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Does Dietary Fiber Reduce Risk of Colorectal Cancer?
October 5, 2009
††††††††† Have you ever been told to incorporate fiber in your diet? Many physicians and health care providers recommend the daily consumption of dietary fiber to relieve constipation, lower cholesterol, and decrease risk of diabetes. Many studies have been conducted that have shown these health benefits to be true. Companies that sell foods high in fiber release marketing campaigns to promote dietary fiber products as healthy choices. Although health professionals and society have acknowledged the many benefits to eating foods with dietary fiber, the effectiveness of fiber in reducing the risk of colorectal cancer still remains unclear. Many Americans believe that dietary fiber can reduce the risk of colorectal cancer, but conflicting results from numerous scientific studies have delayed the affirmation of dietary fiber as a prevention method against colorectal cancer. Physicians and other health professionals continue to encourage the consumption of foods with dietary fiber because of the other believed health benefits. Although studies show conflicting results regarding the role of fiber in reducing the risk of colorectal cancer, the proposed biological rationale for its potential benefit and studies that have shown a negative correlation between fiber and colorectal cancer provide enough evidence to include fiber in our diet. ††††
What is Dietary Fiber?
††††††† Dietary fiber refers to nondigestible carbohydrates that are found in plants (http://medicinenet.com/fiber/page2.htm). Sources of fiber are categorized into soluble and insoluble groups based on their ability to absorb water. Soluble fiber, which absorbs water into a gel, is found in oats, fruits, and beans. Insoluble fiber, which increases stool bulk and helps in moving materials through the digestive tract, is found in cereals and whole grains.†† ††††
What is Colorectal Cancer?
††††††† The purpose of the colon is to complete digestion in the large intestine by extracting excess water from food waste. Colorectal cancer refers to cancer that forms in the colon, the longest part of the large intestine, and cancer that develops in the tissues of the rectum. Colorectal cancer occurs when polyps on the lining of the large intestine become cancerous (http://www.mayoclinic.com/health/colon-cancer/DS00035.). Symptoms of colorectal cancer include rectal bleeding, blood in stool, abdominal discomforts, unexplained weight loss, and weakness or fatigue. If symptoms of colorectal cancer are present, physicians often perform a colonoscopy to take tissue samples of the colon and to locate abnormalities in the lining of the intestine.
††††††† Colorectal cancer can be removed through treatment especially when the cancer is diagnosed during an early stage of progression. If the cancer is small and isolated, a physician may be able to remove infected polyps during diagnostic exams. At later stages in cancer progression, surgery may be required for cancerous areas of the large intestine or chemotherapy and radiation techniques may be used (http://www.mayoclinic.com/health/colon-cancer/DS00035/DSECTION=tests-and-diagnosis). Colorectal cancer is the third most common cancer and the second most deadly cancer in America. Fortunately, a healthy lifestyle free from smoking and alcohol and a balanced diet low in fat, reduces oneís risk of colorectal cancer. †††
How did the perceived benefit of dietary fiber in reducing the risk of colorectal cancer originate?
††††††† The belief that dietary fiber reduces the risk of colorectal cancer has been debated for more than forty years. Denis Burkitt, a surgeon from Ireland, is often credited with proposing that dietary fiber reduces risk of colorectal cancer. He visited Uganda in the 1960s and observed that Ugandan people rarely displayed colon cancer while British residents in Uganda developed colon cancer and heart disease. He attributed these health differences to the rich fiber diet of Ugandan people and the low fiber diet of the British (http://understandingscience.ucc.ie/pages/sci_denisburkitt.htm). Burkittís 1979 book, ďDonít Forget Fibre in your DietĒ led to the popular belief that a healthy diet includes whole grains, beans, nuts, and fruits.
What claims are made about fiber and its role in reducing risk of colorectal cancer?
††††††† Due to conflicting evidence that dietary fiber reduces risk of colorectal cancer, many health information websites have chosen not to make a conclusive statement about the effects of fiber on the risk of colorectal cancer. For example, the Mayo Clinic website suggests that people concerned with preventing colorectal cancer should seek regular colon cancer screening from physicians and should not rely on a high-fiber diet to reduce risk of colorectal cancer (http://mayoclinic.com/health/fiber/NU00033).
