A new clinical study is adding a surprising ally to the fight against breast cancer: vitamin D. Researchers found that women who took a relatively modest daily supplement of vitamin D while undergoing chemotherapy were nearly twice as likely to experience complete tumor elimination compared to those who did not, a finding that is turning heads in the oncology community.

The Study at a Glance

The Brazilian randomized clinical trial, published in March 2025 in the journal Nutrition and Cancer, enrolled 80 women over the age of 45 who had breast cancer and were eligible for neoadjuvant chemotherapy, the type of chemotherapy used to shrink tumors before surgery. The participants were divided into two groups: one received 2,000 IU of vitamin D daily alongside their standard treatment, and the other received a placebo.

After six months, the results were striking. In the vitamin D group, 43% of women achieved a pathological complete response (pCR), meaning no cancer cells were detectable in tissue samples after treatment. In the placebo group, only 24% reached the same outcome. That is nearly double the success rate, from a supplement widely available and generally safe at low doses.

These pCR rates are considered especially impressive given that nearly two-thirds of the participants had hormone-receptor positive (luminal) cancers, which typically show pCR rates of only around 15% with standard chemotherapy alone.

How Might Vitamin D Help?

Scientists believe vitamin D acts on multiple fronts in the tumor environment. It may help regulate how cancer cells grow their own blood supply, influence the sensitivity of cancer cells to chemotherapy drugs, and even reduce the ability of cancer cells to spread to other parts of the body. At the cellular level, vitamin D can inhibit tumor cell proliferation by regulating cell cycle genes and triggering programmed cell death (apoptosis).

This is not the first study to connect vitamin D to improved breast cancer outcomes. A 2024 review published in Nutrients found that serum vitamin D levels between 26 and 54 ng/mL appeared to exert a protective effect against breast cancer. Crucially, the women in the new trial achieved a mean serum level of 28 ng/mL, right in that beneficial range, from just 2,000 IU per day.

What This Means for Patients

Vitamin D deficiency is common globally, and low levels at the time of breast cancer diagnosis have been associated with more aggressive disease progression. While the results of this trial are promising, experts note that larger studies will be needed to confirm reproducibility across different populations and cancer subtypes. As always, patients should consult their oncologist before adding any supplement to their treatment plan.

Still, the potential upside is hard to ignore: a safe, inexpensive supplement that may meaningfully improve the odds of eliminating a tumor before surgery. Further research into vitamin D as an adjunct to standard cancer therapy looks more warranted than ever.

Journal article:

Omodei MS, Chimicoviaki J, Buttros DAB, Almeida-Filho BS, Carvalho-Pessoa CP, Carvalho-Pessoa E, Vespoli HDL, Nahas EAP. “Vitamin D Supplementation Improves Pathological Complete Response in Breast Cancer Patients Undergoing Neoadjuvant Chemotherapy: A Randomized Clinical Trial.” Nutrition and Cancer. Published 17 March 2025. DOI: 10.1080/01635581.2025.2480854. Funded by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).