Get the latest from TODAY

Sign up for our newsletter
By Madelyn Fernstrom, NBC News Health and Diet Editor

Millions of people take multivitamins and minerals daily with the hope of staying healthy and even reducing disease risk. And one big area of interest, especially for women, is breast cancer risk and mortality. But how much scientific evidence actually documents the role of vitamin and mineral supplements in the risk of death from women already diagnosed with breast cancer? A new study published in Breast Cancer Research and Treatment attempted to address this specific question.

Using a combined database of 161,608 postmenopausal women aged 50 to 79 from the Women’s Health Initiative, a group of 7,728 women diagnosed with invasive breast cancer was selected for further study. Known as an observational study, this report compared vitamin and mineral use with breast cancer mortality in women already diagnosed with breast cancer. Their analyses showed an association between multivitamin-mineral use and a 30 percent reduction in mortality rate. While this might sound impressive, it’s important to take a closer look.

The authors acknowledge that the link between supplement intake and reduced breast cancer mortality is an association only -- not a “cause and effect” relationship. But before an association can even be established, it’s important to examine how the original data were collected. For example, how precise was the vitamin/mineral intake of the study participants? For observational studies, a reliance on self-reported lifestyle information, collected by survey results, is the basis for complex statistical analyses. And while the statistical analyses are complex, the actual data analyzed can be highly variable especially when it comes to self-reported recall information.

A closer look at multivitamin/mineral use among the study participants clouds the conclusion of even an association. The information on multivitamin use (with or without minerals) was collected in a standardized manner for supplements used at least once a week in the past two weeks. While the information about the supplements was collected in a systematic manner, and subjects brought in the bottles of supplements they said they took, the authors also point out that they had no way of confirming that these supplements were taken daily, as directed on the bottle, so this introduces major variability in just how many vitamins and minerals the participants actually took. Also, the information was collected infrequently, and based only on the prior two weeks of intake.

And while many factors that might impact breast cancer mortality were corrected for, no information of diet history was compared, a key factor because this is the original daily source of vitamins and minerals. Diet recall is also of questionable accuracy, but this reveals a huge gap of information in attempting to connect the role of vitamins and minerals and breast cancer mortality.

There are many challenges of using observational studies to make associations between vitamin and mineral supplementation and disease risk or mortality. That is why there is such mixed, and oftentimes conflicting, information about the role of vitamins and minerals in health and disease. Earlier studies have suggested that vitamin and mineral use actually increases cancer risk.

With more than one-third of Americans reporting use of vitamins and minerals and more than half using some type dietary supplement, what’s the bottom line for vitamin and mineral supplementation? A simple message: When it comes to vitamin and mineral supplements, more doesn’t mean better, and moderation is key. Supplements are intended to support, not replace, real food, which is nature’s primary source of daily vitamins and minerals. 

For healthy adults, a daily multivitamin/mineral supplement containing 100 percent of the recommended daily intake is easy, convenient and economical. Call it good dietary insurance. Always check with your doctor for personalized advice, if you want to go outside of these recommended guidelines. And remember that when it comes to claims about vitamins, minerals and other supplements, if it sounds too good to be true, it probably is.