††††††† Other health information websites such as WebMD, make claims that a high-fiber diet may reduce risk of colorectal cancer (http://www.webmd.com/colorectal-cancer/guide/risk-factors-colorectal-cancer). The American Institute for Cancer Research also explains that dietary fiber helps protect people from colorectal cancer (http://www.aicr.org/site.PageServer?pagename=elements_fiber). These websites reflect the common belief that fiber may still reduce risk of colorectal cancer despite some studies that show no protective effect. †††
††††††† Inconsistencies in experimental data can be shown in articles available through the American Cancer Society. The American Cancer Society reported on a study conducted by the National Cancer Institute, in which 34,000 people were interviewed about their dietary habits and then examined for cancerous colorectal polyps. It was found that people with more than 36 grams of fiber in their daily diet had the lowest incidence of polyps. Their risk of colorectal cancer was 27 percent lower than people that consumed less than 12 grams of fiber daily (http://www.cancer.org/docroot/NWS/content/NWS_1_1x_Studies_Find_High-Fiber_Diet_Lowers_Colon_Cancer_Risk.asp).
††††††† The American Cancer Society has also issued reports that fiber may not reduce the risk of colorectal cancer. In Cancer Epidemiology, Biomarkers and Prevention, researchers reported that the amount of fiber in oneís diet has no effect on the number of polyps present in the intestine. This conclusion was made after conducting a study of 1304 people in which half the participants were given a wheat bran supplement low in fiber and the other half were given a supplement high in fiber. The daily food consumption of the participants was recorded to determine the total fiber intake of each participant and the participants were evaluated for colorectal cancer. (http://www.cancer.org/docroot/NWS/content/NWS_1_1x_Fiber_May_Not_Reduce_Colon_Cancer_Risk.asp).
††††††† Many sources of health information recommend the daily consumption of fiber. The Harvard School of Public Health explains that fiber can reduce the risk of heart disease, diabetes, and diverticular disease, but does not reduce the risk of colorectal cancer (http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/fiber-full-story/index.html#intro). The Harvard School of Public health as well as other sources of health information choose to focus on the other perceived benefits of a diet rich in fiber.
What is the proposed rationale for how a fiber diet reduces risk of colorectal cancer?
††††††† Fiber increases contractions in the large intestine and expands the walls of the colon to allow waste to pass through more quickly. Soluble fiber also absorbs large amounts of water, which leads to bulkier and softer stool. In theory, the increase in the speed of digestion and the bulkier bowel movements caused by fiber consumption could remove harmful substances from the digestive tract before problems occur (http://www.gicare.com/diets/high-fiber-diets.aspx). Furthermore, an increase in speed of digestion decreases the time that cells in the intestine are exposed to potential carcinogens (http://www.diet.com/g/highfiber-diet). Other people propose that the risk of colon cancer diminishes when soluble fiber breaks down to produce short-chain fatty acids. Short-chain fatty acids, which are low in number during cases of inflammation in the colon, are believed to maintain microbial balances to prevent cancerous cells from forming (http://health.learninginfo.org/fiber.htm). More generally accepted is the idea that softer stool caused by fiber intake prevents constipation and greater strain on the colon. Irritable bowel, caused by defects that weaken the walls of the colon, can be avoided when fiber relieves pressure in the colon.
Do websites provide enough evidence for their claims about fiber reducing the risk of colorectal cancer?
††††††††† Many websites that provide basic health information to the general public are in the business of promoting healthy behaviors throughout society. Mayo Clinic presents information that is based on medical research and scientific literature and does not have an invested interest in promoting fiber consumption. Their website, which discusses the benefits of a high-fiber diet, reflects the medical beliefs at the present time (http://www.mayoclinic.com/health/fiber/NU00033). The website does not provide information on the studies that have led to their conclusions. The American Cancer Society provides people with access to numerous articles that discuss experiments that show conflicting results when testing the potential correlation between a high-fiber diet and a reduced risk of colorectal cancer (http://www.cancer.org/docroot/home/index.asp). Their annual statistics and information about colorectal cancer does not list the ingestion of fiber as a benefit in reducing the risk of colorectal cancer. As shown in these two examples, health information websites are interested in providing people with unbiased information that is well supported by medical research. Although these websites can be trusted, they do not always list the studies and medical sources they rely on for information. Due to inconclusive evidence about the benefits of fiber in reducing colorectal cancer, the Mayo Clinic and the American Cancer Society do not make conclusions about fiber reducing colorectal cancer on their websites. The websites do recommend fiber consumption because of the many health benefits associated with fiber.
