Although breast cancer mortality follows a similar geographic distribution to that of colon cancer, direct evidence of an association between vitamin D nutritional status and breast cancer risk is limited. A prospective study of women who participated in the first National Health and Nutrition Examination Survey (NHANES I) found that several measures of sunlight exposure and dietary vitamin D intake were associated with a reduced risk of breast cancer 20 years later. More recently, a 16-year study of more than 88,000 women found that higher intakes of vitamin D were associated with significantly lower breast cancer risk in premenopausal women but not postmenopausal women.

Adult humans need much more vitamin D than the amount that used to be recommended (400 IU) – probably somewhere around 3000-5000 IU daily. So you cannot get enough vitamin D from the diet alone. Sun exposure without sunscreens is the preferred source of vitamin D. If you need vitamin D supplementation, blood testing of vitamin D level is recommended to know how much supplements to take and not to overdose.

Conclusion: We found strong evidence to support the hypothesis that vitamin D could help prevent breast cancer. However, our results suggest that exposure earlier in life, particularly during breast development, maybe most relevant. These results should be confirmed.