A recent case-control study from Canada found reduced risk of colorectal cancer associated with both calcium and vitamin D [Sun, 2011]. People from two regions, Ontario and New Foundland/Labrador, were involved. In case-control studies, 25(OH)D levels are determined at time of cancer diagnosis. Controls are matched to the cases in all respects, except the presence of cancer.
For vitamin D, those obtaining 650 to 750 IU/day from oral intake had a 15-25% lower risk than those taking 110-125 IU/day.
For those obtaining more than 1064 mg/day calcium had a lower risk of colorectal cancer in conjunction with higher vitamin D intake: those in the highest third (>241 IU/day vitamin D) had a 25% reduced risk of colorectal cancer, while those in the lowest third (<157 IU/day vitamin D) had a 15% reduced risk.
While dairy products are a source of calcium and vitamin D, there was no statistically significant reduced risk associated with any or all dairy products.
The brothers Cedric and Frank Garland first proposed that vitamin D reduced the risk of colon cancer in 1980 based on seeing a map of colon cancer mortality rates in the United States and realizing that the region of lowest rates, the Southwest, was the sunniest, while the least sunny region, the Northeast, had the highest rates [Garland and Garland, 1980]. In 1985, they showed that calcium and vitamin D together reduced the risk of colorectal cancer [Garland et al., 1985].
Garland C, Shekelle RB, Barrett-Connor E, Criqui MH, Rossof AH, Paul O. Dietary vitamin D and calcium and risk of colorectal cancer: a 19-year prospective study in men. Lancet. 1985 Feb 9;1(8424):307-9..
Sun Z, Peizhong Peter Wang, Barbara Roebothan, Michelle Cotterchio, Roger Green, Sharon Buehler, Jinhui Zhao, Josh Squires, Jing Zhao, Yun Zhu, Elizabeth Dicks, Peter T. Campbell, John R. Mclaughlin, Patrick S. Parfrey. Calcium and Vitamin D and Risk of Colorectal Cancer: Results From a Large Population-based Case-control Study in Newfoundland and Labrador and Ontario. Canadian Journal of Public Health. 2011;102(5):392-9. http://journal.cpha.ca/index.php/cjph/article/view/2627