Fiber-based laxatives linked to decreased risk of colorectal cancer: study

A large, prospective study involving more than 75,000 adults in western Washington has found that frequent use of fiber-based laxatives is associated with a decreased risk of colorectal cancer while the use of non-fiber laxatives is linked to significantly increased risk of the disease. The study also found no association between bowel-movement frequency and constipation with regard to colorectal cancer risk.

The findings, published in the Oct. 7 print issue of The American Journal of Gastroenterolgy, are important because an estimated 20 percent of Americans use laxatives, according to the paper’s first and corresponding author, Jessica Citronberg, M.P.H., a predoctoral fellow in the Epidemiology Program of the Public Health Sciences Division at Fred Hutchinson Cancer Research Center. She is also a doctoral student in epidemiology at the University of Washington School of Public Health.

While non-fiber laxatives – those most commonly used in the U.S. – work by forcing the colon to contract, fiber-based laxatives rely on increasing the water content and bulk of the stool to move it through the colon.

Citronberg and colleagues found that people who use non-fiber laxatives five or more times a year had a 49 percent increased risk of colorectal cancer, while those who used fiber-based laxatives at least four days a week for four years had a 56 percent lower chance of the disease as compared to those who did not use them.

“I was just surprised to see such a strong association between laxative use and colorectal cancer risk,” Citronberg said. “I didn’t expect the results to be as strong as they were.”

The analysis, which assessed frequency of bowel movements, constipation and 10-year laxative use, was based on data from 75,214 men and women aged 50-76 years in western Washington who were part of a larger study to investigate the association between vitamins, minerals and other dietary supplements on cancer risk. The study participants were surveyed initially between 2000 and 2002 and followed until 2008, during which time 558 of the study participants were diagnosed with colorectal cancer. People with a history of colorectal cancer, Crohn’s disease and intestinal polyps were excluded from the study.

While previous research in laboratories and in animals has found stimulant laxatives to have carcinogenic and gene-mutating effects, prior cohort studies on laxative use have produced inconsistent results regarding colon cancer risk.

“Given that we found CRC [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,” the authors reported.

The current study is the first of its kind to examine the association between fiber, or bulk, laxatives and colorectal cancer risk. The authors hypothesize that soluble-fiber laxatives may possess some of the protective effects postulated for dietary fiber. Consuming dietary fiber is thought to reduce colorectal cancer risk by diluting carcinogens in the stool, encouraging growth of healthy bacteria in the colon, and, by increasing stool transit time, decreasing the duration by which carcinogens are in contact with the colon.  

The study’s strengths included its large number of participants and the fact that the data took into account various factors that can influence colorectal cancer risks. In addition, its prospective design allowed information on bowel habit and laxative use to be collected before diagnosis of colorectal cancer, which reduced recall bias among the participants. The study’s limitations included the reliance on the participants’ self-report as well as a lack of detail about stool consistency, as previous studies have found loose stools to be related to colon cancer risk. In addition, it is unclear whether the study’s demographics –older, mostly white, fairly healthy, supplement-taking western Washington residents – could be extrapolated to represent the entire U.S. population.

Given the study’s limitations, more research is needed before Citronberg and colleagues can make clinical recommendations regarding laxative use, she said.

“I wouldn’t necessarily jump the gun and say because of this study people should stop taking stimulant laxatives,” she said. “I think the better route to go would just be to have a healthy diet.” Citronberg did, however, emphasize the significance of the null findings around constipation and bowel motility.

“I guess the takeaway is there’s no association between bowel-movement frequency, constipation and colorectal cancer risk, but we did see an association with laxative use,” Citronberg said. “While the study results suggest that non-fiber laxatives increase your risk and fiber laxatives decrease your risk of colorectal cancer, more research is needed.”

The National Cancer Institute and the Office of Dietary Supplements, both branches of the National Institutes of Health, funded the research. Study collaborators included senior author Emily White, Ph.D., of the Fred Hutch Public Health Sciences Division and John Potter, Ph.D., of Massey University in Wellington, New Zealand.

Editor’s note: To obtain a copy of The American Journal of Gastroenterology paper, “A Prospective Study of Bowel Movement Frequency, Constipation, and Laxative Use on Colorectal Cancer Risk,” or to arrange an interview with Citronberg, please contact Kristen Woodward at media@fredhutch.org or 206-667-2210.

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