Calcium supplements may be linked to the development of colon polyps—small, benign growths that could ultimately develop into cancer, according to a new study.

There had been some belief that calcium played a role in colorectal cancer prevention, making it the subject of many studies. While some studies showed that calcium supplements may lower colorectal cancer risk, others suggested no benefit.

“This was an unexpected finding,” said the study’s lead author, Seth D. Crockett, M.D., M.P.H., an epidemiologist at the University of North Carolina School of Medicine. “We sort of expected to see either a protective or no effect ... so we’re surprised to see the opposite.”

The study was published Thursday in the medical journal Gut.

Vitamin D helps the body absorb calcium, and the two nutrients are often taken together in supplement form, in many cases to try to help strengthen bones. So the study looked at both, and the authors noted that roughly 40 percent of Americans take calcium and vitamin D supplements.

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

Taking a vitamin D supplement alone, however, did not increase the risk of polyps, nor did getting one's calcium from food.

Here’s what we know about the health effects of calcium supplements, and how to lower your chances of developing colorectal cancer with some simple lifestyle changes.

The Connection Between Pills and Polyps

The 2,058 adults in the study had a history of colon polyps and were between the ages of 45 and 75. Rather than focusing on adenomatous polyps, one of the most common types, the researchers chose to look at the less common serrated polyps, which are a more recently recognized and less-studied type.

The researchers randomly assigned the participants to take one of the following for three to five years: 1,200 mg of calcium, 1,000 milligrams of vitamin D, both calcium and vitamin D, or a placebo. 

More on Supplements

After this initial three- or five-year “treatment” phase of the study, colonoscopies revealed nothing of note: there appeared to be no major differences in polyp risk in any of the four groups.

But after analyzing the results of colonoscopies performed on over 1,000 participants 3 to 5 years later—after they stopped taking the supplements—the researchers noted the startling increase in serrated polyp risk in the two calcium supplement groups. It was especially pronounced in women and in smokers. Women, the authors noted, make up the majority of calcium supplement users.

A Dose of Perspective

While these results are concerning, there are a few things the study doesn’t show. For one, it doesn’t prove that calcium supplements increase the risk of developing serrated polyps; instead, the researchers noted, they simply found a correlation between the two.

It also doesn’t suggest calcium supplements increase the chances of getting colon cancer itself. "While polyps have the potential to turn into cancer,” said Crockett, “most of them won’t."

Crockett said it’s still unclear how calcium is involved in the formation of polyps in the body. And since the study found no increased risk of serrated polyps from getting calcium from food, more investigations are needed to tease out these complicated biological mechanisms.

"[O]verall the study design is strong, and the findings should be taken seriously," said Edward Giovannucci, M.D., Ph.D., a researcher and professor of nutrition and epidemiology at Harvard’s T.H. Chan School of Public Health who was not involved with the study. "But I think more work needs to be done before we worry about calcium for colorectal cancer risk." Still, he said, "there may be subgroups where we need more caution."

Smokers, for instance, are known to be at higher risk.

The study also only looked at people who already had a history of polyps, so it doesn’t say anything about the risk for those taking these supplements who have never had them.

“I don’t think we want to alarm people,” said Crockett, “but I do think this could be an important part of the calculus when physicians are talking about the benefits and potential risks of calcium supplementation.”

Andrea Wong, Ph.D., vice president of scientific & regulatory affairs at the Council for Responsible Nutrition (CRN), an industry trade group that represents supplement makers, urged a cautious interpretation of this new study.

“CRN is surprised by the results of this new study as previous research has indicated calcium supplementation may have a protective effect against colorectal polyps,” Wong said in a statement. “...Until further research is conducted, we caution the medical community and consumers from haphazardly dismissing the value of calcium supplementation.”

Do You Need Calcium Supplements?

A recent analysis of 33 studies involving more than 50,000 adults over age 50 found that the risk of fractures was similar among those who took calcium, vitamin D, or a combination as in those that did not take these supplements. These findings bolstered the results of a 2015 review of about five dozen randomized controlled trials published in the British Medical Journal, which found that taking calcium pills each day increases bone density in those over age 50 by just one to two percent.

“That’s not enough to prevent fractures,” said Consumer Reports' chief medical adviser Marvin M. Lipman, M.D. “And it doesn’t justify the risks of side effects associated with these supplements, which include an increased likelihood of constipation, heart disease, kidney stones, and gastrointestinal problems.”

Additionally, Consumer Reports advises caution when taking supplements in general because they are loosely regulated, and some may even contain undisclosed ingredients or may interact (sometimes dangerously) with medications. Calcium supplements, for example, should never be paired with certain types of antacids, antibiotics, anti-seizure medications, diuretics, and osteoporosis drugs, as well as blood pressure meds.

“You should always consult with your doctor before taking any kind of supplement, regardless of how harmless you think they are,” said Lipman.

How to Cut the Risk of Colorectal Cancer

Despite colorectal cancer being the third most commonly diagnosed cancer (after lung and breast cancers) studies have shown that lifestyle changes can be protective. Here are some science-backed ways to help mitigate your risk:

Eat whole grains. Last summer, a new report from the American Institute for Cancer Research (AICR) and the World Cancer Research Fund (WCRF) showed that eating about three servings of whole grains each day can lower colorectal cancer risk by 17 percent. (You can get three servings from one cup of ready-to-eat cereal, a slice of bread, and ½ cup of cooked rice or pasta, for example.)

The key may not just be in the fiber, but in the rich mixture of nutrients, minerals, and other compounds found in foods high in fiber—such as whole grains, fruits, and vegetables, experts say, so it’s smart to eat a diet high in fiber-rich fruits and veggies, too.

Cut back on alcohol, tobacco, and processed, red meats.
Drinking more than 30 grams of alcohol per day (the equivalent of about two glasses of wine, or two cocktails or two beers), increases your risk of colorectal cancer, according to the AICR/WCRF report. So if you drink, you should do so moderately.

The same is true for tobacco users, authors of the most recent study noted. “The best thing for smokers to do is to quit smoking,” said Giovannucci, the professor of nutrition and epidemiology at Harvard.

Processed meat (such as bacon, cold cuts, and sausage) and red meat (such as beef and pork) can increase your risk, too. For example, every 1.8-ounce-per-day serving of processed meat can increase your colon cancer risk by as much as 16 percent.

Get regular exercise. According to the AICR/WCRF report, being overweight heightens your risk for colon cancer, and all types of physical activity—even walking—can cut your risk. Studies have found, for example, that people who are most active are about 20 percent less likely to develop colon (but not rectal) cancer than those who are the least active.

Get screened. Colorectal cancer has been described as a “silent killer,” developing slowly over 10 to 15 years with few or no symptoms. But if you can detect and remove polyps from the lining of the large intestine or the rectum before they develop into cancer, you can dramatically cut your risk of developing the disease.

Experts recommend getting a colonoscopy every five to 10 years starting at age 50. If your risk of developing the disease is already high—if you have a parent or sibling with colorectal cancer, for instance—experts recommend that you start colonoscopies at age 40, and get screened more frequently. Talk to your doctor about the best screening plan for you.