A delayed impact on bowel health has been noted with the use of calcium supplements, either on its own or in combination with vitamin D.

A long-term US study looking at bowel polyp growth assessed the impact of supplementation using 1,200€‰mg/day of elemental calcium, 1000 IU/day of vitamin D3, both or neither agent. Treatment was continued for 3 or 5 years, and colonoscopies were conducted during the period of treatment and then in follow up after treatment ended.11,12

While polyps are non-cancerous in themselves, they have the potential to turn cancerous. In this study, the researchers were assessing prevalence of serrated polyps in over 2,000 patients aged between 45-75 in the study.

There was no effect of either calcium or vitamin D on cases of serrated polyps during the treatment phase. However, in the period 6-10 years after treatment began, there were €increased risks of serrated polyps among patients taking calcium alone and among those taking a combination of calcium and vitamin D.€ Women and smokers were found to be at higher risk, but there was no increased risk when taking vitamin D alone.