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The results contradict decades of observational studies that showed that people who take lots of calcium and vitamin D are less likely to develop colon cancer, researchers said in the Oct. 15 issue of the New England Journal of Medicine.
"It suggests that you should not think you should take calcium or vitamin D to reduce your risk of colon cancer," said study co-author Elizabeth Barry, an assistant professor of epidemiology and community and family medicine at Dartmouth's Geisel School of Medicine in New Hampshire.
However, Barry added that this trial focused on a relatively low dose of the two supplements taken over a 3- to 5-year time frame.
"We didn't address whether a higher dose or a longer period of time would have been effective," she said, noting other trials are underway that could shed additional light on the question.
For 15 to 20 years, studies have suggested that vitamin D and calcium are protective against colon cancer, said study lead author Dr. John Baron, a professor of medicine at the University of North Carolina School of Medicine. Baron himself wrote one of those reports.
"For calcium, we conducted one of the earlier trials that showed a benefit," Baron said. "That was roughly 20 years ago. In fact, we used the same agent in the same sort of patients, in many of the same settings."
To test this protective benefit, the researchers behind the new study randomly assigned more than 2,200 participants to take daily either 1,200 milligrams of calcium carbonate or 1,000 (International Units) IU of vitamin D separately, to take both together or to take a placebo.
All participants had recently had precancerous colon polyps, also called adenomas, removed from their colon. They were given colonoscopies to make sure they were polyp-free at the start of the study, and had a follow-up colonoscopy three to five years later to assess how well the supplements had helped prevent a recurrence.
"We studied these polyps because they're an early stage in the cancer pathway, so it takes a shorter amount of time for vitamin D to have an effect, if it has one," Baron said.
About 43 percent of participants had developed a recurring precancerous polyp by the time of their follow-up colonoscopy, researchers found.
But they found that vitamin D, calcium or the two combined had no effect on a person's risk for recurring polyps, when compared to people taking a placebo.
Baron said he was particularly surprised by the results for calcium, given the large body of evidence suggesting its cancer-preventing benefits.
The changing health profile of the average American may have affected the calcium results, Baron said. For example, more of the subjects in the new trial were obese, and fewer smoked.
"We're looking into things like that to see if they explain the differences between the earlier and the current study," he said.
Dr. Arun Swaminath, director of the inflammatory bowel disease program at Lenox Hill Hospital in New York City, called the report an "important negative study."
"Is this the end of the road for vitamin D in the prevention of precancerous colon polyps and, hence, colorectal cancer?" Swaminath said. "Well, the authors suggest that in today's world, that 1,000 IU of vitamin D would be considered a quite low dose of supplementation, so maybe even higher doses would have been beneficial."
Swaminath added that the supplements might have better protective benefits if given to patients who haven't already had precancerous polyps.
"Is it possible that choosing higher-risk patients, ones with adenomas already detected, may be too late in the lifetime of colon health to try this supplement?" he asked. "Maybe it would have been more useful in patients who didn't have adenomas to begin with? Of course, we don't have the answers to that just yet."
Barry and Baron emphasized that this study focused on supplements, and that their findings don't negate the overall healthy benefits of a nutrient-rich diet.
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