Study links multivitamins to higher breast cancer risk

Consumer Reports News: April 13, 2010 10:53 AM

I ditched my multivitamin a couple months ago, and I'm feeling pretty good about it.

I started taking the supplement when I was pregnant, and I kept on taking it when I was breastfeeding, sleep-deprived, and feeling like my body needed all the help it could get post-pregnancy. But even after my sleep, meals, and life had become more routine again, I continued to give myself this extra jolt of nutrients each day.

The turning point came when I realized I was using my multivitamin as a nutritional crutch, to excuse my lack of vegetables at lunchtime, or breakfast in the morning. Since I'm pretty strict about my kids eating a balanced diet, why should I get a pass? And as I well knew, vitamins from a bottle aren't a substitute for good nutrition on the table.

Even so, it's tempting to use these supplements as a nutritional insurance policy of sorts, and many people do, hoping to improve their overall health and reduce their risk of disease.

But it's uncertain whether multivitamins deliver on such expectations. Although daily supplements are recommended for some people (such as strict vegetarians, dieters, and others who may be missing key nutrients), research hasn't shown they provide clear benefits for most people. Some studies have even suggested that multivitamins may actually increase the risk of certain health problems.

The latest such study comes from a group of Swedish researchers who explored a possible link between multivitamins and breast cancer. Their study included more than 35,000 Swedish women ages 49 to 83 years old. Using questionnaires, the researchers recorded the women's use of multivitamins, as well as details about their diet, health, and lifestyle. They then followed the women for an average of 9.5 years, tracking any breast cancer diagnoses through a national database.

Among women taking multivitamins, 3.25 percent were diagnosed with breast cancer during the study. This compared with 2.6 percent of women not taking these supplements. After factoring in several things that might have affected the women's breast cancer risk (such as their age, family history of breast cancer, and use of hormone replacement therapy), the researchers concluded that women who took multivitamins were 19 percent more likely to develop breast cancer.

However, this doesn't mean that the multivitamins actually increased the women's risk of cancer. The study wasn't designed to show cause and effect, so it's possible that other factors were responsible for the increase in risk. For example, it could be that women who took multivitamins were less healthy in other respects, and this is what increased their chances of getting cancer.

Nonetheless, these findings are of concern, especially in light of other research showing that premenopausal women who take multivitamins tend to have denser breast tissue, which is a known risk factor for breast cancer. Studies have also suggested that some nutrients in multivitaminsnotably, folic acid, iron, and zincmay each play a role in breast cancer development, although more research is needed.

What you need to know. Women who regularly take multivitamins may have a higher risk of breast cancer than women who don't. But we don't yet know whether these supplements actually increase a woman's risk, or whether other factors might be at work.

However, we also don't know whether these supplements have clear benefits for most healthy people. If you eat a varied and balanced diet, you might rethink whether you need to take a multivitamin.

—Sophie Ramsey, patient editor, BMJ Group has partnered with The BMJ Group to monitor the latest medical research and assess the evidence to help you decide which news you should use.

Find out when it makes sense to take a supplement, and when it doesn't.

Aaron Bailey

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