Calcium may prevent polyps, but effect on colorectal cancer unclear

Jul 19 2005

Although calcium supplements might prevent development of polyps that sometimes lead to colon cancer, there is not enough evidence that the mineral can prevent colorectal cancer itself, according to a new study.

The study, a review of two earlier well-designed randomized controlled trials, found a moderate protective effect on development of colorectal adenomatous polyps, the small, generally benign types that about 30 percent of middle-aged and older Americans have. But "this does not constitute sufficient evidence to recommend the general use of calcium supplements to prevent colorectal cancer," says an Israeli team of researchers headed by Michael Asher Weingarten of Rabin Medical Centre.

Polyps can lead to colorectal cancer if unchecked. Because it is difficult to conduct studies on the effect of preventive doses of calcium on colorectal cancer itself -- due to the relatively small number of cases and the length of time they could be watched -- the reviewers looked instead at prevention of polyps, which occur frequently in the population, as a potential predictor of later cancer.

Previous experiments in animals and surveys of people who had high calcium diets have indicated a possible protective effect. In looking at the two best studies done so far, Weingarten and colleagues found results that "suggest a clinically relevant protective effect of dietary calcium supplementation on the development of colorectal adenomatous polyps."

"Although it is likely to be safe, this … would need to be more clearly demonstrated in further controlled studies before any attempt at widespread introduction into clinical practice," they say.

The review appears in the current issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.

Dr. Anca Zalmanovici, one of the review's authors, says "calcium supplementation is relatively cheap, likely to be safe, readily available and has other positive metabolic effects on conditions that occur with aging," such as osteoporosis, high blood pressure, kidney stones and weight gain.

If further studies confirm calcium as effective, the review says it could be given to people who had polyps before and who are thus are at higher risk of getting colorectal cancer.

The review cautions that higher doses of calcium taken over longer periods of time, while perhaps needed, also may not be well tolerated by some subjects. At the same time, it notes that no evidence exists that calcium supplements up to 2,000 mg a day would be detrimental to those taking such supplements. As a general rule, people need about 1,750 mg of calcium a day. Standard dosages for over-the-counter elemental calcium supplements range from 500 mg to 1,500 mg a day.

Doubts about the importance of dietary calcium have surfaced in a number of studies, according to the review, which points out that early findings of polyp- and cancer-inhibiting effects of calcium have not been confirmed by recent studies. The review also identified a major fecal occult blood-screening program that found no association between colon cancer and reported calcium intake.

The review focused on two double blind, placebo controlled trials involving 1,346 people, including participants with previous adenomas. A U.S.-based study involving six clinical centers used 1,200 mg of elemental calcium. A second study, conducted in nine European countries and Israel, used calcium supplements of 2,000 mg per day for three years.

Colon cancer is one of the leading cancers in men and women, and dietary factors – including calcium, fiber, sugar, fat, vegetables and meat -- have been considered involved in the increasing incidence of the disease in industrialized countries. The risk begins to increase after age 40 and rises sharply between 50 and 55 years old.

Calcium has been suggested as a protective agent against colorectal cancer because it is thought to inhibit processes considered critical to the growth of colon cancer cells. While calcium has been shown to have a protective effect in some animal and epidemiological studies, according to Dr. Weingarten, the evidence from other studies gave inconsistent results.

The review notes that a large randomized controlled trial by the Women's Health Initiative, which was begun in the early 1990s, will be deserving of attention when its findings are made known. The trial is expected to conclude in 2007. The trial is focused, in part, on the impact of calcium supplements on colorectal cancer and adenomas.