(Journal of the National Cancer Institute) -- A new study suggests that the nutrients calcium and vitamin D work together, not separately, to reduce the risk of colorectal cancer.

Maria V. Grau, M.D., of the Dartmouth Medical School in Lebanon, N.H., and her colleagues examined data on 803 people who participated in the Calcium Polyp Prevention Study, a randomized trial that showed a protective effect of calcium supplementation for preventing the recurrence of colorectal cancer. The new study found that calcium supplements reduced the risk of recurrence only among individuals with baseline vitamin D levels above the median (29.1 ng/mL). Similarly, serum vitamin D levels were associated with reduced adenoma recurrence only among individuals taking calcium supplements.

These findings "provide a strong indication that vitamin D and calcium have a joint antineoplastic effect in the large bowel," the authors write, but they add that "further investigation is needed to understand the mechanistic basis of the vitamin D/calcium interaction and to clarify the amount of intake of each nutrient required for optimum protective effect."