In 2008, breast cancer caused more than 458,000 deaths worldwide (13.7% of cancer deaths in women). Survival rates in the Western world are high; for example, 84% of women in England diagnosed with breast cancer survive for at least 5 years. In developing countries, survival rates are much poorer although the incidence rates in many developing countries are much lower than in the US.
The USA has the highest incidence of breast cancer in the world. In twelve world regions, the incidence rates per 100,000 women are as follows: in Eastern Asia, 18; South Central Asia, 22; sub-Saharan Africa, 22; South-Eastern Asia, 26; North Africa and Western Asia, 28; South and Central America, 42; Eastern Europe, 49; Southern Europe, 56; Northern Europe, 73; Oceania, 74; Western Europe, 78; and in North America, 90. As you see, the rates in The US are more than four times higher than in other areas of the world, although one wonders about proper reporting from the developing world.
Sun exposure lessens a woman’s risk of developing breast cancer, but that may be due to non-vitamin D benefits of sunlight, such as immune modification, melatonin secretion, or yet undiscovered beneficial UV induced compounds in the skin.
Recently researchers at Harvard led by Scott Bauer working under the supervision of Professor Eric Ding found in their study that higher vitamin D levels have no protective effect in premenopausal breast cancer, but a moderate protective effect in postmenopausal breast cancer. This was the first metanalysis of vitamin D and breast cancer that was stratified by menopause.
Bauer SR, Hankinson SE, Bertone-Johnson ER, Ding EL. Plasma vitamin D levels, menopause, and risk of breast cancer: dose-response meta-analysis of prospective studies. Medicine (Baltimore). 2013 May;92(3):123-31.
They did a metanalysis of nine prospective studies that included 5,200 cases. They found that at or above 25(OH)D levels of 27 ng/ml ,there is a 12% reduction in breast cancer for every 5 ng/ml increase in 25(OH)D levels in postmenopausal women only. The cutoff point where the effect flattened out was 35 ng/ml. The relative risk for breast cancer was about 0.7 for women with 25(OH)D levels greater than 35 ng/ml, compared to women with levels of 27 ng/ml or lower. They had no data on women with levels greater than 42 ng/ml, but the curve was flat from 37 to 42 ng/ml.
Based on this meta-analysis, it appears the preventative effect of vitamin D is modest, but real in postmenopausal breast cancer. Few easily modifiable risk factors exist for breast cancer, so it’s important to recognize that better sun exposure and vitamin D supplementation habits may have promise.
To clarify the extent of benefit of vitamin D in preventing breast cancer (if any benefit at all), there is a trial underway called the VITAL study at Harvard, which is studying cancer rates in people supplanted with an extra 2,000 IU/day of D3. However, their results are not due until 2017.