Vitamin D and breast cancer: Is there a link?

Alison Conlin, M.D.
Medical oncologist, Providence Cancer Center
Providence Oncology and Hematology Care Clinic-Westside

Published October 2010

The potential link between vitamin D deficiency and cancer has certainly become a hot topic, especially in the gray and rainy Pacific Northwest.

I had the opportunity to become an “expert” on the subject by researching all the literature on the potential connection between vitamin D deficiency and breast cancer risk for a public talk at the Breast Cancer Issues Conference held by Susan G. Komen for the Cure, Oregon and SW Washington. 

Here is what I learned

The current evidence linking vitamin D deficiency and breast cancer risk is rather inconsistent. What is clearer is that vitamin D deficiency has a definite risk in osteoporosis and in rickets, a bone disease first described in the 1600s. It is characterized by skeletal deformities primarily seen in children.

There also have been relationships proposed between vitamin D deficiency and cardiovascular disease, hypertension, infection, chronic pain, dementia and diabetes. It turns out it is not good to be deficient in vitamin D.

One meta-analysis of nine studies concluded that people who took vitamin D3 supplements had a lower all-cause mortality than those who did not. In other studies it appears that 70 to 80 percent of all hospitalized patients are vitamin D deficient, and potentially 77 percent of all Americans may be deficient (defined as a 25 [OH] D level of 30 ng/ml).

The first suggestions of a potential connection between vitamin D deficiency and breast cancer risk came from observational studies. These studies found that there were higher breast cancer rates in northern latitudes than in southern latitudes.

Indeed, Oregon consistently ranks high for incidence of new breast cancers in the United States. Further, a study reported that women in northern Norway had poorer outcomes when diagnosed with breast cancer than women diagnosed in southern Norway, where there is more sun exposure and potentially higher vitamin D levels.

Other studies have found vitamin D levels to be significantly lower in women diagnosed with locally advanced or metastatic breast cancer than in women with early-stage disease.

The majority of breast cancer patients are vitamin D-deficient. Studies in both northern and southern climates (Toronto and Los Angeles, for example) have found that approximately 75 percent of women diagnosed with breast cancer have deficient levels of vitamin D, and that 42 percent of black women with breast cancer were seriously deficient (defined as <15 ng/ml).

What about other factors?

All of these associations call into question: Is it really the vitamin D or some confounders that are also linked to breast cancer risk? For instance, obesity, an established risk factor for breast cancer, also may be the cause for lower vitamin D. It is known that higher BMI is associated with lower vitamin D levels, and given it is a fat-soluble vitamin, this is plausible.

In addition, we know that physical activity and outdoor exercise raise serum vitamin D levels and potentially protect against breast cancer. The relationship between vitamin D deficiency and cancer is quite biologically possible given that vitamin D is converted to its active form in breast tissue (as well as the kidney, colon, etc.). And vitamin D receptors are nuclear transcription factors for more than 200 genes responsible for proliferation, differentiation, apoptosis and angiogenesis.

Two studies have been conducted addressing the hypothesis: Can you reduce cancers if you supplement vitamin D? The studies disagree on the answer. In the Women’s Health Initiative study, more than 36,000 women were given 400 IU of vitamin D (with 1000 mg calcium), and no difference was noted in breast cancer incidence. This dosage, however, has been criticized for being too low for a notable effect.

In fact a smaller study in Nebraska of 1,179 postmenopausal women who took either placebo, calcium alone (1400 to 1500 mg daily) or calcium with vitamin D (1100 IU daily) found fewer cancers, particularly breast cancer, in the calcium and vitamin D group. The study also found fewer cancers in the calcium-alone group, and the numbers were quite small (36 cases of breast cancer total in all three groups).

Therefore, more work is needed to determine if it is a vitamin D deficiency or other factors that contribute to cancer risk. Current recommendations for vitamin D supplementation are 200, 400 and 600 IU of vitamin D3 daily for people younger than 50, 50 to 70, and older than 70, respectively.

Most experts, however, agree this is too low, especially in a deficient patient. To confuse things even more, the “ideal” level is not truly known and one dose may not fit all.

Clinical articles by Ali Conlin, M.D.