I wouldn't advise giving up multivitamins or calcium supplements on the basis of the study you refer to: an assessment of supplement use and the death rate among more than 38,000 older women enrolled in the long-running Iowa Women's Health Study. Published in the October 10, 2011, issue of the Archives of Internal Medicine, the study reported the possibility of a slightly increased risk of death among women who took multivitamins and other supplements, including vitamin B6, folic acid, iron, magnesium, zinc and copper. The researchers also found a reduced risk of death associated with taking calcium supplements.
First, I should tell you that this study was based on the recall of the women who took part. They were asked in three separate years, 1986, 1997 and 2004, to fill out a 16-page questionnaire on their supplement use and the frequency with which they ate 127 different food items. Of course, there's no way to verify the accuracy of participants' recall. This type of study is observational in nature - it can provide us only with statistical correlations and doesn't tell us anything about cause and effect. Indeed, the most the researchers could say in their conclusion was that taking certain supplements "may be associated" with an increased death rate.
The strongest association seen was between iron supplements and mortality rates. This doesn't surprise me: iron is one of the few minerals we cannot eliminate (except through blood loss), and accumulations in the body can rise to toxic levels. Iron is an oxidizing agent that can increase the risk of cancer and heart disease. No one should take iron supplements or a multivitamin/mineral containing iron. The two exceptions are premenopausal women with heavy menstrual flow and individuals who have had blood tests showing that they're iron deficient.
I'm skeptical about the rest of the study's findings. We don't know the dosages or quality of the supplements the women took. (Different forms of vitamins and minerals have different absorption rates and bioavailability in the body.) However, we do know that the average BMI (body mass index) of women in the study was 27, which means that the majority were overweight. We also know that when the study began between 14 to 17 percent of the women were smokers. (That dropped to three to five percent in the 2004 follow- up.) When the study began, seven to 13 percent of the women were on hormone replacement therapy and between 35 and 38 percent had high blood pressure; by 2004, 43 percent of them had high blood pressure. As my colleague Danna Park, M.D., points out in an analysis, generalizing about this group is questionable since the women were not completely healthy to begin with.
Dr. Park also notes that since there is such a wide range of multivitamin products on the market, it is possible that some individuals may be combining supplements in a way that might be harmful. (More than one quarter of the women were taking four or more supplements.)
I'm sure this won't be the last word on the subject. We need more and better studies to assess the benefits and risks of taking multivitamin/mineral supplements. In the meantime, I wouldn't let the much publicized conclusions of this study frighten you away from taking a good quality daily multivitamin/mineral supplement.
Andrew Weil, M.D.