The Clinical Uses of Dietary Fiber
American Family Physician, Feb 1, 1995 v51 n2 p419(8)
David S. Gray
Author's Abstract: COPYRIGHT American Academy of Family Physicians 1995
Dietary fiber has received considerable attention in both the popular press and the scientific
literature. Fiber is a complex mixture of substances, and research on its effects is difficult to
interpret. Dietary fiber has significant gastrointestinal effects, and it is a mainstay of treatment
for constipation and hemorrhoids. Insoluble fiber, such as wheat bran, is most effective for
treatment of these conditions. Increased intake of soluble dietary fiber appears to benefit patients
with diabetes mellitus and hyperlipidemia. High-fiber, low-fat diets have been recommended by
a variety of authorities to decrease the incidence of heart disease and certain types of cancer. Any
increase in dietary fiber intake should be accompanied by an increase in water intake.
Full Text: COPYRIGHT American Academy of Family Physicians 1995
Dietary fiber has caught the attention of the public and the food industry. Numerous high-fiber
and fiber-supplemented food products are now available, and product labels abound with claims
of health benefits. Dietary fiber has also been the topic of a considerable volume of medical
research. It has been recommended as part of the treatment and prevention of a number of
diseases (Table 1). This article reviews the medical uses of dietary fiber and fiber products.
TABLE 1 Conditions for Which Dietary Fiber Has Been Recommended
Disease treatment
Disease prevention
Diseases of the colon
Heart disease
Constipation
Cancer
Hemorrhoids
Diverticulosis
Hiatal hernia
Varicose veins
Diabetes mellitus
Hyperlipidemia
Obesity
Definition
Dietary fiber has been defined as the portion of plant material that is resistant to degradation by
the enzymes of the human small intestine,(1) specifically the substances that make up the plant
cell walls. Plant fibers are mostly complex carbohydrate polymers of simple sugars such as
cellulose, pectin, hemicellulose, gum and mucilage.
Dietary fiber is often classified according to its solubility in water. While both soluble and
insoluble fiber is resistant to hydrolysis by the enzymes of the small intestine, soluble fiber tends
to be efficiently degraded by bacteria in the colon and insoluble fiber passes through the body
largely unchanged. Dietary fiber is a complex mixture of substances. Most plant foods contain a
variety of fibers, and some types may be concentrated in certain foods (Table 2). For example,
wheat bran tends to be rich in insoluble (hard) fiber, while oat bran contains more soluble
(viscous) fiber.
Pitfalls in Fiber Research
The study of dietary fiber is complicated, since the effects of the refined fiber components that
are often used in controlled experiments may differ from the effects of fiber components in their
natural states and when mixed with other fiber substances in food. Thus, a study evaluating the
effects of pure cellulose on a certain disease state may not predict the effects of a diet high in
cellulose.
The problem is further complicated by the common coexistence of many other nutrients with
dietary fiber in foods. For example, a diet high in dietary fiber may also be high in vitamin A and
carbohydrates and low in fat, making it difficult to determine whether the effects of such a diet
are due to the fiber, the other nutrients, or a combination of the two.
Another problem in the study of dietary fiber is that common methods of food refining and
preparation, such as grinding or boiling, may alter the effects of the dietary fiber. Such
complexities often result in inconclusive and contradictory results in published studies.(2)
Nevertheless, some physiologic and pathologic conditions seem to be helped by certain types of
fiber (Table 3).
TABLE 3 Comparison of Mechanical and Metabolic Effects of Soluble and Insoluble
Dietary Fibers
Effect
Delay gastric emptying
Increase fecal bulk and frequency of
bowel movements
Regulate colonic transit time
Slow glucose absorption from small
intestine and reduce postprandial
blood glucose levels during
clinical tests
Lower serum total cholesterol and
low-density lipoprotein cholesterol
during clinical tests
Soluble
fibers(*)
Yes
Yes
Insoluble
fibers([dagger])
No
Yes
Yes
Yes
Yes
No
Yes
No
(*)--Soluble fibers are gel-forming pectins, gums, mucilages and some hemicelluloses from
beans, fruits, oat bran, psyllium and vegetables.
