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.