The research, funded by FAPESP, included 80 women over the age of 45 who were preparing to begin treatment at the oncology outpatient clinic of the general and teaching hospital ("Hospital das Clínicas") at FMB-UNESP. Participants were split into two equal groups. One group received a daily dose of 2,000 IU (international units) of vitamin D, while the other group was given placebo tablets.

All of the women underwent neoadjuvant chemotherapy, a treatment given before surgery to shrink tumors and make them easier to remove.

Higher Rates of Cancer Disappearance

After six months, the difference between the two groups was notable. Among those taking vitamin D, 43% experienced complete disappearance of their cancer following chemotherapy. In comparison, only 24% of those in the placebo group saw the same result.

"Even with a small sample of participants, it was possible to observe a significant difference in the response to chemotherapy. 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," says Eduardo Carvalho-Pessoa, president of the São Paulo Regional Brazilian Society of Mastology and one of the study's authors, published in the journal Nutrition and Cancer.

Vitamin D's Role in Immunity and Health

Vitamin D is best known for helping the body absorb calcium and phosphorus, which are critical for maintaining strong bones. However, growing evidence shows it also plays a role in immune function, helping the body defend against infections and diseases, including cancer. Many previous studies examining vitamin D and cancer have focused on much higher doses than those used in this research.

The body produces vitamin D mainly through sunlight exposure, and it can also be obtained through certain foods. Current guidelines recommend 600 IU per day for most adults and 800 IU for older individuals. The American Academy of Pediatrics advises 400 IU daily for infants. Excessive intake can be harmful and may lead to symptoms such as vomiting, weakness, bone pain, and kidney stones.

Low Vitamin D Levels and Treatment Response

At the start of the study, most participants had low vitamin D levels, defined as less than 20 nanograms per milliliter (ng/mL) of blood. The Brazilian Society of Rheumatology recommends maintaining levels between 40 and 70 ng/mL.

"With supplementation, levels increased throughout chemotherapy treatment, which reinforces a possible contribution to the patients' recovery," Carvalho-Pessoa told Agência FAPESP. "Vitamin D is an accessible and inexpensive option compared to other drugs used to improve the response to chemotherapy, some of which are not even included in the list of the Unified Health System [the Brazilian national public health network, known as the SUS, its acronym in Portuguese]," he adds.

Promising Results, More Research Needed

The findings point to a possible supportive role for vitamin D in cancer treatment, but researchers caution that more data is needed. Larger studies will help confirm how effective the supplement is and clarify how it influences chemotherapy response.

"These are encouraging results that justify a new round of studies with a larger number of participants. This will allow a greater understanding of the role of vitamin D in increasing the response to chemotherapy treatment and, consequently, in the greater likelihood of breast cancer remission," he concludes.