Calcium supplements increase the risk of colon cancer

Some believe that calcium plays a role in preventing colorectal cancer, making it the subject of much research. While some studies have shown that calcium supplements may reduce the risk of colorectal cancer, others have shown no benefit.

“This was an unexpected finding,” said Dr. Seth D. Crockett, an epidemiologist at the University of North Carolina School of Medicine and lead author of the study. “We expected to see some protective effect or none at all … so it was surprising to see the opposite.”

The study was published in the medical journal Gut.

Vitamin D helps the body absorb calcium, and the two are often taken together in supplement form, in many cases to help keep bones strong. So the study looked at both, and the authors noted that about 40% took calcium and vitamin D supplements.

For people taking calcium alone, the risk of developing polyps was 2.6 times higher; those taking calcium and vitamin D together had a 3.8 times increased risk.

However, vitamin D supplementation alone, as well as calcium from foods, did not increase the risk of polyps.

Here's what we know about the health effects of calcium supplements and how to reduce your risk of developing colon cancer with some simple lifestyle changes.

The relationship between drugs and polyps

The 2,058 adults in the study had a history of colon polyps and were between the ages of 45 and 75. Instead of focusing on adenomatous polyps, one of the most common types, the researchers looked at serrated polyps, a less common type of polyp that has recently been recognized and is little studied.

Study subjects were randomly assigned to take one of the following doses for 3 to 5 years: 1,200 mg calcium, 1,000 mg vitamin D, both calcium and vitamin D, or a placebo.

After this initial 3- or 5-year “treatment” period, colonoscopy revealed nothing remarkable: there appeared to be no major difference in polyp risk in any of the four groups.

However, after analyzing colonoscopy results performed on more than 1,000 of these people three to five years later, after they stopped taking the supplements, the researchers found a surprising increase in the risk of serrated polyps in the two calcium supplement groups. This was especially evident in women and smokers. The authors note that women make up the majority of calcium supplement users.

Risk level

While these results are concerning, there are a few points the study doesn't make clear. First, it doesn't prove that calcium supplements increase the risk of developing serrated polyps; instead, the researchers note, they simply found a correlation between the two.

Nor does it suggest that calcium supplements increase the risk of colon cancer. “While polyps can become cancerous,” says Dr. Crockett, “most of them won’t.”

It is still unclear how calcium contributes to polyp formation in the body. And since research shows no increased risk of serrated polyps from dietary calcium, more research is needed to clarify these complex biological mechanisms.

"Overall the study design is strong, and the findings should be taken seriously," said Edward Giovannucci, PhD, a researcher and professor of nutrition and epidemiology at Harvard T.H. Chan School of Medicine. "But I think more work is needed before we worry about calcium for colorectal cancer risk." However, he added, "there may be groups that need to be more cautious."

For example, smokers are often at higher risk.

The study also only looked at people who already had a history of polyps, so it can't say anything about the risk of people who have never had polyps who take calcium supplements.

Do you need calcium supplements?

A recent analysis of 33 studies involving more than 50,000 adults over 50 found that the risk of fractures was similar among those who took calcium, vitamin D, or both supplements and those who did not. These findings supported the results of a 2015 review of about five dozen randomized controlled trials published in the British Medical Journal, which found that taking calcium supplements daily increased bone density by only 1-2% in people over 50.

"This is not enough to prevent fractures, and it is not worth the risks of side effects associated with this supplement, which include constipation, heart disease, kidney stones, and digestive problems," said Marvin M. Lipman, chief medical adviser for Consumer Reports.

Additionally, Consumer Reports advises caution when taking supplements in general, as they are loosely regulated and some may even contain unlisted ingredients or may interact (sometimes dangerously) with medications. For example, calcium supplements should not be combined with certain antacids, antibiotics, anti-seizure drugs, diuretics, and osteoporosis medications, as well as blood pressure medications.

"You should always consult your doctor before taking any supplement, no matter how harmless you think it is."