In one study, 35,016 women who did not have a history of breast cancer reported their current and past use of supplements. After six years of follow up, women who regularly took fish oil supplements—rich in omega-3 fatty acids—were 32 percent less likely to develop invasive ductal breast cancer, the most common form, compared to those who did not regularly take fish oil supplements.

Researchers in another study compared 636 women with breast cancer to 1,272 women of similar age and menstrual status without breast cancer. Women with the highest blood levels of vitamin D were 27 percent less likely to have breast cancer compared to those with the lowest vitamin D levels. Doctors say more study is needed but believe public policy should encourage women to maintain adequate vitamin D levels.

References:
Cancer Epidemiol Biomarkers; 2010 Jul, Vol. 19, No. 7, 20615886
Cancer Epidemiol Biomarkers Prev; 2010 Sep, Vol. 19, No. 9, 20826834