by Craig Weatherby
Many studies have linked vitamin D levels to a reduced risk of certain major malignancies, including breast cancer.
But we lack the clinical evidence to determine whether vitamin D definitely prevents cancer.
Assuming that vitamin D really does possess real cancer-curbing power, it's been unclear whether the source—diet or sunlight—matters to the vitamin's anti-cancer efficacy.
According to new findings, getting vitamin D from a combination of sun and diet may yield the best results.
Evidence review urges research into vitamin D's
The link between higher vitamin D levels and lower cancer rates dates from the 1940s, when Frank Apperly, Ph.D., discovered that people living in lower latitudes (i.e., closer to the equator) enjoyed lower rates of cancer death.
His findings prompted follow-up studies, most of which have found associations between higher vitamin D levels and lower risks of major cancers.
And two large epidemiological studies that compared dairy intake and breast cancer detected a possible risk reduction, which is presumed but not proven to be related to the calcium and/or vitamin D in milk products (Shin MH et al. 2002; McCullough ML et al. 2005).
However, the results of an evidence review published in August of 2010 characterized the evidence showing beneficial effects of vitamin D in breast cancer as “ambiguous”.
The authors analyzed the combined results of 10 epidemiological studies that looked for associations between women's vitamin D levels and their risk of breast cancer.
While the studies' combined data generally linked higher vitamin D levels with a lower risk, these results fell short of statistical significance, leaving the protective value of vitamin D in doubt.
And the authors of a large Norwegian epidemiological study published this year found no significant associations between vitamin D intake or sun exposure and breast cancer risk in post-menopausal women, concluding, “Our results do not support an association between vitamin D status and breast cancer risk” (Edvardsen K et al. 2010).
The European team behind the meta-analysis called for more large population studies, and controlled clinical trials, to test the small positive indications detected by their review.
Study finds sun plus a D-rich diet better for breast health, versus sun only
French researchers may have uncovered one reason for the mixed findings from the 10 epidemiological studies included in the meta-analysis.
European investigators—led by Dr. Pierre Engel from INSERM, France's counterpart to the U.S. National Institutes of Health—examined data from a 10-year-long study involving 67,721 post-menopausal women living in France (Engel P et al. 2010).
And their analysis revealed what may be a critical distinction in determining the efficacy of vitamin D in supporting breast immunity to cancer.
They compared breast cancer rates among women who lived in the sunniest places in France—below latitude 46°N, such as Provence—to the rates seen among women from higher, less sunny latitudes, such as Paris.
Compared with sun-deprived women, women from lower, sunnier latitudes had lower rates of breast cancer.
And, the women from sunny places who consumed the most dietary vitamin D (from foods and supplements) enjoyed a greater breast-risk reduction than the women who consumed less dietary vitamin D.
To be specific:
Among women who lived in sunnier latitudes, those with the highest vitamin D intakes enjoyed a 45 percent lower risk of breast cancer, compared to women from sun-deprived places.
Among women who lived in sunnier latitudes, those with the lowest vitamin D intakes had a 32 percent lower risk of breast cancer, compared to women from sun-deprived places.
In other words, a combination of ample sun exposure and ample vitamin D intake was associated with the greatest risk reduction, compared with getting vitamin D from either sunlight or diet alone.
To be clear, higher intakes of dietary vitamin D alone were not associated with reduced breast risks.
The French team articulated their core finding:
“Our results suggest that a [minimum] threshold of vitamin D [obtained] from both sun and diet is required to prevent BC [breast cancer] and this threshold is particularly difficult to reach in postmenopausal women at northern latitudes where quality of sunlight is too poor for adequate vitamin D production” (Engel P et al. 2010).
And they made a cogent observation:
“…[the] minimal [dietary intake of vitamin D [needed] to reduce breast cancer risk] is likely to vary with individual ability to metabolize or synthesize vitamin D from both sources [sun exposure and diet]” (Engel P et al. 2010).
The also noted that the average French woman—like the average American woman—has relative low blood levels of vitamin D and gets relatively little sun, and suggested that “…an increase in overall vitamin D intake should be encouraged by food and health agencies” (Engel P et al. 2010).
Fortunately—and although the action fell short of experts' recommendations—the Food & Nutrition Board of the U.S. Institute of Medicine just tripled the recommended daily allowances for vitamin D, from 200IU for adults to 600IU.
For more on that—and the broader subject of adequate vitamin D intake—see “Vitamin D RDAs Raised Substantially” and “Dr. Weil's Take on the New Vitamin D RDAs” and the Vitamin D Sources & Safety section of our news archive.
Edvardsen K, Veierød MB, Brustad M, Braaten T, Engelsen O, Lund E. Vitamin D-effective solar UV radiation, dietary vitamin D and breast cancer risk. Int J Cancer. 2010 May 13. [Epub ahead of print]
Engel P, Fagherazzi G, Mesrine S, Boutron-Ruault MC, Clavel-Chapelon F. Joint effects of dietary vitamin D and sun exposure on breast cancer risk: results from the French E3N cohort. Cancer Epidemiol Biomarkers Prev. 2010 Dec 2. [Epub ahead of print]
McCullough ML, Rodriguez C, Diver WR, Feigelson HS, Stevens VL, Thun MJ, Calle EE. Dairy, calcium, and vitamin D intake and postmenopausal breast cancer risk in the Cancer Prevention Study II Nutrition Cohort. Cancer Epidemiol Biomarkers Prev. 2005 Dec;14(12):2898-904.
Shin MH, Holmes MD, Hankinson SE, Wu K, Colditz GA, Willett WC. Intake of dairy products, calcium, and vitamin d and risk of breast cancer. J Natl Cancer Inst. 2002 Sep 4;94(17):1301-11.