Calcium supplements, taken with or without vitamin D, may increase the risk of polyps growing in the colon, suggest researchers following a large US trial.
As a result, 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.
They noted, however, that further studies were recommended to confirm their results and any possible risks must be weighed against the benefits of supplementation.
They highlighted that some previous studies have suggested that calcium and vitamin D may protect against colon polyps, but results have been mixed.
To investigate further, the researchers set out to determine whether taking daily calcium and vitamin D supplements reduce the risk of serrated polyps.
They noted that serrated polyps were less common than conventional “adenomatous” polyps, but probably had the same risk of developing into cancer.
They analysed findings from a large US trial involving over 2,000 patients aged between 45 and 75 who had a history of polyps and were due to have a follow-up colonoscopsy in three to five years.
Patients were excluded if they had a family history of bowel cancer, inflammatory bowel disease, or other serious health conditions, and several factors were taken into account at the start of the study.
The remaining patients were randomly split into groups to receive either daily calcium supplements, daily vitamin D supplements, both or neither for three or five years until their colonoscopsy.
During the three to five year treatment phase of the trial, there was no effect of either calcium or vitamin D on cases of serrated polyps, according to the researchers.
However, six to 10 years after treatment began, they 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 no association was found between vitamin D alone and the risk of serrated polyps.
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The results also suggest an association with calcium supplements only, not dietary calcium, said the researchers from the University of North Carolina School of Medicine.
Further research was recommended to confirm their results, which they described as “surprising”, said the study authors in the journal Gut.
But they concluded that there would be “important public health implications” if calcium and its combination with vitamin D were truly associated with an increased risk of serrated polyps.
They highlighted that calcium supplements were taken by millions of people around the world and the findings may have important implications for bowel cancer screening and prevention.
In the meantime, 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.