Calcium supplementation, alone or with vitamin D, has been shown to increase the risk of sessile serrated adenomas or polyps (SSA/Ps) in people with a history of polyps.

The findings from the Vitamin D/Calcium Polyp Prevention Study were a surprise for US researchers who expected the supplements to either have no effect or a protective effect on serrated polyps.

The multi-centre trial comprised more than 2,000 participants, 45-75 years of age with a history of at least one adenomatous polyp detected and removed, who were scheduled for surveillance colonoscopy in three to five years.

Patients were randomised to either 1,200mg daily calcium, 1,000 IU daily vitamin D, both, or no supplements. People with known familial colorectal cancer syndromes were excluded from the study.

The study found 1,100 serrated polyps during colonoscopies at the end of the treatment phase of the study but no significant difference in polyp detection rates or polyp sub-type between the four groups.

However subsequent colonoscopies more than three years later identified a significant increase is SSA/Ps in both the calcium and calcium plus vitamin D groups. Women and smokers were at the highest risk.

“The possible mechanisms by which calcium (alone or in addition to vitamin D) could influence serrated neoplasia are uncertain,” the researchers said.

“Our findings of the relatively long latency period would suggest that calcium (with or without vitamin D) likely influences early events in the serrated neoplasia pathway rather than late effects such as progression to dysplasia or cancer.”

The researchers said their findings had important public health implications given the number of people taking the supplements.

“Women represent the majority of those taking calcium supplements in the general population. It is interesting therefore that SSA/Ps are at least as common if not more common in women than men, in contrast to conventional adenomas, which are more common in men.”

They concluded that patients with a history of premalignant serrated polyps, especially women and current smokers, might wish to avoid vitamin D and calcium supplementation.

There was no evidence that dietary calcium intake or vitamin D supplements alone were associated with polyp risk.