The dietary supplement industry in the United States is enormous, profitable, and growing. There is a widespread belief that nutritional supplements are inherently safe and beneficial and that everyone can benefit from taking them regardless of health status or dietary habits. The evidence has generally not supported this belief (for example, and also this summary), though the available studies are mostly observational and so cannot definitively identify risks or benefits in specific populations with particular behavior or medical conditions. However, as usual, the burden of showing safety and efficacy properly lies with those claiming supplements are useful or necessary, and particularly those using these claims to sell supplements. The supplement industry, however, benefits from virtually no regulatory oversight and so is not generally inclined to support rigorous scientific research into their products. So we must make do with the data we can get.
Mursu J, et al. Dietary supplements and mortality rate in older women: The Iowa Women’s Health Study. Archives of Internal Medicine. 2011;17(18):1625-33.
This study examines data drawn from a decades-long longitudinal study of over 38,000 women in Iowa. It examines self-reported supplement use and correlates this with total risk of mortality over time. The study illustrates the growth of supplement use. At baseline in 1986, the mean age of the women was 61.6 years, and about 63% of the women reported using supplements. By 2004, 85% of the women reported taking a dietary supplement.
Of the many supplements reported and analyzed, the only one correlated with a decreased risk of death was calcium. This is widely recommended supplement for the demographic in this study, post-menopausal women, and there is a sound rationale and some supporting evidence for using it in this group. All other supplements examined either had no association with mortality or were associated with an increased risk of death, including multivitamins, vitamin B6, folic acid, iron, magnesium, zinc and copper. Iron, in particular, showed a strong association with increased mortality risk that increased as the dose of the supplement increased.
It is not possible to demonstrate from such a study that these supplements cause an increased risk of death. But if untargeted supplementation truly had a beneficial effect on older women, it would be very unlikely to see the pattern of results shown in this study. And considering that several other similar studies have also failed to find a beneficial effect from such supplement use, it seems pretty clear that unless there is a specific deficiency or risk factor that suggests supplementation is needed, the non-specific use of single or multiple vitamin and mineral supplements is not likely helpful and may very well be harmful.
In a commentary on this study, a pair of nutritional experts remarked that the results
add to the growing evidence demonstrating that certain antioxidant supplements, such as vitamin E, vitamin A, and beta-carotene, can be harmful….
Dietary supplementation has shifted from preventing deficiency to trying to promote wellness and prevent disease….Until recently, the available data regarding the adverse effects of dietary supplements has been limited and grossly underreported. We think the paradigm ‘the more the better’ is wrong….
We cannot recommend the use of vitamin and mineral supplements as a preventive measure, at least not in a well-nourished population…
This should not be a surprising or controversial conclusion. The myth that vitamins are inherently benign and always beneficial likely can be traced to the 19th and 20th centuries, when vitamins, and diseases associated with then common vitamin deficiencies, began to be identified and understood. The effects of supplementation when there is a deficiency are remarkable and dramatic. Unfortunately, there is no free lunch in physiology. If something ahs benefits, it also has possible harms. And most nutrients have an optimal range, with diseases associated with excesses as well as deficiencies. Unfortunately, we don’t often know what this optimal range, or the appropriate level of intake is, which leaves a lot of room for baseless claims about widespread deficiencies and the need for supplements. Hopefully, as data like this accumulates, we will be better able to dispel the “more is better” myth that supports untargeted dietary supplement use, and sales.