Their conclusions are based on a review of previously published scientific studies on multivitamins. The researchers presented their findings at the National Institutes of Health's State-of-the-Science Conference on Multivitamin/Mineral Supplements and Chronic Disease Prevention held in Bethesda, Md., May 15 to 17, 2006.
Background information contained in the study stated that over-the-counter multivitamin and mineral supplements are commonly consumed by Americans in the hope of promoting overall health and preventing chronic disease. Multivitamin/mineral supplements typically contain 10 or more vitamins and minerals at varying doses.
"We found very little scientific evidence to demonstrate that multivitamin use prevents chronic disease. The only possible benefit was seen for cancer prevention in people with poor nutrition," said Han-Yao Huang, PhD, MPH, assistant professor in theDepartment of Epidemiology at the Johns Hopkins Bloomberg School of Public Health and the principal investigator of the review project conducted at the Johns Hopkins Evidence-Based Practice Center.
Using an objective, comprehensive and reproducible review approach, Huang and her colleagues identified only five clinical trials that specifically examined the effects of routine use of multivitamin supplements in preventing cancer, hypertension, cardiovascular disease or age-related eye disease. No clinical trial looked at other chronic diseases. One large clinical trial conducted in approximately 30,000 nutritionally deprived Chinese showed that a beta-carotene, vitamin E and selenium supplement reduced the risk for gastric cancer and overall cancer deaths. Another clinical trial in approximately 13,000 French showed that a vitamin C, vitamin E, beta-carotene, selenium and zinc combination reduced the risk of cancer in men, but not in women. Blood levels of certain nutrients suggest that women in the French trial may have consumed more fruits and vegetables than men. They were also less likely to be smokers. Participants in these clinical trials used the study supplements for 5 to 8 years. Both trials used low doses (one to two times the U.S. Recommended Daily Allowances, the average daily dietary intake level sufficient to meet the nutrient requirement of nearly all apparently healthy individuals). In the few studies that looked at the effects of antioxidant supplement use on hypertension, cardiovascular disease or cataracts, no consistent significant benefit was found. However, one study showed that combined antioxidants and a high dose of zinc slowed the progression of age-related macular degeneration in people who already have the disease.
The research team is uncertain whether the findings from the Chinese and French studies are applicable to the United States population because of differences in diet and other lifestyle factors among the three populations. In addition, the multivitamins used in the studies were different from the types of multivitamins commonly used by Americans.
"According to the Dietary Guidelines for Americans, Americans should consume a sufficient amount of and a wide variety of fruits and vegetables each day to maintain adequate intake of vitamins and minerals. We do not have evidence to show that multivitamin supplement use is effective or ineffective for promoting health or preventing chronic diseases among healthy Americans. We also do not have good evidence to say whether multivitamin supplement use will harm people who regularly consume vitamins and minerals in an amount much more than the Recommended Daily Allowances, such as those who take multivitamin supplements in addition to eating foods fortified with vitamins and minerals or taking single vitamin or mineral supplements. It is biologically possible, though, that adverse effects may occur with over-dosing," said Huang.
The study was requested and funded by the National Institutes of Health and conducted by the Johns Hopkins Evidence-Based Practice Center, under contract to the Agency for Healthcare Research and Quality (AHRQ). The Johns Hopkins Evidence-Based Practice Center is based in the Department of Epidemiology at the Johns Hopkins Bloomberg School of Public Health. Its mission is to generate, assemble, and synthesize knowledge and evidence necessary for the effective and efficient application of medical and public health practices.
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