Studies reveal mixed opinions on fiber intake’s relation to colorectal cancer.
For decades, researchers thought that a high fiber diet lowered the risk of colorectal cancer and cardiovascular disease. And while studies have shown fiber’s benefit in the cardiovascular realm, the results of recent research have exposed varying opinions to its relationship to colorectal cancer.
In a pooled analysis, published in 2005 in the Journal of the American Medical Association, of more than 725,000 men and women followed for 6 to 20 years in 13 studies, scientists found no association between dietary fiber intake and colorectal cancer risk, says lead researcher Yikyung Park, ScD, from the Division of Cancer Epidemiology and Genetics at the National Cancer Institute.
Results of the Polyp Prevention Trial revealed the same. In this study, half of participants adopted a high-fiber, low-fat diet and half did not. After four years, the polyp recurrence rate was about the same in both groups. (Adenomatous polyps are considered precursors to colon cancer.) In 2008, a follow-up on the study failed to show any difference after eight years on the diet.
The mechanisms at which fiber affects the gastrointestinal tract for its protective role continue to remain unclear.
And yet, results of the European Prospective Investigation into Cancer and Nutrition showed a lower incidence of colorectal cancer with a high-fiber diet, and a 2007 consensus report from the World Cancer Research Fund and the American Institute of Cancer Research concluded that dietary fiber may lower the incidence of colorectal and esophageal cancers.
Park says these mixed results may lie in the differences in study design, populations, and data analyses methods. “One of the challenges the observational studies confront is that people who consumed a large amount of dietary fiber tended to have [healthier] lifestyles than those who consumed less fiber,” observes Park. “Thus, it is difficult to tease out the effect of fiber from that of the general healthy lifestyle.”
Park adds that definitions of fiber vary as well, as fiber isn’t “a single entity, but a mixture of plant substances with different properties.” In order to clarify, the Institute of Medicine recommended changing the definitions of soluble and insoluble fibers to dietary and functional fibers.
Dietary fiber includes plant foods, such as apples or oatmeal, that combine fiber and other nutrients and can be soluble, insoluble, or both. Functional fibers are extracted from their natural sources, such as pectin taken from citrus peel. When dietary and functional fibers are added together, their sum is total fiber. Recommended total fiber intakes differ based on age but are about 38g/day for men and 25g/day for women 19 to 50 years of age.
These differing properties are being studied to further define fiber’s role in cancer prevention, says Clare McKindley, RD, LD, a clinical dietitian at MD Anderson Cancer Center in Houston. “Fiber fermentation [in the gastrointestinal tract] leads to the release of butyrate, and this compound can induce apoptosis, cell-cycle arrest, and cell differentiation, limiting risk of the development of cancer.” McKindley adds other compounds present in fiber, such as antioxidants and nutrients like folate, also may play a role in neutralizing cancer cells.
Combine that with the estimated 100 trillion microflora in the gut that also react with fiber and other substances, and the complexity of pinpointing this research becomes understandable. “The mechanisms at which fiber affects the gastrointestinal tract for its protective role continue to remain unclear,” says McKindley.
These confusing results only leave questions for the cancer patient or survivor; however, experts say do not abandon high-fiber diets. Fiber has many other health benefits, such as helping to lower the risk of cardiovascular and diverticular disease. Fiber is also prescribed for constipation, a side effect of some cancer treatments.
Both Park and McKindley agree that much work in the area of fiber and colorectal cancer still needs to be done. “The consensus may change in the future depending on the results from more studies,” says Park. “Thus, more research is needed.”