Taking calcium supplements may increase the odds of developing small growths in the colon called polyps, says a study published in the journal Gut. Supplements increased the risk with or without the addition of vitamin D.

Although polyps are non-cancerous, some could eventually turn into cancer if they are not removed. They come in different shapes and sizes, and this study specifically focused on serrated polyps, which are less common than adenomatous polyps, but probably carry the same cancer risk.

Some studies have suggested that calcium and vitamin D may protect against colon polyps, but results have been mixed.

For the new study, researchers analyzed findings from a large U.S. trial involving over 2,000 patients aged between 45 and 75 who had a history of polyps and were due to have a colonoscopy in 3 to 5 years.

Patients were excluded if they had a family history of bowel cancer, inflammatory bowel disease, or other serious health conditions, and factors such as sex, diet, weight, and use of anti-inflammatory drugs were taken into account.  

The remaining patients were split into groups to receive either daily calcium supplements, daily vitamin D supplements, both, or neither until their colonoscopy.

During the treatment phase, there was no effect of either calcium or vitamin D on cases of serrated polyps. However, during the later observational phase that was 6 to 10 years after treatment began, the researchers found increased risks of serrated polyps among patients taking calcium alone and among those taking a combination of calcium and vitamin D.

Evidence indicated that women and smokers who took calcium supplements were at higher risk, but no link was found between vitamin D alone and the risk of serrated polyps.

The results suggested the risk was associated only with calcium supplements and not calcium obtained from the diet.

Although the researchers said more research needs to be done, they suggested that patients with a history of pre-cancerous serrated polyps, especially women and smokers, may wish to avoid vitamin D and calcium supplementation.

Calcium supplements have also been linked to an increased risk of artery-clogging plaque buildup in arteries, although calcium-rich foods are heart-healthy, says a Johns Hopkins Medicine study. Participants, who ranged in age from 45 to 84, answered questionnaires about their dietary habits and underwent cardiac CT scans to measure their coronary artery calcium scores.

While people with high calcium scores due to foods reduced their risk of heart disease by 27 percent when compared to those with low calcium intake, those who used calcium supplements increased their risk by 22 percent.