Clinically sufficient vitamin D levels at breast cancer diagnosis and survival outcomes

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Published: 10 Jun 2021
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Prof Song Yao - Roswell Park Comprehensive Cancer Center, Buffalo, USA

Prof Song Yao speaks to ecancer about the relationship between Vitamin D levels and breast cancer survival outcomes as observed from the pathways study.

This study suggested better cancer survival with vitamin D supplementation. It investigated survival outcomes in a prospective cohort of 3,995 patients after a median follow-up of 10 years.

Prof Yao discusses the methodology and key findings of this study. In the end, he talks about the impact of the study and mentions that this study proves that Vitamin D levels should be maintained in patients for a better outcome.

In this study we looked at the vitamin D levels measured at the time of patients who were just diagnosed with breast cancer and we tried to understand how the vitamin D levels are related with breast cancer survival outcomes. We had a previous study that was published back in 2017 where we showed in a smaller patient population that lower vitamin D levels at time of breast cancer diagnosis were associated with a poor patient survival. So now with this larger study, more than 4,000 patients, and after a median follow-up time of almost three years we replicated our previous findings, that the lower vitamin D levels at cancer diagnosis were indeed associated with poorer patient survival.

We measured a blood-bound marker called 25-hydroxyitamin D which is an indicator of the sufficiency of vitamin D levels in our body. We did this in over 4,000 patients, breast cancer patients who were enrolled into a study called the Pathways Study. This is a study established at the Kaiser Permanente Northern California lab by Dr Larry Kushi and Dr Christine Ambrosone here at Roswell Park. We did a number of statistical analyses trying to understand how vitamin D levels measured at this time were associated with projected survival outcomes. We looked at several patient outcomes including overall survival, breast cancer specific survival. We also tried to control for a number of factors that may affect vitamin D levels because of confounding factors. Because vitamin D levels can be impacted by a number of factors such as the body mass index which measures the obesity and it can be impacted by the season of blood collection because in summertime under sun exposure vitamin D levels tend to be higher. So we tried to control all those factors and after we had done those careful statistical analyses we found that the association between lower vitamin D levels and poorer patient outcomes remained.

We also further stratified our analysis to look at several patient subgroups and we found that the association was stronger in patients with more advanced stage disease so that means the patients were diagnosed at stage 3 or 4 and those patients tend to… the association turned out to be even stronger in this subgroup of patients.

One particularly interesting thing we did look at in this study is how it is related with racial disparities in breast cancer survival because in the US African American patients tended to have poorer survival after breast cancer diagnosis than white women. Also because of the higher skin pigmentation in African American women, they tend to have lower vitamin D levels because we know vitamin D is synthesised under sun exposure by our skin. So we thought those two may be related and we did look at this and it turned out, yes, lower vitamin D levels in African American patients explains some of the poorer survival we have seen in comparison to white women.

It would be helpful to put our findings in the context of the literature because our study is an observational study. So although we have this outstanding Pathway Study, the prospective cohorts where we enrol the patients at the time of diagnosis and we follow up the patients for a median of ten years so we know their outcomes prospectively, this will help us to control for some biases in those observational studies but then this is still not a clinical trial where we can experimentally test whether giving patients vitamin supplementation will help to improve the outcome of their cancer diagnosis. There are several recent trials with vitamin D supplementation on cancer patient outcomes; so those analyses were not done as a primary analysis, it’s not the primary endpoint of the clinical trials but secondary outcomes but they did show very interesting and promising results that vitamin D supplementation improves cancer outcomes after diagnosis. Now our study from a large observational study showed consistent results. There are also a number of meta-analyses in the literature where it’s basically a literature review like they put a number of studies, mostly from clinical trials, together and they synthesise the evidence based on separate studies. Their conclusion is also that vitamin D supplementation, especially daily supplementation, is beneficial for patient outcomes after cancer diagnosis.

So to put our study in the context of the literature, we have come up with pretty strong evidence to support the use of vitamin D supplementation after breast cancer diagnosis, especially for those patients with higher stage of cancer at diagnosis and also African American women who are known to have lower vitamin D levels.