Calcium prevents recurrence of colon cancer more effectively in people with higher levels of vitamin D than in people with lower levels, reports a new study published in the Journal of the National Cancer Institute.
The results of some animal and human studies have suggested that vitamin D might protect against colon and other cancers, but other studies have found no such effect. A number of studies have demonstrated that high intake of calcium reduces the risk of colon cancer.
There is some evidence that vitamin D and calcium might have a synergistic cancer-preventive effect in the colon, but the interactions between these two nutrients in the colon are not well understood.
The current study examined the effect of calcium supplementation and vitamin D levels on the risk of recurrence of colon cancer. The 803 participants had all had colon cancer in the past, but had no detectable cancer at the beginning of the study.
Participants were randomly assigned to receive either 1,200 mg of calcium per day or a placebo. They were monitored for recurrence of colon cancer over a four-year period. Vitamin D levels in the blood were measured at the beginning and end of the study and a median vitamin D level was calculated.
The risk of recurrence of colon cancer was evaluated separately for those whose vitamin D levels were at or below the median and for those whose levels were above the median.
In people with higher vitamin D levels, those receiving calcium had a significantly lower risk of recurrence than those receiving placebo, but in those with lower vitamin D levels, calcium showed no significant protective effect.
Similarly, in people taking calcium, those with higher vitamin D levels had a significantly lower risk of recurrence than those with lower vitamin D levels, but in people receiving placebo, vitamin D levels had no significant effect on risk.
The results of this study suggest that calcium supplementation and higher vitamin D levels together, but not independently, protect against recurrence of colon cancer. Future studies are needed to evaluate the joint effect of calcium and vitamin D on colon cancer risk in people who have never had colon cancer. The effect of vitamin D supplementation and optimal levels of calcium and vitamin D intake remain to be determined.