Vitamin D supplements may reduce risk of developing advanced cancer, study finds

In a secondary analysis of the Vitamin D and Omega-3 Trial (VITAL), a team led by researchers at Brigham and Women's Hospital found that vitamin D was associated with an overall 17 percent risk reduction for advanced cancer, according to results published in JAMA Network Open.  

The Vitamin D and Omega-3 Trial (VITAL), which concluded in 2018, found that vitamin D did not reduce overall incidence of cancer, but hinted at a decreased risk of cancer deaths. In the new analysis, when the team looked at only participants with a normal body mass index (BMI), they found a 38 percent risk reduction, suggesting that body mass may influence the relationship between vitamin D and decreased risk of advanced cancer, according to the study.

VITAL was a placebo-controlled study that took place over a span of about five years. The VITAL study population included men who were 50 or older and women 55 or older who did not have cancer when the trial began. VITAL 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 (2000 IU/day) plus omega-3s; vitamin D plus placebo; omega-3s plus placebo; and placebos for both. Primary endpoints were major adverse cardiovascular events and incidence of cancer. VITAL did not find a statistical difference in overall cancer rates, but researchers did observe a reduction in cancer-related deaths, the study said.

In the new analysis, researchers 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, such as breast, prostate, colorectal, and lung. Of the nearly 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 BMI of less than 25 taking vitamin D, only 58 were diagnosed with advanced cancer compared with 96 taking the placebo, according to the study.

While the team's findings on BMI could be due to chance, the researchers said there is previous evidence that body mass may affect vitamin D action. Obesity and associated inflammation may decrease the effectiveness of vitamin D, possibly by reducing vitamin D receptor sensitivity or altering vitamin D signaling. In addition, randomized trials of vitamin D and type 2 diabetes have found greater benefits of vitamin D in people with normal weights and no benefit among those with obesity, they said.

Vitamin D deficiency is common among cancer patients, with one study reporting rates of vitamin D deficiency as high as 72 percent among cancer patients. There is also evidence that higher amounts of body fat are associated with increased risk for several cancers, according to the researchers.

"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 Paulette Chandler, MD, MPH, corresponding author and a primary care physician and epidemiologist in the Brigham's Division of Preventive Medicine, in a statement. "Additional studies focusing on cancer patients and investigating the role of BMI are warranted."