A new systematic review and meta-analysis has found a clear link between vitamin D deficiency and poor prognosis in breast cancer.
The research team, led by Dr. April Rose of McGill University, were the first to study the association of vitamin D status at diagnosis and risk of breast cancer recurrence and death from breast cancer years later. In 2009, their group looked at a cohort of Canadian women with early stage breast cancer and found that vitamin D deficiency increased the risk of cancer recurrence and death.
Since then, several more research groups have examined this association. So Dr. Rose and colleagues wanted to see, what does all the research say combined?
The researchers searched three databases for studies, including MEDLINE, Abstracts of the American Society of Clinical Oncology and the San Antonio Breast Cancer Symposium.
They looked for studies that drew vitamin D levels at time of non-metastatic breast cancer diagnosis (immediately or within 6 months) and then measured the clinical outcomes of recurrence, distant recurrence, death and breast cancer-specific death. The researchers only selected studies that reported survival or recurrence data and reported hazard ratios.
They found 8 studies in all that met this criteria that included 5,691 patients, 6 studies of which reported recurrence data and 6 studies of which reported data on death (with some of the studies obviously reporting on both).
Here is what the researchers found:
- All 6 recurrence studies found that vitamin D deficiency was associated with increased risk of breast cancer recurrence. Three of the studies looked at recurrence, while the other three looked at distant recurrence.
- When pooled together, there was a 2.13 times increased associated risk of having a recurrence of breast cancer if you were vitamin D deficient compared to sufficient (HR=2.13, 1.64-2.78).
- All 6 death studies found that vitamin D deficiency was associated with increased risk of death. Five of the studies looked at breast cancer-specific death. One looked at death overall.
- When pooled together, there was a 1.76 times increased associated risk of death if you were vitamin D deficient compared to sufficient (HR=1.76, 1.35-2.30).
The researchers concluded,
“We have identified statistically significant associations of low levels of vitamin D, measured around the time of breast cancer diagnosis, with an increased risk of recurrence and death in early stage breast cancer patients.”
The researchers noted, however, that since all the studies in the meta-analysis were observational, a causal connection between vitamin D deficiency and risk of recurrence and death could not be inferred. For instance, low vitamin D levels might be a marker for poor overall health.
The next step is to study if vitamin D supplements help in breast cancer for these very outcomes in the form of a randomized controlled trial. The researchers noted that setting up this kind of trial has been difficult and unfeasible. The reason being, many women diagnosed with breast cancer begin taking vitamin D supplements, and they have shown an unwillingness to be randomized at risk of having to take placebo. Dr David Feldman of Stanford University has reported similar barriers, including in this interview with Carole Baggerly.