Several observational studies have shown that lower vitamin D levels are associated with a greater risk of developing several types of common cancers such as breast, prostate, colorectal and even skin.
There have two controlled trials that have looked at the effect of vitamin D supplementation and the incidence of cancer. One trial in 36,000 post-menopausal women found that vitamin D and calcium supplementation over a period of 7 years had no effect on the incidence of cancer. The other trial found a significant reduction in overall cancer risk by 60% with calcium and vitamin D supplementation over a period of 4 years in 1,179 post-menopausal women.
Observational studies looking at the relationship between vitamin D status and risk of cancer death have also been conflicting. Recently, a review of prospective epidemiological studies found no association between cancer mortality and vitamin D concentrations in the general population. Therefore, the relationship between vitamin D and the risk of the developing cancer as well as the risk of death from cancer is still uncertain.
The aim of the current study was to investigate the association between vitamin D status and both the risk of cancer and the risk of cancer death in elderly women.
Australian researchers obtained data from a cohort of elderly women recruited to the Calcium Intake Fracture Outcome Study (CAIFOS) and conducted an analysis. CAIFOS was a randomized controlled trial initiated in 1998 that examined the effects of oral calcium supplementation on the prevention of osteoporotic fractures in elderly Australian women.
Data from a total of 1500 women were used in the study. Included in the analysis were baseline vitamin D measurements as well data on the incidence of cancer and deaths from cancer during the follow-up period between 1998 and 2008.
The researchers wanted to see how vitamin D status at initiation of the study was related to the risk of developing cancer or the risk of dying from cancer.
After adjusting for confounders such as age and smoking status, their analysis revealed the following:
- Median vitamin D level was 25.6 ng/ml. A total of 591 participants had levels below this.
- The risk of developing cancer was not significantly associated with lower vitamin D levels (p = 0.6).
- There were a total of 274 deaths during the follow-up period, 84 of which were due to cancer.
- Compared to participants with levels above 33.2 ng/ml, participants with levels between 25.7 ng/ml and 33.2 ng/ml, levels between 18.4 ng/ml and 25.6 ng/ml, and levels less than 18.4 ng/ml had 123%, 155%, and 163% increased risk of death from cancer, respectively.
- For every 12 ng/ml reduction in vitamin D levels, there was a 33% increase in cancer mortality risk.
The researchers concluded,
“In this large study of Australian women, with over 12,000 person-years of follow-up, we have shown that women with lower baseline were at an increased risk of overall cancer death but not for incident cancer.”
One of the ways vitamin D is thought to protect against cancer by reducing the activity of a protein called c-myc that is critical in promoting cell proliferation and the transformation of pre-malignant cells to malignant cells. Moreover, in vitro studies show that activated vitamin D can inhibit the proliferation of tumor cells as well as limit the ability of cancer cells to form new blood vessels and metastasize.
One important feature of the results to note is the high threshold level (33.2 ng/ml) that predicted the increased risk in cancer mortality. Previous literature has observed a cutoff point of 20 ng/ml and its association with increased risk of overall cancer mortality. The researchers admitted that the difference could be due to having only a single measurement of vitamin D levels in their analysis.
“Our findings suggest future trials may consider higher doses of vitamin D to achieve a target serum 25 (OH) D concentration of [24 ng/ml] or greater when evaluating the benefits and harms of vitamin D supplementation in older women with cancers.”
The study’s strengths include its long follow-up period and detailed data on potential confounders. Limitations include the lack of measurements of vitamin D levels over time and lack of generalizability to other populations.
Additional prospective studies looking at the relationship between vitamin D status and cancer mortality are needed in different populations of both genders. Randomized trials will be needed to see if any potential association is causal and potentially reversible.