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In this study, 38,772 women from Iowa, whose average age was 62 years, filled out questionnaires three times over an 18-year period regarding dietary supplement use.
After a total of 22 years, researchers followed up and report that the risk of dying from any cause appeared to be 6% higher among women who took a multivitamin supplement than among women who did not take a multivitamin. Additional supplementation with vitamin B6, folic acid, iron, magnesium, zinc, and copper was also said to be associated with increased mortality rates.
Two factors should be taken into consideration while interpreting these results, the method used for calculating the results and the type of study.
The media coverage did not note a potentially serious problem with this study: that researchers looked at “adjusted” mortality rates rather than actual mortality rates in the population of women who took supplements, adjusting for a wide range of factors including caloric intake, cigarette smoking, body mass index, blood pressure, educational level, diabetes, use of hormone-replacement therapy, physical activity, and intake of fruits and vegetables.
Studying health events to find patterns in a population (epidemiology) is a relatively inexact science, and it is quite possible that the assumptions upon which the researchers based their adjustments were not entirely correct. When they adjusted the data only for age and caloric intake, there was no statistically significant difference in mortality rate between supplement users and nonusers.
The study was observational, meaning that while it might show a relationship between certain supplements and mortality, it does not provide evidence that one causes the other.
In observational studies, scientists correlate various lifestyle factors with health outcomes. Such studies help researchers develop hypotheses that can be investigated further, but the only type of study that can prove cause and effect is a randomized controlled trial, in which participants are randomly assigned to receive either a particular treatment or a placebo (an inert dummy pill) without knowing whether they are getting the treatment or not.
In the history of medical research, results of observational studies have sometimes eventually been contradicted by randomized controlled trials. In a famous example, numerous observational studies suggested that the use of hormone-replacement therapy by postmenopausal women prevents heart disease, but subsequent randomized controlled trials demonstrated that hormone-replacement therapy either has no effect or actually increases the risk of heart disease.
The new study does not negate previous research demonstrating that vitamins and minerals can have a wide range of health benefits. However, as with all substances that can affect your health, talk to your doctor about which dietary supplements are right for you.
(Arch Intern Med 2011;171:1625–33)