Vitamin D Supplements May Boost Breast Cancer Treatment Success

Vitamin D has been credited with a variety of important health benefits, from strengthening bones and boosting immunity to potentially counteracting the development of depression and heart disease. Now, a new Brazilian study published in the journal Nutrition and Cancer suggests that supplementing with relatively low doses of vitamin D may improve response to chemotherapy for breast cancer.

So, how does vitamin D seem to help in the battle to fight breast cancer, and who might best benefit from taking supplements of the “sunshine vitamin”?

In the study, 80 women over age 45 with breast cancer who were eligible for neoadjuvant chemotherapy (the type of chemotherapy that helps shrink tumors so they are easier to be surgically removed) were split into two groups. One group received 2,000 IU of vitamin D daily, while the other group received a placebo. Ultimately, 75 women followed this regimen for six months while undergoing treatment. After six months, the researchers found that 43% of the women in the vitamin D group saw their tumors disappear, while about half as many (24% of the women) in the placebo group saw the same result.

In cancer treatment terms, doctors call this elimination of cancer in tissue samples that have been removed during surgery or biopsy after treatment the pathological complete response (pCR) rate. “These pCR rates are relatively high, especially considering that 49 of the 75 patients [who completed the study] had luminal [hormone-receptor positive] cancers, which traditionally have low pCR rates [about 15%] following standard neoadjuvant chemotherapy,” says Jason Mouabbi, M.D., an assistant professor of breast medical oncology in the Division of Cancer Medicine at The University of Texas MD Anderson Cancer Center in Houston, who was not associated with the study. “The increased pCR in the vitamin D arm is particularly striking but also raises questions about biological plausibility and reproducibility.”

While this study is not the first to link vitamin D to potential benefits during treatment for breast cancer, it does give more credence to its use as an adjunctive therapy. “There have been many investigations into vitamin D and its role in cancer,” notes oncologist Daniel Landau, M.D., the medical director of virtual hematology at the Medical University of South Carolina in Charleston, who was also not associated with the study.

Still, the role D seems to play isn’t certain. “While exact answers remain elusive, it is felt that vitamin D helps to regulate the tumor microenvironment, or the way the cancer cell grows its own blood supply,” explains Dr. Landau, who is an expert contributor with The Mesothelioma Center at Asbestos.com, an advocacy and support organization for people with mesothelioma. It may affect the sensitivity to chemotherapy, he adds.

In addition, vitamin D may affect the ability for cancer cells to travel to other areas of the body. “Vitamin D can inhibit tumor cell proliferation by regulating cell cycle genes and inducing cell death [apoptosis],” adds Dr. Mouabbi.

The impact of vitamin D can be felt both in its presence and absence: While adequate levels of vitamin D appear to be beneficial in prevention, management, and treatment of breast cancer, according to a review published in the Journal of International Medical Research, a deficiency in the vitamin is also believed to be a risk factor for developing breast cancer in the first place.

Having low levels of vitamin D is not a new problem for people undergoing cancer treatment, notes Dr. Mouabbi. “Past studies have shown that vitamin D deficiency is associated with lower pCR rates, suggesting that the benefit may be limited to deficient individuals,” he says.

While this study is indeed promising and offers a simple strategy—supplementing with low-dose vitamin D—that may improve your response to breast cancer treatment, it has two limitations that are worth mentioning. The first, as was true with previous studies on the topic, is that all the women in the study were deficient in vitamin D at the beginning of the trial. “Correcting a deficiency may optimize immune and cellular function, but further supplementation beyond sufficient levels is unlikely to add additional benefit,” Dr. Mouabbi says. In other words, if you’re already getting enough vitamin D, there’s not much additional benefit you can get from taking more.

The other has to do with the types of tumors that the people in the study had. Dr. Mouabbi explains that in general, different types of tumors have different pCR rates and different prognoses. “This study included a large proportion of luminal (HR+) tumors, which are known to have low pCR rates but favorable long-term survival,” he says. So while taking vitamin D did increase the pCR rate for many women with these tumors, which is certainly a positive outcome, Dr. Mouabbi suggests that pCR rate may not be the best term of measurement to define “successful treatment” for this type of breast cancer.

Physicians and researchers have long suspected that vitamin D could play a role in fighting cancer. This study, despite its limitations, confirms that ensuring you are getting adequate amounts of vitamin D can make a difference. “As oncologists, we will often check vitamin D levels and recommend replacement if we see low levels,” Dr. Landau says.

The women in this study took 2,000 IU of vitamin D daily to achieve a mean serum vitamin D level (or how much is available for use by the body) of 28 ng/mL. This aligns with the results published in a 2024 review in Nutrients, which found that serum levels of vitamin D between 26 ng/mL and 54 ng/mL seemed to exert a protective effect against breast cancer. If you think vitamin D could help you or a loved one who is undergoing breast cancer treatment, talk to your doctor or healthcare provider about it today.