A study at the University of California, San Diego School of Medicine looked at the blood levels of vitamin D in 1200 healthy women.
They found that women whose serum vitamin D level was low during the three-month period just before diagnosis had approximately three times the risk of breast cancer as women in the highest vitamin D group.
Several previous studies have shown that low serum levels of vitamin D are associated with a higher risk of premenopausal breast cancer. "While the mechanisms by which vitamin D could prevent breast cancer are not fully understood, this study suggests that the association with low vitamin D in the blood is strongest late in the development of the cancer, "said principal investigator Cedric Garland, professor in the Department of Family and Preventive Medicine at UC San Diego.
A 2011 meta-analysis by Garland and colleagues estimated that a serum level of 50 ng/ml is associated with 50 percent lower risk of breast cancer. While there are some variations in absorption, those who consume 4000 IU per day of vitamin D from food or a supplement normally would reach a serum level of 50 ng/ml.
Researchers say ask your doctor to check your blood levels of vitamin D before increasing the dose.