††††††† Many physicians especially gastroenterologists recommend fiber in peopleís diets despite inconclusive evidence that fiber reduces risk of colorectal cancer. A website, sponsored by a group of gastroenterologists, explains the many health benefits of fiber ingestion and discusses the hypothesized mechanisms for which fiber could reduce the risk of colorectal cancer (http://www.gicare.com/diets/high-fiber-diets.aspx). These physicians are motivated to keep their patients healthy. By recommending fiber in each personís diet they can reduce each personís risk of other diseases (such as cardiovascular disease and diabetes) even in the event that fiber does not reduce the risk of colorectal cancer.
What does scientific literature reveal about the effects of Dietary Fiber on risk of Colorectal Cancer?
††††††† In a prospective study published in The New England Journal of Medicine, researchers found no association between the ingestion of dietary fiber and reduced risk of colorectal cancer. The study conducted by Fuchs et al. (1999) involved 88,757 women between the ages of 34 and 59 who had no history of cancer, bowel disease, or familial polyposis. The research participants filled out a questionnaire regarding their diet and over a sixteen year period cases of colorectal cancer were documented among the participants. Variables such as age, risk factors, and total energy intake were controlled for in order to more accurately analyze the relationship between fiber consumption and risk of colorectal cancer. During the follow-up period, 787 cases of colorectal cancer were diagnosed in the 88,757 possible participants. Fuchs et al. (1999) revealed that women, who consumed more fiber, ate less red meat, were less likely to smoke cigarettes, were more likely to exercise, and had more calcium in their diet. Although the presence of cardiovascular disease in the participants was not documented in the results, one would expect the participants that ate fiber to have a lower incidence of heart disease (based on lowering risk through a healthy lifestyle).
††††††† The large number of participants in the experiment helps legitimize the results by eliminating possible variation in the tested population. Despite the advantages of having a larger cohort, surveys were only given every two years to measure fiber consumption. Furthermore, the validity of the research results relies on participants to accurately monitor their fiber consumption over many years. The results of the study depend heavily on participants documenting their eating habits and truthfully reporting in surveys. The experiment seems to have been carried out well even with possible sources of error.
††††††† Park et al. (2005) recently published a study in the Journal of the American Medical Association which incorporates experimental analysis from thirteen prospective cohort studies involving dietary fiber intake and risk of colorectal cancer. In order for a study to have been included in this pooled study, fifty cases of colorectal cancer must have been reported, dietary intake and fiber ingestion must have been assessed, and the dietary assessment method must have been used. The pooled study showed that dietary fiber lowered the risk of colorectal cancer when only age was controlled. When the results were adjusted for other colorectal cancer risk factors, an association no longer existed between dietary fiber and risk of colorectal cancer. Although Park et al. (2005) has received great consideration in the debate over whether or not fiber reduces risk of colorectal cancer, some inconsistencies such as the range of dietary fiber intake, across the studies considered, point to potential bias.
††††††† Both of the previous studies discussed found no association between fiber and a lower risk of colorectal cancer, but a recent study published in the Lancet, Bingham et al. (2003), provides evidence against these conclusions. The study included 519,978 individuals aged 25-70 and the participants were required to fill out a dietary questionnaire to determine fiber consumption. Bingham et al. (2003) concluded that people who ate the most fiber, at least 33 grams a day, had a 25% lower incidence of colorectal cancer than people that ate the least fiber or 12 grams a day. Although this study has produced promising results, many confounding variables need to be considered that may have influenced the results. Individuals that have high-fiber diets are also consuming vitamins (in fiber products) that may protect people from colorectal cancer. As shown in the study from the New England Journal of Medicine, people with high-fiber diets also tend to eat less red meat and exercise more frequently; thus, reducing the risk of cancer. Therefore, the results of Bingham et al. (2003) can not conclusively show that fiber intake alone reduces risk of colorectal cancer.
