Calcium is thought to inhibit processes critical to the growth of colon cancer cells. But although the mineral has been shown to have a protective effect in some animal and epidemiological studies, say Israeli researchers, evidence from other studies have thrown up inconsistent results.
The researchers also found two well-designed, randomized controlled trials, showing a moderate protective effect on development of colorectal adenomatous polyps, the small, generally benign types occurring in about 30 per cent of middle-aged and older Americans.
Polyps can raise the risk of cancer.
However the study, led by Michael Asher Weingarten of Rabin Medical Centre, notes that "this does not constitute sufficient evidence to recommend the general use of calcium supplements to prevent colorectal cancer".
Weingarten and colleagues say that doubts about the importance of dietary calcium have surfaced in a number of studies. The review also identified a major faecal occult blood-screening program that found no association between colon cancer and reported calcium intake.
But if further studies confirm the protective effects of the mineral on polyps, calcium could help those who have already had polyps reduce their raised risk of colorectal cancer, say the authors.
Previous studies 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 caution in the current issue (number 3) of The Cochrane Library, a publication of The Cochrane Collaboration that evaluates medical research.
But Dr Anca Zalmanovici, one of the review's authors, adds that "calcium supplementation is relatively cheap, likely to be safe, readily available and has other positive metabolic effects on conditions that occur with ageing," such as osteoporosis, high blood pressure, kidney stones and weight gain.
A large randomized controlled trial by the Women's Health Initiative, begun in the early 1990s, could throw more light on the issue when it concludes in 2007. The trial is focused, in part, on the impact of calcium supplements on colorectal cancer and adenomas.