Vitamin D is an essential nutrient that supports healthy bones, teeth, and muscles.
It also plays an important role in regulating the immune system and reducing inflammation.
Beyond these well-established functions, growing research suggests that vitamin D may also influence cancer risk and outcomes.
Several studies have linked vitamin D deficiency to an increased risk of developing certain cancers, particularly breast and colorectal cancer, raising questions about whether vitamin D could help prevent or even support the treatment of cancer.
In a recent review, researchers at University College Dublin examined existing evidence on the relationship between circulating vitamin D levels—measured as 25-hydroxyvitamin D [25(OH)D]—and cancer risk, as well as cancer-related outcomes.
The review focused primarily on colorectal and breast cancer, which are among the most extensively studied cancers in relation to vitamin D.
Links between vitamin D and cancer risk
Multiple studies have found that low levels of vitamin D in the blood are associated with a higher risk of developing several types of cancer, particularly colorectal and breast cancer.
In a meta-analysis of 25 studies, individuals with the lowest circulating vitamin D levels had a significantly higher risk of colorectal cancer compared with those who had higher levels.
Similarly, a meta-analysis of 38 observational studies reported that women with lower vitamin D levels were more likely to develop breast cancer.
Together, these findings suggest that maintaining adequate vitamin D levels may play a role in reducing the risk of colorectal and breast cancer.
Effects of vitamin D supplementation on cancer risk
Despite consistent associations between low vitamin D levels and increased cancer risk, evidence supporting vitamin D supplementation for cancer prevention in healthy individuals remains limited.
Several randomized controlled trials have examined whether vitamin D supplements can reduce cancer risk, but most have not found a significant protective effect.
In one randomized trial involving more than 2,000 older women, vitamin D supplementation did not reduce breast cancer incidence over a four-year follow-up period. Likewise, a large randomized trial involving more than 25,000 men and women found no strong reduction in colorectal cancer risk after five years of vitamin D supplementation.
However, some studies suggest that vitamin D supplementation may be beneficial in specific populations. For example, a large prospective cohort study of more than 33,000 individuals in Japan found that higher vitamin D intake was associated with a reduced risk of liver cancer.
Effects of vitamin D supplementation on cancer outcomes
While vitamin D supplementation does not appear to substantially reduce cancer risk in healthy populations, emerging evidence suggests it may improve outcomes in people who have already been diagnosed with cancer.
A meta-analysis of 25 randomized controlled trials found that vitamin D supplementation was associated with improved overall survival in patients with colorectal cancer.
Another meta-analysis of 10 randomized controlled trials reported improved disease-free survival among breast cancer patients who received vitamin D supplements.
The researchers caution that many of these studies were small and varied in design. Larger, well-controlled clinical trials are needed to confirm whether vitamin D supplementation meaningfully improves cancer outcomes and to determine optimal dosing strategies.
Vitamin D is essential for overall health, supporting skeletal function, immune regulation, and inflammation control. Research suggests that low vitamin D levels are associated with a higher risk of developing certain cancers, particularly colorectal and breast cancer.
Although vitamin D supplementation does not appear to significantly reduce cancer risk in healthy individuals, there is growing evidence that it may improve survival and disease outcomes in patients who already have cancer.
Further research is needed to clarify these effects and establish appropriate supplementation guidelines.
The review was conducted by Michael Duffy and colleagues and published in Nutrition and Cancer.
