Gilberto Santana-Rios, Ph.D.
Q. What is fiber?
Dietary fiber is not a single chemical entity that can be easily categorized. Fiber can be defined as the fraction of plant material that is not broken down by human gastrointestinal microbes. Dietary fiber is divided into two main groups: "soluble" and "insoluble." These two types of fiber vary in foods and probably have quite different effects on human health. Insoluble fiber can bind to carcinogenic compounds and bile acids, helping in their excretion from the body. Insoluble fiber also adds bulk to the intestinal content and decreases fecal transit time, leading to the suggestion that it might offer protection against cancer by acting as a "colonic broom" and speeding material through the intestines. Soluble fiber participates in fermentation processes in the colon, yielding products (short-chain fatty acids) that have been proposed to help optimize colonic function. The ratio of soluble to insoluble may be important to consider when evaluating the cancer chemoprotective effect of dietary fiber.
Q. Does fiber protect against colorectal cancer?
A recent study published in The New England Journal of Medicine concluded that, contrary to popular belief, dietary fiber does not protect against cancer of the colon. The research team, led by Charles S. Fuchs of the Dana-Farber Cancer Institute in Boston, conducted a prospective study of 88,757 women over a 16 year period. This group of women is part of the Nursesí Health Study, an epidemiological study begun in 1976 with 121,700 female registered nurses with the goal of analyzing their nutritional habits over a long period of time and then making correlations with disease incidence.
Q. How much fiber did the nurses consume each day?
In the analysis, women were divided into 5 groups based on their fiber intake. The daily consumption of fiber by some women was 10 grams, and the group with the highest intake of fiber consumed an average of 25 grams of fiber per day, which is at the lower end of intake recommended by the National Institutes of Health (25 to 35 grams per day). A further subdivision of cohorts might have resulted in a group that consumed close to the actual recommended dietary intake of fiber, possibly providing more insight into the relation to disease.
Q. What type of fiber did the nurses consume?
Since different types of fiber have different effects, the relative amounts of soluble and insoluble fiber are important to assess accurately during the entire 16-year period. For example, cereals and whole grains are good sources of insoluble fiber, which have been proposed to be the most protective against colon cancer by Dr. David Kritchevsky, an expert in dietary fiber at The Wistar Institute in Philadelphia. We do not know if cereal composition has changed significantly in the last 16 years during the Nursesí Health Study. As can be seen from the table below, different foods have different ratios of soluble to insoluble fiber. Did some nurses eat more broccoli and tomatoes, which contain about 35% soluble and 65% insoluble fiber, whereas others preferred oranges and papayas that contain almost the opposite ratio? Changes in dietary habits and food choices over the course of the study may have been confounding factors and made the studyís result more difficult to interpret.
Q. How was the nursesí food intake measured?
A common problem in nutritional epidemiological studies is the way that the data is collected. Usually, this is done using questionnaires that are based on an estimation of food intake over a period of time. Since these estimations depend on the individualís ability to remember the composition of previous meals, they are not as accurate as the measurement of actual food composition or the administration of a standardized food supplement, as in clinical trials. The Nursesí Health Study used a food frequency questionnaire for the determination of dietary fiber intake. Such questionnaires are often self-administered and can be easily completed by the study subject with minimal effort. Although this particular type of questionnaire is convenient and has been validated for the measurement of some dietary factors (e.g. antioxidants), it may not discriminate between specific fiber types. Therefore, it may be difficult to specifically correlate soluble or insoluble fiber intake with protection from disease.
The study of Fuchs and his colleagues is an important contribution to our knowledge about fiber, but represents only one of many epidemiological studies. More data is needed, particularly from well-designed clinical trials combined with laboratory studies, before we can make conclusions about the effect of dietary fiber on colorectal cancer. However, many studies have supported the beneficial role of fiber in other areas (see table), thus is would seem advisable to continue to follow the NIH recommendation of 25 to 35 grams of fiber per day.
For more information on dietary fiber, see the Linus Pauling Institute's Micronutrient Information Center.
Last updated May, 1999
Honoring a Scientific Giant with Research Toward Longer, Better Lives