A new study has reported that older women who take certain supplements are at higher overall risk of death (i.e., from any cause, not just breast cancer). Although many women take dietary supplements to prevent chronic disease, the long-term health consequences are unknown. The study included 38,772 women in the Iowa Women's Health Study. The average age at baseline in 1986 was 61.6 years. The women reported their supplement use at baseline and in 1997 and 2004 and were followed through December 31, 2008. Use of supplements increased between 1986 and 2004, with 62.7% of the women reporting use of at least one supplement daily at baseline, compared to 75.1% in 1997 and 85.1% in 2004. A total of 15,594 (40.2%) of the study participants had died through 2008.

Use of multivitamins was found to be associated with a slight (2.4%) increase in the absolute risk of death compared with nonuse. Use of zinc (3.0%), magnesium (3.6%), iron (3.9%), vitamin B6 (4.1%), and folic acid (5.9%) each were associated with a somewhat higher risk of death compared with corresponding nonuse. However, the increased risk of death was found at progressively lower doses of iron as women aged throughout the study. Copper supplementation was associated with a relatively high risk of death (18.0%) compared to nonuse. On the other hand, use of calcium was inversely related with mortality (-3.8%). The findings for iron and calcium were confirmed in separate, shorter-term analyses. The author conclude that in older women, several commonly used dietary vitamin and mineral supplements may be associated with increased total mortality risk; this association is especially strong with supplemental iron. In contrast to the findings of many studies, calcium is associated with reduced risk.

Comments regarding the study

As the studies below show, excess levels of most of the micronutrients identified with higher overall mortality in the study have also been found to be associated with increased risk of breast cancer. Our suggestion is that women should have their levels of these vitamins and minerals (and others such as vitamin D) checked periodically (with blood tests) and take supplements only as necessary to correct deficiencies that cannot be corrected by diet. Both abnormally low and excessively high levels of vitamins and minerals, including calcium, should be avoided.