The Calcium Follow-up Study, an observational study that followed the Calcium Polyp Prevention Study, found that people from the original calcium supplementation group had a significant 12 per cent lower risk of any adenoma five years after the original supplements were stopped, compared to people from the placebo group.
"The protective effect of calcium supplementation on risk of colorectal adenoma recurrence extends up to five years after cessation, even in the absence of continued supplementation," wrote lead author Maria Grau from Dartmouth Medical School in the US.
Colorectal cancer accounts for nine per cent of new cancer cases every year worldwide. The highest incidence rates are in the developed world, while Asia and Africa have the lowest incidence rates.
It remains one of the most curable cancers if diagnosis is made early.
In the Calcium Polyp Prevention Study, 930 people with a recent adenoma were randomly assigned to receive four years of daily 1200-milligram calcium supplements or a placebo. The study revealed that those assigned to calcium supplements had a 17 per cent lower relative risk of an adenoma recurrence than those who got the placebo.
The new research, published in the Journal of the National Cancer Institute, used data on 822 of the original 930 subjects from the Calcium Polyp Prevention Study. Of these subjects, 597 underwent at least one colonoscopy exam, and completed follow-up questionnaires.
Grau and her colleagues report that, during the first five years after the end of the original study, those randomly assigned to the calcium supplementation group had a 12 per cent lower risk of all adenomas than those from the placebo group (31.5 versus 43.2 per cent).
During the next five years (five to ten years after the end of the Calcium Polyp Prevention Study), no significant difference was observed between subjects from the calcium or placebo groups, said the researchers.
"Our study provides further evidence of the potential of calcium as a chemopreventive agent against colorectal adenomas among individuals with a history of these tumours," wrote the researchers. "Our data indicate that, in these patients, the protective effect of calcium may extend for up to 5 years after the cessation of active treatment."
While no mechanistic study was performed in this study, the researchers did propose two mechanisms to explain the apparent beneficial benefits of calcium. The first concerned calcium's ability to bind and precipitate bile acids in the bowel, thereby making them inert and protecting against inflammation. The second possible mechanism involved activation of the so-called calcium sensing receptor (CSR) which has been reported to have anti-cancer effects in vitro.
In an accompanying editorial, Maria Martinez and Elizabeth Jacobs from the Arizona Cancer Center in Tucson asked: "Where do we go from here--and, more important, what public health recommendations related to calcium do we provide for risk reduction of colorectal cancer?"
Martinez and Jacobs note that current US guidelines recommend that people simply consume recommended levels of calcium (1000 mg/day for adults up to age 50 years and 1200 mg/day for those older than 50 years). "Because no protection for colorectal cancer is apparent at higher levels of calcium intake, this recommendation is justified," they said.
"Large clinical trials of calcium and colorectal cancer are unlikely to be launched in the near future. However, should the opportunity arise, we should consider taking into account that, as is the case for many nutrients, individuals with lower rather than higher nutrient intakes are likely to benefit the most from supplementation and that those who have already exceeded the threshold of prevention may experience no added protection," said Martinez and Jacobs.
Source: Journal of the National Cancer Institute Volume 99, Pages 129-136 "Prolonged effect of calcium supplementation on risk of colorectal adenomas in a randomized trial" Authors: M.V. Grau, J.A. Baron, R.S. Sandler, K. Wallace, R.W. Haile, T.R. Church, G.J. Beck, R.W. Summers, E.L. Barry, B.F. Cole, D.C. Snover, R. Rothstein, J.S. Mandel
Editorial: Journal of the National Cancer Institute Volume 99, Pages 99-100 "Calcium supplementation and prevention of colorectal neoplasia: lessons from clinical trials" Authors: M.E. Martinez, E.T. Jacobs