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					             Position of the American Dietetic Association:
                             Health Implications of Dietary Fiber
Journal of the American Dietetic Association, Oct 1997 v97 n10 p1157(3)
Judith A. Marlett and Joanne L. Slavin

Abstract: The American Dietetic Assn (ADA) is calling on the public to consume sufficient
amounts of dietary fiber from various plant foods. Dietary fiber is mostly the storage and cell
wall polysaccharides of plants that human digestive enzymes cannot hydrolyze. It greatly
benefits the gastrointestinal tract, and may offer protection against colon cancer. Fiber-rich foods
may help prevent breast cancer and may be valuable in prevention of obesity. The ADA proposes
to health professionals to help promote consumption of fiber-rich foods.

Dietary fiber has demonstrated benefits for health maintenance, disease prevention, and as a
component of medical nutrition therapy. Except in certain therapeutic situations, dietary fiber
should be obtained through consumption of foods. In addition to fiber, minimally processed
fruits, vegetables, grains, and legumes provide micronutrients that are essential components of
healthful diets. Plant foods also contain many other nonnutritive components (eg, xenobiotics,
antioxidants, phytoestrogens) that have implications for health.


It is the position of The American Dietetic Association (ADA) that the public should consume
adequate amounts of dietary fiber from a variety of plant foods.


Recommended Fiber Intakes Through the Life Cycle

Recommendations for dietary fiber intake for adults generally fall in the range of 20 to 35 g/day
or 10 to 13 g dietary fiber per 1,000 kcal (1,2). Nutrition Facts labels use 25 g dietary fiber per
day for a 2,000 kcal/day diet or 30 g/day for a 2,500 kcal/day diet as goals for American intake.
However, dietary fiber intake continues to be at less-than-recommended levels in the US
population, with usual intakes averaging only 14 to 15 g/day (3). Most popular American foods
are not high in dietary fiber. Servings of commonly consumed grains, fruits, and vegetables
contain only 1 to 3 g dietary fiber (4,5). Legumes and high-fiber cereal products supply more
dietary fiber but are not commonly consumed.

Recently, attempts have been made to define recommended dietary fiber intakes for children and
adolescents. Although based on limited clinical data, the recommendation for children older than
2 years is to increase dietary fiber intake to an amount equal to or greater than their age plus 5
g/day (7), to achieve intakes of 25 to 35 g/day after age 20 years. To our knowledge, no studies
have defined desirable fiber intakes for infants and children younger than 2 years. Until there is
more information about the effects of dietary fiber in the very young, a rational approach would
be to introduce a variety of fruits, vegetables, and easily digested cereals as solid foods are
brought into the diet. Specific recommendations for the elderly have not been published,
although a safe recommendation would be to encourage intake of 10 to 13 g dietary fiber per
1,000 kcal. All recommendations need to recognize the importance of adequate fluid intake and
caution should be used when recommending fiber to those with gastrointestinal diseases,
including constipation.

Benefits of Adequate Fiber Intake

The primary action of fiber in the body is in the gastrointestinal tract, but not all fiber sources
have the same physiological effects. Generally, concentrates of water-soluble fibers delay transit
through the stomach and small intestine (8). Soluble fibers are rapidly broken down (fermented)
by bacteria in the large intestine and do not promote laxation (1). Fibers that are predominantly
water insoluble promote laxation and are either slowly or not fermented (1,9). Two notable
exceptions to these general guidelines are oats, which contain up to 50% soluble fiber (primarily
as 1 [approaches] 3, 1 [approaches] 4 [Beta]-glucans) and psyllium seed husks (10), which also
analyze as a soluble fiber. Both of these fiber sources promote laxation and modulate gastric and
small intestine physiology.

Some soluble fiber sources slow the appearance of glucose in the blood. For a soluble fiber to be
effective, it also must be viscous. Viscosity slows transit of chyme in the upper gastrointestinal
tract, resulting in slower absorption rates, lower blood concentrations of nutrients, and altered
hormonal responses to these absorbed nutrients (8). Viscosity also appears to be a requirement
for fiber to lower blood cholesterol concentrations (11). Most of these effects of soluble fibers
were demonstrated using fiber concentrates (8,9). There is no evidence that the soluble fiber in a
mixed-food diet would have the same effect on upper gastrointestinal tract physiology.

Stool weight, an important measure of the laxative effect of fiber, increases with fiber intake,
although the addition of insoluble fiber sources modulates other measures of large-bowel
physiology. Defecation frequency is increased only when there is less than the normal one bowel
movement daily. Consuming more food fiber decreases gastrointestinal transit time (the time
required for the residue from an ingested meal to be excreted) only when it is longer than about 2
1/2 to 3 days. The concentration of water in stools is not usually affected by larger fiber intakes.

More than 75% of the dietary fiber in an average American's diet is broken down in the large
intestine, where carbon dioxide, hydrogen, methane, short-chain fatty acids, butyrate, propionate,
and acetate are produced by bacterial fermentation. In some studies short-chain fatty acids have
been shown to promote growth and healing of the mucosa (12). Research using isolated human
coloncytes suggests that butyrate is a major energy source for the colonic mucosa, while acetate
and propionate enter the portal circulation (13,14). In healthy adults, about 60% of the produced
short-chain fatty acids is acetate and only 10% to 20% is butyrate (14).

