Chapel Hill, NC—Pharmacists might want to warn customers using calcium supplements that the products, however harmless they might seem, aren’t without risk.

A new study in the journal Gut finds that calcium supplements, taken with or without vitamin D, may increase the chance of developing colon polyps. University of North Carolina School of Medicine researchers led the large U.S. trial.

Background information in the study notes that serrated lesions such as sessile serrated adenomas or polyps (SSA/Ps) are important colorectal cancer precursors. Study authors sought to determine the effects of calcium and vitamin D supplementation on the incidence of serrated polyps (SPs) in general, and hyperplastic polyps and SSA/Ps, specifically.

To do that, researchers randomized participants with one or more adenomas at baseline to receive 1,200?mg/day of elemental calcium, 1,000 IU/day of vitamin D3, both or neither agent. With treatment extending from 3 to 5 years, risk of polyps was determined from surveillance colonoscopy during the treatment phase. Outcomes after treatment ceased were also assessed in an observational phase.

The study states that SPs were diagnosed in 565 of 2,058 participants (27.5%) during the treatment phase and 329 of 1,108 (29.7%) during the observational phase. Overall, 211 SSA/Ps were identified during follow-up.

Results indicate that, during the treatment phase, no effect was determined for either calcium or vitamin D on incidence of SSA/Ps. During the observational phase, however, researchers report elevated risks of SSA/Ps associated with calcium alone, as well as with calcium plus vitamin D treatment (aRR (95%?CI): 2.65 (1.43-4.91) and 3.81 (1.25-11.64), respectively).

“In a large multicenter chemoprevention study, we found evidence that calcium and vitamin D supplementation increased the risk of SSA/Ps,” study authors conclude. “This appeared to be a late effect: 6-10 years after supplementation began. These possible risks must be weighed against the benefits of calcium and vitamin D supplementation.”

Researchers call for further studies to confirm their results and also suggest that any risks be balanced against the benefits of calcium supplementation.

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