Research from BMC Cancer found that higher vitamin D levels were associated with a decreased risk of postmenopausal breast cancer in white women, but not in any other ethnic groups surveyed, who reside in low latitudes.
Worldwide, breast cancer is the most common invasive cancer in women, comprising 16% of all female cancers. It is strongly correlated with age and 95% of all breast cancers occurs in women over the age of 40 years old.
Epidemiological studies on the association between vitamin D and various health outcomes are complex because vitamin D levels are determined by a myriad of factors, including sun exposure, skin color, and latitude of residence.
That being said, multiple epidemiological studies have shown a link between vitamin D levels and risk of developing breast cancer. Ecological studies, specifically, have reported a correlation between lower UVB exposure and higher incidences of breast cancer.
However, results from some cohort studies and intervention trials, which are of higher quality than most epidemiological studies, have been null and inconsistent.
Most studies examining the relationship between vitamin D and breast cancer have been conducted in white females. Yet, vitamin D levels have been known to differ substantially between different race and ethnic groups.
A research team led by Dr. Yeonju Kim recently conducted a nested case-control study within the Multiethnic Cohort (MEC) in Hawaii at a latitude of 21° N and Los Angeles at a latitude of 34° N on the relationship between vitamin D levels before diagnosis and risk of breast cancer and how this relationship differs between different race and ethnic groups living in areas with relatively high year-round levels of sunlight.
The MEC is a prospective study established between 1993 and 1996 of more than 215,000 adults from five race/ethnic groups (Euro-American, African American, Native Hawaiian, Japanese, Latin American) in Hawaii and Los Angeles to examine the association of lifestyle and genes with chronic disease risk.
Between 2001 and 2006, 35,458 postmenopausal women between the ages of 45 and 75 year had their blood tested and vitamin D levels measured. From this cohort, the researchers matched 707 women who developed breast cancer with 707 healthy controls. The participants were matched according to location (Hawaii or California), race/ethnicity, age, date of blood draw, hormone replacement therapy use, and even time of blood draw.
After running their statistical analyses, the research team found the following results:
- Mean total vitamin D level was highest among whites (36.15 ng/ml) and lowest among African Americans (26.05 ng/ml). Native Hawaiians, Japanese, and Latinos had average total vitamin D levels of 33 ng/ml, 32.65 ng/ml, and 27.4 ng/ml, respectively.
- White participants in group with breast cancer with a total level below 20 ng/ml had a 7.5 times increased risk of breast cancer compared to white participants with a total level above 20 ng/ml.
- Both higher total vitamin D and vitamin D3 levels were associated with a decreased risk of breast cancer among white women.
- For participants in the other ethnic groups, the association of total vitamin D and vitamin D3 levels with breast cancer was insignificant.
The researchers summarized the results by saying,
“In conclusion, plasma vitamin D was inversely associated with postmenopausal breast cancer risk among white women who reside in latitudes where levels of ultraviolet radiation are comparatively high. It is likely that a minimum threshold of vitamin D exposure from both sun and diet is required to achieve a reduction in breast cancer risk among postmenopausal white women.”
They also touched upon the subject of genetics, as genes may also explain the puzzling results.
These results are very interesting and add a new layer of complexity to the vitamin D and breast cancer investigation. As the researchers mention, the fact that an association between vitamin D3 and total vitamin D levels was only seen in whites, who had the highest levels of any ethnicity, suggests that a threshold exists for a protective effect of vitamin D on breast cancer development.
Furthermore, this threshold may be relatively high, as no group had an average vitamin D level below 27.4 ng/ml. In this study, it is hard to attribute the lack of association observed in the other ethnic groups to differences in skin type, latitudinal location, or vitamin D production.
This study highlights the need for more research on the relationship between breast cancer, sun exposure, and vitamin D across different ethnic groups. Yet, it supports previous research suggesting a high vitamin D cutoff of 40 ng/ml needed for breast cancer protection.