Colorectal adenomas are precancerous polyps that occur in the lower digestive tract. Various kinds of evidence indicate use of calcium supplements can help prevent colon polyps and also reduce colon cancer risk.
An article published in the Journal of the National Cancer Institute in 2007 adds weight to this hypothesis. A total of 930 people with a history of a previous colon polyp were given either placebo or calcium carbonate (three grams daily) for four years. At the end of the four-year trial period, people given calcium showed a significantly lower rate of polyp recurrence compared to those given placebo. Treatment was then stopped, but researchers continued to follow the participants. Remarkably, the protective effect of calcium was maintained for as long as five years after the end of the active part of the study. Even if people did not continue to take calcium supplements, they remained relatively less likely to develop polyps simply on the basis of having had taken it.
There are two caveats, however. First, this dose of calcium supplementation is very high, and should not be undertaken without a doctor’s supervision. Furthermore, there is weak but still potentially worrisome evidence that, when taken by men, calcium supplements may raise risk of cancer of the prostate.