A new study suggests that vitamin D supplements may help improve the effectiveness of breast cancer treatment.

Researchers in Brazil found that taking a low-dose vitamin D supplement during neoadjuvant chemotherapy — a treatment given before surgery to shrink tumors — may enhance treatment response in women with breast cancer.

In the randomized study of 80 women ages 45 and older, 43% of those who took 2,000 IU of vitamin D daily experienced a complete disappearance of invasive cancer in tissue samples after six months of chemotherapy. By comparison, 24% of women who received a placebo saw the same result.

According to ScienceDaily, researchers from the São Paulo Research Foundation (FAPESP) described the difference as meaningful, even in a relatively small study. They also noted that the vitamin D dose used was modest compared with higher doses sometimes prescribed to correct deficiency.

Because vitamin D is inexpensive and deficiency is common, the researchers suggest that improving vitamin D levels could become a practical add-on to standard treatment. However, they emphasize that larger studies are needed before this approach becomes routine.

Vitamin D is best known for supporting bone health, but it also plays a role in immune function and cell signaling — processes that may influence how tumors respond to treatment. Researchers say supplementation during chemotherapy may work primarily by correcting low vitamin D levels, rather than acting as a direct cancer treatment.

The findings need to be confirmed in larger and more diverse groups of patients.

Experts caution that patients should consult their oncology team before considering starting high-dose vitamin D supplements during chemotherapy.