In the most recent, largest study (meta-analysis) of vitamin D, calcium, and the prevention of breast cancer, both vitamin D and calcium seemed to be protective for the development of breast cancer. The best results were among women with the highest intakes of vitamin D and calcium as compared to the lowest levels of consumption. The top quarter of women having the highest blood 25(OH)D levels had a 45% decreased risk of breast cancer.
Another study of 562 women found that the 142 women with a diagnosed breast cancer had, on average, lower blood levels of 25(OH)D. Women with the highest levels of 25(OH)D had a significantly reduced risk for breast cancer. Surprisingly, use of vitamin D supplements, sunbathing, and fish intake did increase blood levels of 25(OH)D -- but the lifestyle factors did not directly impact the risk of breast cancer.
Conversely, McCullough and colleagues, studying almost 22,000 women, found no impact of blood levels of 25(OH)D on the risk of breast cancer. A study of almost 42,000 Swedish women did not identify linkages between breast cancer risk and sun exposure, nor vitamin D intake through diet or multivitamin use.
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