Women who regularly take multivitamins have a higher risk of breast cancer than women who don’t, according to a large new study. However, it’s not clear whether these supplements actually increase a woman’s risk, or whether other factors might be at work.

What do we know already?

Many people take multivitamins as an insurance policy of sorts, to make up for any nutritional gaps in their diet. By upping their daily intake of vitamins and minerals, they hope to improve their overall health and reduce their risk of disease.

But it’s uncertain whether multivitamins deliver on such expectations. Although certain supplements are recommended for some groups, such as folic acid for pregnant women, research hasn’t shown they provide clear benefits for most healthy people. Some studies even suggest that multivitamins may increase the risk of certain health problems.

One area of concern is a possible link between multivitamins and breast cancer. Some studies (but not all) have reported a higher risk of this cancer among women taking multivitamins. Also, a recent study found that women who take multivitamins tend to have denser breast tissue, which is a known risk factor for breast cancer.

Researchers have now done a large study with more than 35,000 Swedish women aged 49 to 83 years old. Using questionnaires, the researchers recorded the women’s use of multivitamins, as well as details about their health and lifestyle. They then followed up with the women for an average of 9.5 years to assess their breast cancer risk.

What does the new study say?

Among women taking multivitamins, 3.25 percent developed 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.

The researchers can’t say which nutrients, or combination of nutrients, in the multivitamins might be related to the increase in breast cancer risk. However, other studies have suggested that folic acid, iron, and zinc may each play a role in breast cancer development.

How reliable are the findings?

This study was very large and carefully done. However, this type of research can’t show cause and effect. So although it suggests a link between multivitamins and breast cancer, 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.

Where does the study come from?

The study was done by researchers in Sweden and was funded by the Swedish Cancer Foundation and the Swedish Research Council for Infrastructure. It was published in the American Journal of Clinical Nutrition, which is owned by the American Society for Nutrition.

What does this mean for me?

If you are a woman and regularly take a multivitamin, you may be concerned by these findings. However, we don’t yet know whether multivitamins actually increase the risk of breast cancer. We need more studies.

However, we also don’t know whether these supplements have benefits for most healthy people. If you eat a well-balanced diet, you might reconsider whether you need to take a multivitamin.

What should I do now?

If you want to boost your health through improved nutrition, eating a varied and balanced diet is a better bet than taking a multivitamin. And if you’re concerned about breast cancer, ask your doctor about what you might do to lower your risk, such as stopping smoking, taking regular exercise and eating more healthily.