When the research team looked at only participants with a normal body mass index (BMI), they found a 38 per cent risk reduction, suggesting that body mass may influence the relationship between vitamin D and decreased risk of advanced cancer.
"The findings suggest that vitamin D may reduce the risk of developing advanced cancers," said study author Paulette Chandler from the Brigham and Women's Hospital in the US.
"Vitamin D is a supplement that's readily available, cheap and has been used and studied for decades. Our findings, especially the strong risk reduction seen," Chandler added.
The researchers conducted the Vitamin D and Omega-3 Trial (VITAL) study, which was a rigorous, placebo-controlled study that took place over a span of more than five years.
VITAL study was designed to test the independent effects of vitamin D and Omega-3 supplements as well as to test for synergy between the two.
Participants were divided into four groups: vitamin D plus Omega-3s, vitamin D plus placebo, Omega-3s plus placebo, and placebos for both. Primary endpoints were major adverse cardiovascular events and the incidence of cancer.
VITAL did not find a statistical difference in overall cancer rates, but researchers did observe a reduction in the cancer-related deaths.
In their secondary analysis, the research team followed up on the possible reduction in cancer deaths with an evaluation of advanced (metastatic or fatal) cancer among participants who did or did not take vitamin D supplements during the trial.
They also examined the possible modifying effect of BMI.
Among the more than 25,000 participants in the VITAL study, 1,617 were diagnosed with invasive cancer over the next five years.
This included a broad mix of cancers (breast, prostate, colorectal, lung and more). Of the almost 13,000 participants who received vitamin D, 226 were diagnosed with advanced cancer compared to 274 who received the placebo.
Of the 7,843 participants with a normal body mass index (BMI less than 25) taking vitamin D, only 58 were diagnosed with advanced cancer compared with 96 taking the placebo.
"Our findings, along with results from previous studies, support the ongoing evaluation of vitamin D supplementation for preventing metastatic cancer -- a connection that is biologically plausible," said Chandler.