Some earlier research has suggested that vitamin D and calcium supplements might reduce the risk of developing breast cancer, as well as improve outcomes for women diagnosed with breast cancer.
A very large study found that postmenopausal women who took vitamin D and calcium supplements had the same risk of developing breast cancer as women who took a placebo (dummy pill). So vitamin D and calcium supplements didn't seem to reduce breast cancer risk in this study.
These results are from a large study called the Women's Health Initiative (WHI). More than 36,000 postmenopausal women took part in the WHI. The WHI looked at many aspects of women's health, including bone health, cardiovascular health, and cancer risk.
Half of the women in the WHI took vitamin D and calcium supplements. The women took 400 IU (international units) of vitamin D per day and 1,000 milligrams of calcium per day. The other half of the women took placebo pills that looked like the supplements. The women were followed for about 7 years.
The vitamin D and calcium supplements didn't reduce breast cancer risk. The number of women who developed either invasive or non-invasive breast cancer was the same in both the supplement group and the placebo group.
The vitamin D and calcium supplements didn't influence breast cancer prognosis or outcome. Among the women who developed breast cancer, the stage of the cancer at diagnosis and survival during and after treatment were the same in both the supplement and the placebo group.
The researchers think that more research is needed before we're absolutely sure that vitamin D and calcium supplements don't lower breast cancer risk. It might be that a higher daily dose of vitamin D could lower risk. Even though vitamin D and calcium supplements didn't lower risk in this study, taking these two supplements can be important to maintain bone health. If you're a postmenopausal woman, you might want to ask your doctor whether vitamin D and calcium supplements make sense for you.
Also, other diet and lifestyle choices CAN affect breast cancer risk. Visit the Breastcancer.org Lower Your Risk section to learn about diet and lifestyle choices you can make to keep your risk as low as it can be.