
This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Most dietary supplements are safe as long as you follow the label instructions, but large doses of certain nutrients can have strong biological effects on the body. While that may be beneficial in some cases, there are times when taking large doses of individual supplements can be dangerous.
For example, the fat-soluble vitamins A and D can build up to toxic levels in your body when taken in large doses over long periods of time. (Here's a list of safe dosages of fat-soluble vitamins.) Vitamin B-6 is a water-soluble vitamin, so your body doesn't store it as easily as a fat-soluble vitamin, but extended use of vitamin B-6 in large amounts can cause nerve damage. Large doses of vitamin C may cause diarrhea.
Mineral supplements can be dangerous as well. For example, both selenium and iron supplements can be toxic in large amounts.
Some dietary supplements can interact with over-the-counter or prescription medications, or even with each other, and some supplements should be avoided before undergoing surgery. Some dietary supplements are best taken on an empty stomach.
Do you really need to take dietary supplements? Ask your health care provider about supplements before taking anything beyond basic multiple-vitamins; some dietary supplements, like raspberry ketone, have little to no research evidence to back their health claims.
Vitamin and Mineral Facts
National Institutes of Health, Office of Dietary Supplements. "Dietary Supplement Factsheets." Accessed April 8, 2011. http://ods.od.nih.gov/factsheets/list-all/.
National Institutes of Health, Office of Dietary Supplements. "Dietary Supplements: Background Information." Accessed April 8, 2011. http://ods.od.nih.gov/factsheets/DietarySupplements/.
U.S. Food and Drug Administration. "Tips For The Savvy Supplement User: Making Informed Decisions And Evaluating Information." Accessed April 8, 2011. http://www.fda.gov/Food/DietarySupplements/ConsumerInformation/ucm110567.htm.