Patients with a history of nonmalignant polyps took either 1,200 milligrams of calcium in supplement form or a placebo daily for four years in a study previously reported by researchers from Dartmouth Medical School. Calcium use was associated with a 17% lower relative risk for polyp recurrence.
This risk reduction not only persisted in the years after treatment ended, but it seemed to strengthen, the Dartmouth researchers report in a newly published follow-up that included 822 of the 930 original study subjects.
During the first five years after the end of treatment, 31.5% of patients in the calcium group developed new polyps, compared with 43.2% of the study participants who did not take calcium. The protection did not appear to extend beyond five years, however.
The findings are published in the Jan. 17 issue of the Journal of the National Cancer Institute.
"It really does look like calcium interferes with carcinogenesis [cancer development] in the large bowel," researcher John A. Baron, MD, tells WebMD. "The fact that this reduction in risk persisted for years after people stopped taking calcium is really amazing."
Polyps are growths in the colorectal area. Some polyps can become cancerous tumors.
But Baron says it is still not clear if the benefits of taking calcium supplements outweigh the risks among men because some studies have linked the treatment to an increase in prostate cancer risk.
The American Cancer Society recommends that adults aged 19 to 50 take in 1,000 milligrams per day of calcium and that those over 50 get 1,200 milligrams. But the guidelines stress that the calcium should come primarily from food sources and not supplements.
The nutrition guidelines, which were updated in 2006, also state that because of the possible increased risk of prostate cancer with high calcium intake, "it may be wise for men to limit their daily calcium intake to less than 1,500 milligrams per day until further studies are done."
University of Arizona epidemiology professor Maria Elena Martinez, PhD, worries that the findings from the Dartmouth follow-up study will drown out this warning.
"Americans tend to think that if some is good then more is better," she tells WebMD. "But for men at least, we have to keep in mind that more isn’t better in this case and that taking calcium supplements in high doses may even be dangerous."
Colonoscopy Best Risk Reducer
Another concern, Martinez says, is that people will get the idea that all they need to do to prevent colorectal cancer is take calcium supplements or eat calcium-rich foods.
Though the Dartmouth findings show calcium to be modestly protective against colorectal cancer, another major study published in 2006 failed to show a protective benefit.
Calcium and vitamin D supplements proved to be somewhat protective for bone density in the Women’s Health Initiative (WHI) trial. But no difference in colorectal cancer risk was seen for an average of seven years of follow-up among women who took calcium and vitamin D and those who did not.
In an editorial accompanying the Dartmouth study, Martinez points out that despite the fact that polyp removal through screening is a highly effective way to prevent colorectal cancer, most people either aren’t getting the message or are ignoring it.
"If you want to prevent colon cancer, the best thing you can do is get a colonoscopy when you turn 50 and make sure that you have follow-up colonoscopies as needed," she tells WebMD. "That isn’t as easy as popping a pill, but it is true."