It is interesting that the Iowa Women's Health Study (IWHS) data, suggesting that many commonly used dietary supplements do not decrease or might even increase mortality,1 were supported by the Kuopio Ischaemic Heart Disease Risk Factor Study as described by Tuomainen et al. However, the authors were concerned about potential reverse causality. Some of the IWHS women may have started the use of supplements because of health issues, and thus disease would be a cause for the increased mortality and supplement use, just a marker. However, sensitivity analyses excluding those women who self-reported cardiovascular disease, diabetes, or cancer at baseline yielded results similar to our main findings. Furthermore, supplement users tended to have healthier lifestyle compared with nonusers. Except in the case of iron, where the relatively high dosages indirectly argue that they were used to combat anemic conditions, these findings argue against reverse causality bias. However, we cannot fully exclude the possibility because the IWHS did not assess the motives for the supplement use and incidence of disease was not exhaustively assessed.
Mursu J, Jacobs DR. The Importance of Translating Research Skillfully to Benefit the Public—Reply. Arch Intern Med. 2012;172(5):449–450. doi:10.1001/archinternmed.2012.39
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