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Still, the researchers believe the findings are important because about 20 percent of Americans use laxatives.
According to a team led by Jessica Citronberg, a predoctoral fellow at Fred Hutchinson Cancer Research Center in Seattle, non-fiber laxatives are the most widely used in the United States and work by forcing the colon to contract.
On the other hand, fiber-based laxatives boost the water content and bulk of the stool in order to move it through the colon, they said.
The new study involved data on more than 75,000 adults, aged 50 to 76, in western Washington state. The investigators found that people who used fiber-based laxatives at least four days a week for four years were 56 percent less likely to develop colorectal cancer than those who didn't use them.
In contrast, people who used non-fiber laxatives five or more times a year had a 49 percent increased risk for colorectal cancer, according to the findings published in the Oct. 7 issue of The American Journal of Gastroenterology.
"I was just surprised to see such a strong association between laxative use and colorectal cancer risk. I didn't expect the results to be as strong as they were," Citronberg said in a Hutchinson news release.
The researchers theorize that fiber-based laxatives may provide some of the same protective effects believed to be offered by dietary fiber, which is thought to reduce colon cancer risk in a number of ways, such as encouraging the growth of healthy bacteria in the colon and diluting levels of cancer-causing agents in the stool.
Dietary fiber also speeds stool transit time, thereby decreasing the amount of time that cancer-causing agents are in contact with the colon, the study authors noted.
According to the researchers, prior studies on laxatives have yielded inconsistent findings about colon cancer risk.
"Given that we found colorectal cancer risk to be dependent on laxative type, findings from the current study would help to explain the inconsistencies in previous studies that grouped all laxative types together," Citronberg and colleagues wrote.
The authors stressed, however, that further research is needed before any recommendations about laxative use can be made.
"I wouldn't necessarily jump the gun and say because of this study people should stop taking stimulant laxatives," Citronberg said. "I think the better route to go would just be to have a healthy diet."
Two experts in colon cancer said the new study offered intriguing data.
"This study now seems to show that there is a clear association between the use of fiber-based laxatives and a decreased risk for the development of colon and rectal cancer," said Dr. Jules Garbus, attending colorectal surgeon at Winthrop-University Hospital in Mineola, N.Y. "More research needs to be done; however, this is a very exciting start."
Dr. Gina Sam is a gastroenterologist and director of the Motility Center at the Mount Sinai Hospital in New York City. She agreed with Garbus, saying that, "this is a very remarkable study and we need more research to investigate why this [association] happens."
-- Robert Preidt
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