( Health.com ) -- Older women have a slightly increased risk of dying at an earlier age if they take multivitamins or certain other dietary supplements , according to a new study published today in the Archives of Internal Medicine . Of the 15 supplements included in the study , iron was most strongly linked to an increased risk of earlier death . Women who took iron supplements were 10 % more likely to die during the 22-year study than women who did not take them , even after the researchers controlled for several other health and demographic factors . Folic acid , vitamin B6 , magnesium , zinc , copper , and multivitamins were also associated with an increased risk of earlier death . Calcium was the only supplement associated with a lower risk of death during the study . `` I do n't think we really know enough to be prescribing supplements , '' says David Jacobs Jr. , Ph.D. , the senior author of the study and a professor of public health at the University of Minnesota , in Minneapolis . Jacobs stresses , however , that people who have been prescribed dietary supplements for a specific vitamin or nutrient deficiency should not stop taking them without consulting their doctor . Health.com : Which vitamins do you need ? The study -- which shows only an association , not cause and effect -- has some key shortcomings . The researchers measured supplement intake using questionnaires that relied on the memory of the participants , for instance . In addition , the researchers do n't know whether the women were taking supplements for a health problem . The increases in death risk might therefore be due to underlying health conditions rather than the supplements themselves , says Susan Fisher , Ph.D. , a professor of community and preventive medicine at the University of Rochester Medical Center , in Rochester , N.Y. `` People [ often ] go to the vitamin store because they feel something is n't right -- their memory is slipping , they do n't have as much energy , '' says Fisher , who was not involved in the new study . `` These could be subconscious symptoms of a disease that is eventually going to take their life . '' Health.com : Heart trouble ? 30 herbal remedies to avoid . The use of dietary supplements has exploded in the United States in recent decades . According to the study , roughly half of all adults reported taking supplements in 2000 . The rates were even higher among the 38,772 women in Iowa who participated in the study . Sixty-three percent were taking at least one supplement when the study began , in 1986 , but by 2004 that number had jumped to 85 % . And more than one-quarter of those women reported taking four or more supplements on a regular basis . The women were 62 years old at the beginning of the study , on average , and none had chronic health conditions ( with the exception of diabetes and high blood pressure ) . Just over 40 % of the women had died by the time the study ended in 2008 . Women who took calcium -- which is known to reduce the risk of bone fractures associated with aging -- were 9 % less likely to die during the study than women who did not take it , but most of the other supplements were associated with an increased risk of death . Some supplements , including beta-carotene , vitamins A and C , and selenium , did n't appear to affect death risk either way . Health.com : 11 foods for healthy bones . However , the increases in death risk ( or decrease , in the case of calcium ) were extremely small and may not be clinically meaningful , says Mary L. Chavez , a professor of pharmacy practice at the Texas A & M Health Science Center , in Kingsville . Several factors besides supplement use may have shaped the results . Fisher points out , for instance , that the use of hormone therapy to treat the symptoms of menopause was nearly twice as common among supplement users as among nonusers . Some forms of hormone therapy have been linked to an increased risk of heart disease and breast cancer . The researchers did take hormone therapy into account in their analysis , along with several other potentially mitigating factors ( including age , educational attainment , body mass index , diet , and physical activity ) . But the wide disparity in hormone use between supplement users and nonusers may indicate that the two groups of women were different in subtle yet important ways , Fisher says . Health.com : Can supplements ease menopause symptoms ? `` People choosing to take [ hormone ] replacement therapy often have underlying different characteristics and lifestyle behaviors , which you ca n't necessarily balance out with a mathematical model , '' she says . It 's not clear that the study results apply to the population of older women as a whole . The study participants were white and tended to live in rural areas , so the findings ca n't necessarily be generalized to other races and geographic areas . Still , the study is a reminder that supplements may not be as harmless as they seem , Fisher says . `` Taking more of what we think is a good thing may not be so good , '' she says . `` High levels of nutrient supplements may be harmful , or at least not helpful . '' Copyright Health Magazine 2011 .
The use of dietary supplements has increased in the United States in recent decades . Folic acid , vitamin C , magnesium , zinc and copper pose an increased risk . Iron was most strongly linked to an increased risk of earlier death -- calcium was the least . Increases in mortality risk were extremely small and may not be clinically meaningful .