††††††† Peters et al. (2003) also showed a correlation between high dietary fiber consumption and a decreased risk of colorectal cancer. The study evaluated 33,971 participants who initially showed no sign of polyps in their colon. A food questionnaire was distributed to the participants to measure their fiber consumption. At the end of the study 3591 participants had at least one adenoma polyp, which have the potential to become cancerous. After adjusting for dietary and non-dietary factors, data analysis revealed that people who ate the most fiber in their diet had a 27% less chance of developing colorectal adenomas than participants that ate the least amount of fiber. These results have led to continued debate over the effects of fiber on the risk of colorectal cancer. By measuring the presence of colorectal adenomas and not the incidence of colorectal cancer, the researchers can not be sure that all the patients with adenomas would have developed full blown colorectal cancer. Furthermore, the types of fiber consumed (soluble or insoluble) must also be considered when conducting these studies because one type may have more of an effect on protecting both the colon and the rectum.
††††††† The inconsistencies found in the conclusions of these studies can be attributed to many factors including the type of fiber being ingested by participants, confounding lifestyle factors, and differences in experimental design. According to Barron (2005), insoluble fibers such as wheat fiber have been found to be protective against colorectal cancer in some studies and insignificant in other studies. In addition, soluble fibers such as fruits and vegetables have wavered in their effectiveness at reducing risk of colorectal cancer in various studies. Controlling for the types of fibers that people ingest in a prospective study would be difficult to achieve (the different types could be documented). Confounding variables involving the lifestyle of participants adds difficulty in measuring the actual effect of fiber on the risk of colorectal cancer. Sometimes lifestyle factors such as exercise can be identified and adjusted for in statistical analysis, but there are often some unidentified factors that can influence results. Barron (2005) also identifies differences in how studies present their statistics. Experimental design can attribute to some of the discrepancies in the data displayed.† †
††††††† ††††††† †† †
††††††† Colorectal cancer seems to result from multiple genetic, environmental, and dietary factors. Considering the many factors that can influence the onset of colorectal cancer or reduce an individualís risk for colorectal cancer, it may be impossible to determine experimentally if fiber alone reduces risk of colorectal cancer. As shown by the differing conclusions of medical research studies, the effect of fiber on the risk of colorectal cancer has not been determined with any consistency. There are many benefits to consuming fiber which include lowering cholesterol levels, lowering risk of digestive conditions, improving blood sugar levels, and helping in weight loss. Due to the known benefits of fiber, physicians recommend that individuals consciously consume fiber in their diets. If in fact dietary fiber reduces risk of colorectal cancer, then people who consume high amounts of fiber in their diet will benefit further from their already healthy behavior.††† †††
††††††††††††††††††††††††††††††††††††††† Works Cited
Baron, J.A. (2005). Dietary Fiber and Colorectal Cancer: An Ongoing Saga. Journal of
††††††† the American Medical Association, 294 (22), 2904-2906.
Fuchs, C.S., Glovannucci, E.L., Colditz, G.A., Hunter, D.J., Stampfer, M.j., Rosner, B., † et al. (1999). Dietary Fiber and the Risk of Colorectal Cancer and Adenoma in Women. The New England Journal of Medicine, 340 (3), 169-176.
Bingham, S.A., Day, N.E., Luben R., Ferrari P., Slimani, N., Norat T., et al. (2003).
††††††† Dietary fibre in food and protection against colorectal cancer in the European ††††††† Prospective Investigation into Cancer and Nutrition (EPIC): an observational ††††††† study. The Lancet, 361 (9368), 1496-1501.
Park, Y., Hunter, D.J., & Spiegelman, D. et al. (2005). Dietary Fiber Intake and the Risk ††††††† of Colorectal Cancer: A Pooled Analysis of Prospective Cohort Studies. Journal †††† of the American Medical Association, 294 (22), 2849-2857.
Peters, U., Sinha R., Chatterjee, N., Subar, A.F., Ziegler, R.G., Kulldorff M., et al. ††††††† (2003). Dietary fibre and colorectal adenoma in a colorectal cancer early detection ††††††† programme. The Lancet, 361 (9368), 1491-1495.†
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