The report appears in the latest issue of the Journal of the National Cancer Institute.
"Our results suggest that calcium supplementation may have a more pronounced ... effect on advanced [colon polyps] than on other types of polyps," writes lead researcher Kristin Wallace, MS, with Dartmouth Medical School in Lebanon, N.H.
While some research has looked at this link, few have addressed it in any detail -- to look for the effects of calcium on different types of colon polyps, Wallace explains. These studies make no distinction on the effects of calcium supplements on polyp size or other characteristics.
Also, what are the effects of calcium in the diet and from pills? One study has suggested that a 700 mg supplement daily may prevent polyps. However, it's not been clear whether a high-calcium diet boosts or hinders that effect, she writes.
The researchers analyzed data from patients involved in the large Calcium Polyp Prevention Study. The analysis involved 913 patients whose average age was 61 and who were followed for at least four years.
They had been randomly assigned to take either 1,200 mg calcium supplements or a placebo. Each volunteer was asked about the calcium, fat, and fiber they typically got in their diet. Each participant had a history of having a polyp removed at least three years prior to the start of the study. They also had a colonoscopy at the beginning of the study to document no remaining polyps in the colon.
After four years:
- The calcium group had 18% fewer noncancerous polyps and 35% fewer advanced polyps -- those with features that have a higher potential to become colorectal cancer -- compared with the placebo group.
There was another interesting pattern: Those with fewest polyps ate a high-calcium, high-fiber, low-fat diet. However, the numbers did not tally up as a definitive finding, notes Wallace.
In all, her study suggests that total calcium intake over 1,200 mg daily is necessary for colon protection -- and that a high-fiber diet with modest levels of fat will boost the protective effects, she writes.
Wallace's findings are in line with similar studies but fall short of proving a preventive link between calcium, colon polyps, and colon cancer, writes Arthur Schatzkin, PhD, with the National Cancer Institute, in an editorial.
However, studies are in place that could prove that this one nutritional factor -- calcium -- could offer protection against colon cancer. "That would be a tremendous advance," writes Schatzkin.
It's not clear how calcium acts to reduce colon polyps, writes Wallace. It may be that calcium binds "irritants" like bile acids and other fats in the bowel that are carcinogenic -- acting as a sort of "soap," possibly preventing colon cancer.