Bowel cancer: Can calcium prevent it?

Trials have shown that calcium supplements 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 immediate 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 1 out of 20 of these bowel polyps ever becomes cancerous. It takes about 5 to 10 years for polyps to turn into cancer.

Researchers have assumed for decades 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 dairy 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.

Research on bowel cancer prevention with calcium

The current state of scientific knowledge was described in a systematic review from the Cochrane Collaboration, an international research network. The researchers searched for all trials that tested whether calcium supplements could prevent bowel cancer. The results were limited: two trials, in which altogether more than 1,300 people took part. Both were randomized controlled trials (RCTs). The basic principle: Volunteers agree to be divided into two groups by lottery. 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. You can read more about randomized controlled trials here (URL: http://www.informedhealthonline.org/index.61.en.html) .

The people taking calcium in one trial took 1,200 mg a day for about 4 years. In the other trial, it was 2,000 mg a day for about 3 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 an increased risk of developing bowel cancer later on.

Trials indicate a preventative effect

The result: Hardly anyone had developed bowel cancer in the 3 or 4 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 developed at least 1 new bowel polyp (23%) compared to 29 who developed at least 1 new polyp in every 100 people taking placebos (29%). This difference suggests calcium has a preventative 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 would be worthwhile.

Calcium in combination with vitamin D

A large American trial, the Women's Health Initiative (WHI) trial, 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 menopause participated in this trial. This means that their risk of bowel cancer was not particularly high. The women took the supplement for around 7 years on average. That did not appear to have a protective effect against bowel cancer.

The Cochrane researchers concluded that the WHI trial was not ideally suited to answer the question of whether or not calcium supplements can protect against bowel cancer. One reason for this is that participants took calcium only in combination with vitamin D. It is not possible to find out what influence calcium and vitamin D would have individually. You can read more about bowel cancer and bowel cancer prevention in our feature (URL: http://www.gesundheitsinformation.de/bowel-cancer-prevention.208.56.en.html) .

Some researchers have found a possible risk of heart attack for people taking calcium supplements who already take in more than 800 mg of calcium per day in their usual diet. Combining calcium supplements with vitamin D could reduce this effect. You can find out if you are getting enough calcium in your diet or too much by using our calcium calculator (URL: http://www.gesundheitsinformation.de/calcium-calculator.569.56.en.html) .

Author: German Institute for Quality and Efficiency in Health Care (IQWiG)