In a study of more than 700 women, taking multivitamin tablets in the past five years was associated with 31% lower odds of having breast cancer. The use of calcium supplements was linked to a 40% reduced risk.
The study involved 268 women with breast cancer and 457 women without breast cancer, all in Puerto Rico. The women filled out detailed questionnaires asking which supplements they took during the past five years, how frequently they took them, and whether they still took them.
The women also gave blood samples so the researchers could measure the ability of their DNA to repair damage, a complex biological process that is critical to preventing cancer.
The findings were presented at the annual meeting of the American Association for Cancer Research.
When DNA repair capacity was taken into consideration, calcium was no longer protective against breast cancer, suggesting that calcium supplements act to enhance DNA repair, says Jaime Matta, PhD, professor of pharmacology, physiology and toxicology at Ponce School of Medicine in Ponce, Puerto Rico.
Multivitamins remained protective even after taking into account DNA repair, however, suggesting it is associated with other anticancer benefits as well, he says.
Taking supplements of vitamins A, E, or C alone was associated with a slightly lower risk of breast cancer, but the finding could have been due to chance.
All the analyses were adjusted to take into account the effect of other breast cancer risk factors such as age, family history, and having been pregnant.
Matta tells WebMD the finding suggests that vitamins may work better together than individually to lower cancer risk.
Other studies have had conflicting results. Some have suggested that supplement forms of single vitamins such as A and E don't protect against breast cancer. Others have suggested that vitamins are protective.
"The jury is still out," says Victoria Seewaldt, MD, director of the Duke Early Protection Program at Duke University in Durham, N.C.
The dose and timing may explain the disparity, she tells WebMD. "It could be that taking high doses later in life gives very different effects than taking the recommended amount of vitamins earlier on," Seewaldt says. Most of the women in the study were aged 41 to 60.
One weakness of the study is that the researchers asked women to recall their vitamin intake instead of measuring vitamin levels in the blood.
Another drawback, Matta says, is that "we don't know the exact dose that women took. But generally women told us that they took the inexpensive one-a-day supplements you get at your local drug store."
Also, even though the researchers tried to adjust for risk factors, women who take supplements may be more likely to engage in behaviors such as eating right and exercising that are associated with a lower risk of breast cancer.
Still, Matta says there's no harm in taking a daily multivitamin. "If it comes with calcium, even better."
"There's no toxicity at normal doses and it may help protect against cancer," Matta says.
Seewaldt cautions against making recommendations based on one study alone.
"The importance of the study is that it's addressing normal doses, the recommended amount of vitamins, not high-dose supplements. It underscores the role good nutrition plays in cancer prevention," she says.