It was found that taking calcium supplements, with and without vitamin D, may increase the risk of polyps in the colon or rectum.
The researchers, who report their findings in the journal Gut, recommend that additional studies should now be done to validate their results.
Should the findings be confirmed, however, the study authors believe that they will have "important implications" for the screening and prevention of colorectal cancer.
Previous studies investigating the effect of calcium and vitamin D supplements on polyp prevention have yielded inconsistent results.
The American Cancer Society (ACS) estimate that in the United States in 2017, there were 95,520 newly diagnosed cases of colon cancer and 39,910 of rectal cancer, and that 27,150 men and 23,110 women died from one of these diseases.
Cancer starts as a polyp
Colorectal cancer usually starts as a polyp, which is a noncancerous growth that develops in the lining of the colon or rectum. Polyps grow very slowly, often taking up to 20 years to form.
There are different types of polyp, depending on their shape, size, and tissue characteristics. The most common type, which is usually referred to as an adenoma, is an adenomatous polyp.
The new study, led by researchers from several U.S. research centers, focused on another type of polyp known as serrated polyps — which, although not as common as adenomatous polyps, are thought to carry the same risk of becoming cancerous.
While regular colonoscopic screening with removal of suspicious polyps is the ideal way to reduce colorectal cancer rates from the clinical point of view, population-wide implementation is limited in practice for various reasons, including "suboptimal adherence, access, and expense."
Thus, as the authors of the new study note, "In addition to screening, chemoprevention with various agents has been frequently studied as a means of reducing the incidence of [colorectal cancer] and its precursors."
Higher risk with calcium supplementation
The new analysis utilized the data from a recently completed chemoprevention trial of calcium and vitamin D in 2,259 patients aged 45–75. The participants had already had at least one colorectal adenomatous polyp removed during a baseline screening and were due to have a follow-up colonoscopy in 3–5 years.
Once those subjects with a family history of colon or rectal cancer, inflammatory bowel disease, or other serious conditions were excluded, the remainder were randomly assigned to four treatment groups.
The first received daily calcium supplements, the second received daily vitamin D supplements, the third received both supplements, and the fourth group did not receive any supplements.
All the groups remained in the treatment phase until the next screening colonoscopy in 3–5 years. After the second colonoscopy, there followed the observation phase, during which there was no supplementation.
This phased ended with a third screening colonoscopy, which took place 6–10 years after the start of supplementation.
The colonoscopy results at the end of the treatment phase showed that neither vitamin D nor calcium nor both seemed to affect the incidence of cases of serrated polyps.
However, the results of the third colonoscopy — which was performed some 6–10 years after the start of supplementation — revealed a higher incidence of serrated polyps in the patients who took calcium, either on its own or with vitamin D.
No such link was found for vitamin D on its own. Women and people who smoked were those at higher risk of serrated polyps from taking calcium supplements.
The study authors note that although the data that they used came from a large randomized trial, their results came from a secondary analysis, and some of the results might have occurred by chance. Thus, they conclude that:
"Further studies are recommended to confirm these results, which may have important implications for [colorectal cancer] screening and prevention."
Meanwhile, they suggest that people who have, or have ever had, precancerous serrated polyps — particularly women and those who smoke — may prefer to avoid taking calcium and vitamin D supplements.