Calcium supplements, taken with or without vitamin D, may increase the risk of small growths in the bowel called polyps, a new trial of more than 2,000 people shows.
“This study examines the risk of a really interesting class of colon polyps called serrated polyps,” says lead author Seth D. Crockett, assistant professor of gastroenterology and hepatology in the University of North Carolina School of Medicine.
“Specifically, we were interested in ‘sessile serrated polyps’, which have only recently been recognized as important colon cancer precursors that give rise to 20-30 percent of sporadic colon cancer cases.”
“For some patients… data from this study could alter the balance of risks and benefits of calcium supplementation.”
“Sessile serrated polyps are an important target of colon cancer screening,” Crockett says, “but are more difficult to detect on colonoscopy compared to adenomatous polyps because of their flat shape and subtle appearance.”
“The findings from this study were somewhat unexpected,” he says. “There is some evidence from epidemiologic studies that people who have calcium rich diets are at lower risk of colon polyps, including serrated polyps. So it stands to reason that calcium supplementation might have beneficial effects in terms of preventing colon cancer or polyps.”
Some earlier studies have shown calcium can help prevent adenomatous polyps. But the news study showed evidence that calcium supplementation (with or without vitamin D supplementation) appeared to be associated with an increased risk of precancerous serrated polyps.
“It’s important to put these findings in perspective and to not cause alarm,” Crockett says. “Calcium and vitamin D supplementation are taken by lots of people, and do have some beneficial effects on bone health. Many people take low doses of calcium in multivitamins (lower than what was used in our study) that are unlikely to be harmful. This possible association does not necessarily negate the other benefits of these supplements.
“But for some patients, including those with a history of serrated polyps and/or smoking, data from this study could alter the balance of risks and benefits of calcium supplementation.”
Polyps are small growths in the lower part of the large bowel (colon). They are non-cancerous, but some could eventually turn into cancer if they are not removed.
Polyps come in different shapes and sizes, with serrated polyps more likely to turn into cancer than conventional polyps. Some studies have suggested that calcium and vitamin D may protect against serrated polyps, but results have been mixed.
So to investigate further, researchers set out to determine whether taking daily calcium and vitamin D supplements reduce the risk of serrated polyps.
They analyzed findings from a large US trial involving over 2,000 patients aged between 45 and 75 who had had at least one serrated polyp detected and removed—and were due to have a follow-up colonoscopy in three to five years.
Researchers excluded patients if they had a family history of bowel cancer, inflammatory bowel disease, or other serious health conditions—and researchers took several factors into account at the start of the study, including sex, diet, body mass index (BMI), and use of anti-inflammatory drugs.
Researchers randomly split the remaining patients into groups to receive either daily calcium supplements, daily vitamin D supplements, both, or neither for three or five years (treatment phase) until their colonoscopsy. Researchers also recorded the effects three to five years after treatment ended (observational phase).
During the treatment phase, neither calcium nor vitamin D had any effect on cases of serrated polyps. However, during the later observational phase (six 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.
There was evidence that women and smokers were at higher risk when exposed to calcium supplements, but researchers found no association between vitamin D alone and the risk of serrated polyps. The results also suggest an association with calcium supplements only, not dietary calcium.
The researchers recommend further studies to confirm these results, but if calcium and its combination with vitamin D are truly associated with an increased risk of serrated polyps, “this has important public health implications,” they say.
In the meantime, the researchers suggest that patients with a history of pre-cancerous serrated polyps, especially women and smokers, may wish to avoid vitamin D and calcium supplementation.
The results appear in the journal Gut.
Source: UNC-Chapel Hill