A large, randomized study at 11 US hospitals has found that vitamin D and calcium supplements fail to protect against developing colorectal cancer.

The study results, published in the New England Journal of Medicine, did not support earlier observational studies suggesting a link between lower colorectal cancer risk with higher vitamin D blood levels or greater calcium intake.

More than 2,200 adults at 11 academic medical centers and affiliated medical practices took part in a randomized, placebo-controlled trial of supplementation with vitamin D3 (1000 IU/day) and/or calcium (1200 mg/day) for the prevention of colorectal adenoma. Commonly known as polyps, colorectal adenomas are benign tumors that can develop into cancer. Adenomas are detected and removed during colonoscopy performed for routine health screening for patients over 50.

Each study participant, aged 45-75, had a history of colorectal adenoma without any remaining polyps after colonoscopy. Patients were randomly assigned to take either vitamin D3 or calcium; or both; or neither. Contrary to the expectations of the study's investigators, daily supplementation for 3-5 years did not reduce the risk of recurrent colorectal adenomas among study participants.

"What makes this study significant is that clinicians are routinely screening for vitamin D in routine health checks, and often recommending that patients take supplements," said Elizabeth L. Barry, PhD, a researcher at Dartmouth-Hitchcock's Norris Cotton Cancer Center and at the Geisel School of Medicine at Dartmouth. "For prevention of colorectal adenoma, our study showed no benefit in using these doses of daily supplements, much to our surprise. Prior observational studies suggested that low levels of vitamin D in humans may increase risk of colorectal cancer, but in the more rigorous setting of a randomized placebo-controlled trial, we found that vitamin D and calcium supplementation did not reduce the risk of colorectal adenomas, which are cancer precursors."

Researchers are now looking more closely at data suggesting that differences in participants' genetic makeup might influence response to treatment with vitamin D and calcium, and whether longer-term treatment with vitamin D might be needed for prevention. In the meantime, Barry said, "It is important to emphasize the importance of colonoscopy and polyp removal as the most effective way to protect against colorectal cancer."