Oncology | Published in Nutrition and Cancer, 2025 | Study from São Paulo State University, Brazil
Breast cancer is the most commonly diagnosed cancer in the world. One in eight women will develop it in their lifetime. Every year, around 2.3 million new cases are diagnosed globally, and despite decades of progress in treatment, it still kills more than 670,000 people annually.
When a woman is diagnosed with breast cancer and her tumor is large enough that surgery cannot happen immediately, doctors often use chemotherapy first. This is called neoadjuvant chemotherapy: treatment given before the operating table, designed to shrink the tumor, make it easier to remove, and ideally, destroy as many cancer cells as possible before the surgeon goes in.
The most important word in that last sentence is “ideally.”
In the best possible outcome, the chemotherapy kills every last cancer cell, and when the surgeon operates and sends the tissue to a pathologist for examination, the pathologist finds nothing. No residual tumor. No cancer cells hiding in the surrounding tissue or lymph nodes. This outcome has a name in oncology: pathological complete response, or pCR.
Patients who achieve pCR have significantly better long-term outcomes than those who do not.
The problem is that not everyone achieves it. Across different breast cancer subtypes and different chemotherapy regimens, pCR rates typically range from around 20% to 60%, depending heavily on the biology of the tumor.
That means a substantial proportion of women undergo months of difficult treatment and still have cancer cells remaining when surgery happens.
Researchers have been searching for ways to push more patients into pCR for years. Most of the drugs developed for this purpose are expensive, complex, and inaccessible to patients in lower-income health systems.
A new study from Brazil suggests that part of the answer might be sitting in the supplement aisle of any pharmacy, for a few dollars a month.
The Study
Researchers at the Botucatu School of Medicine at São Paulo State University (FMB-UNESP), funded by FAPESP, conducted a randomized controlled trial involving 80 women over the age of 45 who were about to begin neoadjuvant chemotherapy for breast cancer.
The women were divided into two equal groups of 40. One group received a daily supplement of 2,000 IU of vitamin D3 (cholecalciferol) throughout their chemotherapy treatment. The other group received placebo tablets that looked identical. Neither group knew which they were taking. All 80 women received the same standard chemotherapy protocol.
After six months, the researchers examined the surgical specimens and assessed pCR rates between the two groups.
43% of women taking vitamin D achieved complete response vs 24% on placebo.
That is a difference of 79% in relative terms between the two groups.
“Even with a small sample of participants, it was possible to observe a significant difference,” said Eduardo Carvalho-Pessoa, one of the study authors.
Before You Run to the Pharmacy
A finding like this demands careful reading.
The study enrolled 80 women. That is a small trial by clinical standards. Larger trials typically involve hundreds or thousands of participants.
The 79% figure refers to relative difference. The absolute difference is 19 percentage points.
This is an early signal, not a final verdict.
The researchers themselves emphasize that larger studies are needed.
More Evidence Exists
A larger randomized trial in Turkey (227 patients) found similar results.
Patients taking vitamin D were more than twice as likely to achieve complete response.
Older studies also show vitamin D deficiency is linked to poorer outcomes.
Multiple independent studies point in the same direction.
What Vitamin D Actually Does
Vitamin D is not just a vitamin. It behaves like a hormone.
Its active form, calcitriol, binds to the vitamin D receptor (VDR) and directly regulates gene expression.
This affects cell growth, apoptosis, immune response, inflammation, and angiogenesis.
Vitamin D can slow tumor growth and make cancer cells more sensitive to chemotherapy.
Why Deficiency Is Common
Vitamin D deficiency affects over a billion people worldwide.
Cancer patients are especially likely to be deficient due to reduced sunlight exposure and metabolic changes.
The Accessibility Question
Many cancer drugs are expensive and inaccessible globally.
Vitamin D is cheap, widely available, and easy to use.
If validated, this could be one of the most accessible ways to improve cancer outcomes worldwide.
Final Thought
Medicine often looks for complex solutions. But sometimes, the answer might be simple.
Vitamin D is not a cure, but it may help chemotherapy work better — and that possibility deserves attention.
Primary source:
Nutrition and Cancer (2025)
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