Can vitamin D enhance breast cancer response to chemotherapy?

A healthcare professional assists a patient undergoing a mammogram, highlighting research that shows combining vitamin D with chemotherapy enhances tumor response. Image credit – National Cancer Institute

CANCER DIGEST – May 16, 2026 – A small Brazilian study has found that low doses of vitamin D can enhance the effectiveness of chemotherapy given to women being treated for breast cancer. The study suggests the need for further investigations in larger groups of women being treated for the disease.

The study led by graduate student Michelle Sako Omodei, was conducted at the Botucatu School of Medicine at Sao Paulo State University in Brazil, and involved 80 women 45 and older being treated for breast cancer. The women were randomly assigned to either receive 2,000 IU (international units) of vitamin D for six months while undergoing chemotherapy before surgery, or a placebo.

After six months, 43 percent of the women receiving the vitamin D with chemotherapy saw their tumors disappear, compared to 24 percent of the women receiving the placebo with chemotherapy. The results were published in the the March 17, 2025 journal Nutrition and Cancer. 

“Even with a small sample of participants, it was possible to observe a significant difference in the response to chemotherapy," co-author Eduardo Carvalho-Pessoa, president of the São Paulo Regional Brazilian Society of Mastology said in a press release. "In addition, the dosage used in the research [2,000 IU per day] is far below the target dose for correcting vitamin D deficiency, which is usually 50,000 IU per week.” 

Vitamin D is known to help the body absorb calcium and phosphorus needed to maintain strong bones, however evidence of its role in boosting the body’s immune function is growing. The body produces vitamin D primarily from sunlight and from certain foods. Current guidelines recommend 600 IU per day for most adults and 800 IU for older adults. Excessive consumption, however can be harmful leading to nausea, weakness, bone pain, and kidney stones.

The researchers say the study points to a possible support role for vitamin D in breast cancer therapy, but much more data in larger studies are needed to confirm this study’s results.

Sources: Agencia FAPESP press release and the journal Nutrition and Cancer