The research builds on a multi-centre study led by Dartmouth Medical School professor John Baron that found that people who take calcium supplements have a lower risk of adenoma polyps - benign tumours that are precursors to cancer of the colon or rectum.

Baron and colleagues re-analysed the study to examine how calcium and vitamin D interact to lower the possibility of colorectal cancer. They found that calcium supplements prevented adenomas only among individuals with baseline vitamin D levels that were higher than average (29.1 ng/mL).

Similarly, serum vitamin D levels were associated with reduced adenoma recurrence only among individuals taking calcium supplements.

More than 800 people participated in the Calcium Polyp Prevention Study, a four-year randomised trial that documented the benefits of calcium supplementation for preventing the growth of the benign tumours.

The new findings "provide a strong indication that vitamin D and calcium have a joint anti-neoplastic effect in the large bowel," but are essentially ineffective without the other, the authors write.

They add that further investigation is needed to understand the mechanism behind the vitamin D/calcium interaction and to clarify the amount of intake of each nutrient required for optimum protective effect.

The authors noted that calcium and vitamin D are inter-related metabolically in bone and in the normal intestine but their potential interactions in large-bowel carcinogenesis are not well understood.

The study is published in the Journal of the National Cancer Institute, vol 95, no 23, pp1765-1771.