A recent study was at first glance, alarming to users of dietary supplements. However, understanding the details of the study tells a different story. The authors report that the use of multivitamins and select nutrients was assessed in relation to total mortality in 38,722 older women in the Iowa Women’s Health Study. Over a period of 22 years, the risk of dying from any cause was 6% higher in women who took a multivitamin supplement compared with women who did not. The use of folic acid, vitamin B6, iron, magnesium, zinc and copper were also associated with increased risk of total mortality compared with women who did not use these supplements. The use of calcium was inversely related to mortality.
One the potentially big problems with this study is that the researchers did not report the actual mortality rates. Instead they compared what is called "adjusted" mortality rates between supplement users and nonusers. This was done by adjusting for a wide range of factors including weight, intake of calories, cigarette smoking, blood pressure, educational level, diabetes, use of hormone-replacement therapy, physical exercise and fruit and vegetable intake. For each of these factors, those who took supplements were in the categories that would be considered healthier– for example— less diabetes, less obesity, more physically active, less smokers and more fruits and vegetables in their diet. These healthier people would be expected to have lower death rates than those individuals who did not take vitamins. What this does statistically is that the mortality rate of the supplement users would then be adjusted upward compared to the mortality rate of non supplement users. It is very possible that the researchers “over-adjusted” the collection of data, skewing the death rate among supplement users look higher than it really was. This conclusion is supported by the fact that when the researchers adjusted the data based only on age and intake of calories, there was in fact no statistically significant difference in mortality rate between supplement users and nonusers.
Studies that are observational, as this one was, are always weaker studies than randomized controlled trials. You can never prove cause and effect with observational studies, and it would be a mistake to make meaningful conclusions from this study due to its observational nature and possible over adjustment of the data. Another issue to ponder is that the individuals taking supplements were not more likely healthy, but perhaps less healthy. In other words… we might wonder why they were taking supplements to begin with. Perhaps they had a chronic health problem or a family health history that the researchers did not use as an identifier. What if they had a family history of heart disease for example and that is why they were taking supplements. These individuals could then easily have an increased mortality rate due to their family history.
The scientific literature is robust with randomized clinical trials demonstrating the diverse range of benefits of taking vitamins and minerals. It is always important to recognize the potential benefit and risk of any intervention whether it be over the counter or prescription drugs, vitamins, minerals or herbs. For now, women should not be discouraged to take vitamins and minerals, but individual assessment and need is best determined by a licensed practitioner trained in the use of these therapies. The medical degree that offers the most training in this area of medicine is a naturopathic doctor degree. Licensed graduates from the accredited naturopathic medical schools receive extensive training in nutrition and the use of vitamins and minerals for prevention and treatment.
Reference
Mursu J, Robien K, Harnack L, et al. Dietary supplements and mortality rate in older women. The Iowa Women’s Health Study. Arch Intern Med 2011;171(18): 1625-1633.
