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Dietary Fiber


Dietary fiber is traditionally defined as plant material that cannot be digested by human enzymes. That means that when we consume fiber it is not broken down as is food, and not absorbed into the blood. Instead, fiber passes into the large intestine where it may be fermented by the bugs (called colonic microflora) that reside there. When fermented, fiber produces gases (carbon dioxide, methane and hydrogen) and short chain fatty acids. Gases may produce distension and other unpleasant side effects, whereas one of the short chain fatty acids, butyrate, is the primary fuel for colon cells. There has been much debate in recent months as to whether or not fiber is truly protective against colon cancer. Although more than 80% of population studies show a protective effect of fiber-containing diets, the two large clinical trials did not show a positive benefit to colon polyp recurrence. Colon polyps are considered precursors to colon tumors. In these two trials, all polyps were removed from colons of participating individuals, the intervention groups were provided with high fiber diets (compared to controls) and then 3 years later the participants underwent colonoscopies to detect polyps. There was no difference between the control group and the high fiber groups in polyp recurrence, which appears to contradict the previous population studies. Unfortunately, because of these two studies, popular articles have suggested that fiber does not play a role in human health. This is far from true. First, results of these two clinical trials should be interpreted cautiously since only a small proportion of polyps go on to become tumors and the trials may not have detected the premalignant polyps. Also, colon cancer takes many years to develop.A relatively short intervention period may not be of sufficient length to have a protective effect. Finally, and most importantly, fiber containing foods are also high in phytochemicals (substances known to protect against cancers) and high in many vitamins and minerals which are necessary to human health. Fibers such as oat bran and pectin (found in oranges and apples) lower serum cholesterol and protect against heart disease, and these same fibers flatten blood glucose curves, thus positively affecting insulin response and are recommended for diabetics. Importantly, fiber dilutes the energy density of the diet and thus enhances satiety, helping to protect against overeating and potential obesity. The sum knowledge from all of the animal studies, clinical intervention trials, and years of population studies strongly suggest that fiber-containing foods provide multiple benefits for a healthy life.

References for further information:

American Dietetic Association (1993). “Position of the American Dietetic Association: health implications of dietary fiber.” J Am Diet Assoc, 93, 1446-1449.

Anderson, J. W., Chen, W. L., and Sieling, B. (1980). “Hypolipidemic effects of high- carbohydrate, high-fiber diets.” Metabolism, 29, 551-558.

Baghurst, B. A., and Rohan, T. E. (1994). “High fibre diets and reduced risk of breast cancer.” Int J Cancer, 56, 173-176.

Blundell, J. E., and Burley, V. J. (1987). “Satiation, satiety and the action of fibre on food intake.” Int J Obesity, 11, 9-25.

FAO/WHO. (1997). “Expert consultation on carbohydrates in human nutrition.”, FAO/WHO, Rome.

FASEB Life Sciences (1987). “Physiologic Effects and Health Consequences of Dietary Fiber.”, US Department of Agriculture, Center for Food Safety, Washington, DC.

Fuchs, C. S., Giovannucci, E. L., Colditz, G. A., Hunter, D. J., Stampfer, M. J., Rosner, B., Speizer, F. E., and Willett, W. C. (1999). “Dietary fiber and the risk of colorectal cancer and adenoma in women.” N Engl J Med, 340, 169-76.

Giovannucci, E., and Willett, W. C. (1994). “Dietary factors and risk of colon cancer.” Ann Med, 26, 443-52.

Jenkins, D. J. A., Popovich, D. G., Kendall, C. W. C., Vidgen, E., Tariq, N., Ransom, R. P. P., Wolever, T. M. S., Vuksan, V., Mehling, C. C., Boctor, D. L., Bolognesi, C., Huang, J., and Patten, R. (1997). “Effect of a diet high in vegetables, fruit and nuts on serum lipids.” Metabolism, 46, 530-537.

Life Sciences Research Office, (1987). “Physiological effects and health consequences of dietary fiber.” , Federation of American Societies for Experimental Biology, Bethesda, MD.

National Institutes of Health. (1984). “Diet, nutrition, and cancer prevention: a guide to food choices.” NIH publication no.l 87-2711, National Institutes of Health, Bethesda, MD.

Potter, J. D. (1990). “The epidemiology of fiber and colorectal cancer. Why don't the analytical epidemiology data make better sense?” , D. Kritchevsky, C. Bonfield, and J. W. Anderson, eds., Plenum Press, New York, New York, 431-445.

Willett, W. C., Hunter, D. J., Stampfer, M. J., Colditz, G., Manson, J. E., Spiegelman, D., Rosner, B., Hennekens, C. H., and Speizer, F. E. (1992). “Dietary fat and fibre in relation to risk of breast cancer.” <>JAMA, 268, 2037-2044.

Joanne Lupton, Ph.D., is a Regent Professor and William W. Allen Endowed Chair in Human Nutrition at Texas A & M University.

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