Definition and Sources of Fiber

Dietary fiber is primarily the storage and cell wall polysaccharides of plants that cannot be
hydrolyzed by human digestive enzymes (1). Lignin, which is a complex molecule of
polyphenylpropane units and present only in small amounts in the human diet, is also included as
a component of dietary fiber (1,15). Soluble fiber is that fraction of the total fiber that is
suspended in water during analysis (16). Soluble dietary fiber may vary 15% to 50% of the total
fiber using different analytic methods (16). Nearly all fiber-containing foods have more insoluble
than soluble dietary fiber. About two-thirds to three fourths of the dietary fiber in typical mixed-
food diets is water insoluble (1). Most fruits and vegetables contain less than 2 g/serving total
fiber and most refined grain products contain less than 1 g/serving (5). Only legumes, whole
grains, and concentrated grain products typically have more fiber (5).

Resistant starch (the sum of starch and starch-degradation products not absorbed in the small
intestine [17]) contributes to the pool of dietary fiber when the starch is undigested in the small
intestine. The amount of resistant starch in a typical Western diet is not known. Legumes appear
to be the single most important source of resistant starch, with as much as 35% of legume starch
escaping digestion (18). No meaningful tables of the resistant starch content of foods are
available, although in vitro measures of this starch fraction that has been defined by its in vivo
behavior are being developed (19).

Fiber concentrates are frequently not all dietary fiber. Fiber concentrates such as pectins and
gums (eg, gum arabic, carob bean gum, guar gum), wood pulp cellulose, psyllium seed husk, and
oat hull fiber are more than 90% fiber (10). In contrast, citrus fiber, pea fiber, corn bran, and soy
polysaccharide are 60% to 85% fiber (10). Oat bran is about 15% to 20%; wheat bran, 40% to
50%; barley, 15%; and rice bran, 20% to 30% dietary fiber (10,20).

A variety of low molecular carbohydrates are not digested by human digestive enzymes (sugar
alcohols such as sorbitol and mannitol, polydextroses, and fructooligosaccharides). The US Food
and Drug Administration (21) states that because these materials do not analyze as dietary fiber
in the official Association of Official Analytical Chemists methods they are not to be reported on
nutrition labels as dietary fiber, but as "other carbohydrate."

Disease Risk Reduction and Therapeutic Uses of Fiber

Correlation studies that compare colorectal cancer incidence or mortality rates among countries
with estimates of national dietary fiber consumption suggest that fiber intake may be protective
against colon cancer (22). When results of 13 case-control studies of colorectal cancer rates and
dietary practices were pooled, it was concluded that the results provided substantive evidence
that intake of fiber-rich foods is inversely related to risks of both colon and rectal cancer (23). It
is estimated that the risk of colorectal cancer in the US population could be reduced by about
31% if fiber intake from food sources were increased by an average of about 13 g/day (23).
Clinical trials using the incidence of adenomatous polyps as a marker for colon cancer risk have
not supported these epidemiologic associations (24), although studies have not measured fiber
intake adequately.

Limited epidemiologic evidence has been published on fiber intake and risk of breast cancer in
human beings. International comparisons show an inverse correlation between breast cancer
death rates and the consumption of fiber-rich foods (25). A meta-analysis of 12 case-control
studies of dietary factors and risk of breast cancer revealed that high dietary fiber intake was
associated with a reduced breast cancer risk (26). Not all studies, however, including a
prospective cohort study (27) find a relationship between dietary fiber intake and breast cancer
incidence. Foods high in fiber, namely vegetables and fruits (28) and whole grains (29), have
been linked to decreased risk of developing cancer at other sites. Fiber may also protect against
prostate cancer (30).

No clear relationship has been demonstrated between treatment for obesity and dietary fiber.
However, a diet with adequate amounts of fiber-containing foods is usually less energy dense
and its greater bulk has a short-term satiety effect.

A low-fiber diet may increase the risk for diabetes because such diets are usually more energy
dense and promote obesity (1). Fiber has a role in the treatment of diabetes because it slows the
absorption of glucose from the small intestine. If postprandial blood glucose levels are elevated,
incorporating soluble viscous fibers into the diet will minimize this abnormal glucose spike (8).

The role of fiber in the prevention and treatment of cardiovascular disease is related to its effect
on blood cholesterol levels. Typically, viscous, soluble fiber sources lower blood cholesterol
levels by 5% or more (31). Greater decreases are attainable when initial blood cholesterol
concentrations are elevated. A variety of measurements indicate that the major mechanism by
which fiber drains cholesterol from the body pool is through the interference of viscous fibers in
the absorption of bile acids, not the absorption of cholesterol in the diet (11). Most of the
cholesterol in the body is synthesized in the liver, not supplied by the diet. Measuring fetal bile
acids does not always detect this decrease in bile acid absorption. Other compounds in fiber
sources may also affect sterol balance. For example, the gamma analog of tocotrienol found in
oats, barley, and rice bran (11) is a potent inhibitor of endogenous cholesterol biosynthesis and
will lower blood cholesterol concentrations (32). Fatal and nonfatal myocardial infarctions have
been inversely associated with total fiber retakes m some studies (33).

