But women using conventional HRT aren’t as protected by vitamin D.

The relationship between low vitamin D levels and breast cancer has been strongly suggested in a number of studies released over the past few years, but an excellent new study out of Germany strengthens the evidence. This is the first large study where blood levels of vitamin D were measured in a group of women with breast cancer and compared to a similar group of women without breast cancer. Those with breast cancer had significantly lower levels of vitamin D.

In an interesting twist, the researchers compared vitamin D levels and breast cancer risk between women who used hormone replacement therapy (HRT) and those who hadn’t. Those on HRT had higher vitamin D levels, yet weren’t as protected from breast cancer. Vitamin D may protect against breast cancer because, like progesterone, it blocks estrogen’s stimulation of cell growth by encouraging cell differentiation and apoptosis (programmed cell death). Cancer cells don’t differentiate and don’t die when they’re supposed to.

Since many Europeans use progesterone instead of progestins in their HRT, it’s unfortunate that the researchers did not distinguish between types of HRT in this study.

If you're at all concerned about your vitamin D levels, it's probably a good idea to test your vitamin D levels.

Abbas S, Linseisen J, Slanger T et al, “Serum 25-hydroxyvitamin D and risk of post-menopausal breast cancer – results of a large case-control study,” Carcinogenesis 2008, Vol. 29 No. 1, page 93.

Another Sun Exposure and Breast Cancer Study
Researchers at the University of California in San Diego compared exposure to sunlight with studies on breast cancer risk in 107 countries. After accounting for known breast cancer risk factors, they found that women who got more sun had less breast cancer.

Mohr SB, Garland CF, Gorham ED et al, “Relationship between low ultraviolet B irradiance and higher breast cancer risk in 107 countries,” Breast J 2008 May-Jun;14(3):255-60.