Bowel cancer: Can calcium prevent it?

Trials have shown that daily calcium can prevent bowel polyps. There is still no final answer on whether or not this also prevents bowel cancer.

Bowel or colorectal cancer usually occurs in the large bowel or colon. The chances of getting bowel cancer are low for most people. People can be at much higher risk if this cancer is common in their close family.

Bowel cancer usually develops slowly over years. Small harmless growths start in the mucus lining on the inside of the bowel. These are called polyps or adenomas. Almost all of these stay small and harmless. Some can grow though, and then the risk increases that they might develop into cancer. It has been estimated that only about 5 out of 100 of these bowel polyps (5%) ever become cancerous. It takes about 5 to 10 years for polyps to turn into cancer.

Researchers have assumed for years that there is a connection between diet and the risk of bowel cancer. However it is still unclear if there are particular foods or nutrients that can prevent bowel cancer.

The mineral calcium is one candidate that researchers have studied. Laboratory and animal studies show that calcium could possibly interfere with the development of cancerous cells in the bowel. Calcium occurs naturally, especially in milk and milk products. To try to prevent bowel cancer, though, relatively high levels of calcium supplementation have been studied. It is not possible to reach levels this high through diet alone.

The current state of scientific knowledge was described in a systematic review from the Cochrane Collaboration. The researchers searched for all trials that tested whether calcium supplements could prevent bowel cancer.

The results were limited: two trials, in which more than 1,300 people took part. Both were randomised controlled trials. The basic principle: Volunteers agree to be divided into two groups. Only one of these groups would take calcium supplements, while the other would get a calcium-free placebo or dummy supplement. Both men and women were included in both the trials. All had already had bowel polyps identified and removed in the past.

The people taking calcium in one trial took 1,000 mg a day for about four years. In the other trial, it was 2,000 mg a day for about three years. After this, all participants in the trials had a colonoscopy to examine their bowels. This enabled the researchers to see how many had developed new polyps. These people could have at increased risk of developing bowel cancer.

The result: Hardly anyone had developed bowel cancer in the three or four years, regardless of whether they were in the calcium group or not. There were too few to be sure that calcium had reduced the risk of getting bowel cancer.

However the results for polyps were clear. For every 100 people who took the calcium supplements, 23 new bowel polyps developed (23%) compared to 29 new polyps in every 100 people taking placebos (29%). This difference suggests calcium has a protective effect, but the Cochrane researchers remain cautious. A supplement that slows down the development of harmless polyps is not guaranteed to be able to prevent cancer. But more research is worthwhile.

A large American trial, the Women's Health Initiative (WHI), studied whether bowel cancer could be prevented with a comparatively lower dose of calcium, that also included vitamin D. More than 36,200 women who had been through the menopause participated in this trial. This means that their risk of bowel cancer was not particularly high.

The women took the supplement for around seven years on average. That did not appear to have a protective effect against bowel cancer. However, the women's bowel health was not assessed as carefully before they started taking the supplements, and there was no bowel examination at the end of the trial either. The dose of calcium was lower than the dose included in the Cochrane review. So the Cochrane researchers concluded that the WHI trial was not ideally suited to answer the question of whether or not calcium supplementation can protect against bowel cancer.

Source:

Wactawski-Wende J, Kotchen JM, Anderson GL, Assaf AR et al (Women's Health Initiative Investigators). Calcium plus vitamin D supplementation and the risk of colorectal cancer. N Engl J Med 2006; 354: 684-696. [Full text]

Weingarten MA, Zalmanovici A, Yaphe J. Dietary calcium supplementation for preventing colorectal cancer and adenomatous polyps. Cochrane Database of Systematic Reviews 2008, Issue 1.