Dietary fiber has been used to treat constipation and promote laxation for thousands of years.
The gastrointestinal tract is remarkably sensitive to dietary fiber. Foods and other sources of
insoluble fiber have demonstrated positive effects on laxation. Fiber in wheat bran and oat bran
have comparable effects on stool weight. These two brans are more effective at increasing stool
weights than the same amounts of fiber provided from fruits and vegetables, mixed-food diets, or
purified cellulose (8,9,34).

Long-term animal feeding studies indicate that diverticular disease of the colon is an outgrowth
of a lifelong pattern of low fiber intake (35). Although the diverticula remain anatomically intact,
a higher fiber diet eliminates exacerbation of diverticulitis. Because they may become trapped in
one of the diverticula, whole pieces of fiber (hulls of nuts and corn and seeds such as those of
tomato and cucumber) may be excluded from a high-fiber diet to treat diverticulosis. To our
knowledge, no controlled studies have been conducted to demonstrate that skins and seeds are
harmful (36).

Two types of enteral formulas that contain dietary fiber are currently marketed: blenderized
formulas made from whole foods and formulas supplemented with purified fiber sources.
Purified fiber sources used in enteral products include soy, oat, pea, hydrolyzed guar gum, and
sugar beet fibers. Some formulas use a mixture of fiber sources. In healthy subjects, dietary fiber
normalizes bowel function, and success stories exist in which diarrhea in enterally fed patients
was reduced using fiber-containing liquid formulas. Perhaps because studying the physiologic
effects of dietary fiber in a sick population is extremely difficult, no convincing data have been
published to document that fiber-containing enteral formulas prevent diarrhea in tube-fed
patients. The results of clinical studies with dietary fiber also have been disappointing (37).

Despite lack of compelling clinical data, dietary fiber is the treatment of choice for many
enterally fed patients with many bowel disorders (38). In the enterally nourished population,
increased dietary fiber has been recommended for therapy for constipation and diarrhea and as a
treatment for diverticular disease. Fiber-containing formulas tend to be more expensive than
standard formulas, which makes them a difficult choice without compelling clinical data to
support their use.


ADA recommends that health professionals:
   promote food intake patterns consistent with the Food Guide Pyramid (39) that make use
      of a wide variety of plant foods to achieve adequate fiber intakes in healthy children and
      adults. Include at least 2 to 3 servings of whole grains as part of the daily 6 to 11 servings
      of grains (40), 5 servings of fruits and vegetables daily, and legumes at least once or
      twice a week;
   recognize that diets containing excess bulk may not contain sufficient energy to support
      normal growth in children;
   recognize that 1 to 2 servings daily of higher fiber foods leg, legumes, whole grains,
      cereal brans) or concentrated fiber sources may be necessary to achieve adequate intakes
      in those persons whose energy needs are low relative to body mass, such as the elderly,
      hospitalized or ill patients, or those in long-term-care facilities;
   consider the use of concentrated sources of dietary fiber (9) to treat chronic constipation
      when a limited variety of food is consumed or the amount of food consumed is
   incorporate new sources of dietary fiber into diet plans for specific diseases only when
      benefit claims are documented and the overall diet is consistent with the medical nutrition
      therapy appropriate for the disease;
   recognize that viscous concentrated fiber sources with documented health benefits do
      have demonstrated blood cholesterol-lowering effects and can be part of lifestyle that
      includes a heart-healthy diet and exercise;
   use a diet pattern for persons with diabetes mellitus that is moderate in fat and contains a
      wide variety of fiber-containing foods to lower abnormal elevations in postprandial blood
      glucose levels and promote body weight normalization;
   Use enteral feedings containing dietary fiber to promote normal enterocyte function, but
      recognize that clinical benefits may be modest.


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* ADA Position adopted by the House of Delegates on October 18, 1987, and reaffirmed on
September 12, 1992, and September 6, 1996. This position will be in effect until December 31,
2001. ADA authorizes republication of the position statement/support paper, in its entirety,
provided full and proper credit is given. Requests to use portions of the position must be directed
to ADA Headquarters at 800/877-1600, ext 4896, or

* Recognition is given to the following for their contributions:

Judith A. Marlett, PhD, RD, and Joanne L. Slavin, PhD, RD

James W. Anderson, MD; Barbara F. Harland, PhD, RD; David J. A. Jenkins, MD, PhD;
Joanne R. Lupton, PhD; Oncology Nutrition dietetic practice group (Barbara L. Eldridge, RD;
Karen P. Kulakowski, MA, RD); Public Health Nutrition dietetic practice group (Laura Kruse,

                        Full Text COPYRIGHT 1997 American Dietetic Association

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