([dagger])--Insoluble fibers are structural cellulose, lignins and some hemicelluloses from wheat
bran products and whole wheat.
From Hunt R, Fedorak R, Frohlich J. McLennan C, Pavilanis A. Therapeutic role of dietary
fibre. Can Fam Physician 1993; 39:897-910. Used with permission.
Diseases of the Gastrointestinal Tract
High-fiber diets and fiber supplements have been tried in the treatment of many pathologic
conditions. The best evidence for their effectiveness is apparent with regard to problems of the
large intestine. Clinical experience clearly indicates that increasing dietary fiber improves
constipation, and experimental studies have shown that fiber increases stool bulk and decreases
intestinal transit time. Both soluble and insoluble fiber sources reduce constipation, but their
mechanisms of action may be different. Insoluble fiber increases stool bulk by taking on water in
the intestine, while much of the increase in stool bulk caused by soluble fiber is due to the
bacterial mass that accumulates during its degradation.(1)
Once other causes of constipation have been ruled out,(3) a trial of high-fiber diet is a safe and
reasonable therapeutic approach. Americans typically consume 11 g of dietary fiber per day. An
intake of 30 g of dietary fiber per day has been recommended.(3) Table 4 contains suggestions
for attaining this level of fiber intake.
TABLE 4 How to Increase Dietary Fiber
Add one or more servings of fruits and vegetables per day (at least five servings total daily).
Change to whole-grain breads and cereals and brown rice from white, refined varieties.
Have a bran cereal for breakfast. All-Bran and 100% Bran are the highest in fiber (8 to 10 g per
serving); bran flakes and others have less (4 to 6 g per serving).
Add 1/4 cup or more of miller's bran to some food daily (e.g., cooked cereal or applesauce).
Add cooked beans to the diet each week.
Always increase fluid intake with increases in dietary fiber.
Fiber supplements can be used as an adjunct to a high-fiber diet, and many patients find them
acceptable and convenient. The usual dose of psyllium seed preparations is 1 to 2 tsp in 8 oz of
water or juice twice daily (Table 5). Methylcellulose (Table 6) and polycarbophil (Table 7)
preparations are also available. Because of fiber's water adsorbing effect, increased dietary fiber
can actually increase constipation if fluid intake is inadequate. Therefore, when increasing
dietary fiber, it is important to recommend adding at least two 8-oz glasses of water or other
beverage to the diet each day.
TABLE 7 Contents of Fiber Supplements Containing Polycarbophil(*)
Supplement
FiberCon
Equalactin
Mitrolan
Fiber-Lax
Fiberall
Polycarbophil (mg)
500
500
500
500
1,000
Sodium (mg)
0
-<0.46
-<1
Calories
----<6
Other
Sorbitol
Sucrose
Dextrose
(*)--Typical dose is 1 g one to four times per day.
Derived from Barnhart ER. Physician's desk reference for nonprescription drugs. 12th ed.
Oradell, N.J.: Medical Economics, 1991, and Drug facts and comparisons. St. Louis: Lippincott,
1991.
While the evidence of benefit is most clear for the treatment of constipation, evidence also
suggests that increasing dietary fiber may improve or prevent hemorrhoids, diverticulosis and
diverticulitis, hiatal hernia and varicose veins.(4)(5) Increased colonic pressure due to small,
hard stools and increased intra-abdominal pressure due to straining at stool have been
incriminated in the pathogenesis of these disorders. Increasing dietary fiber has also been
commonly recommended for irritable bowel syndrome.
Fiber-supplemented formulas for enteral tube feeding have recently been introduced with the
goal of decreasing the diarrhea frequently associated with this type of nutritional support.
However, studies to date have failed to confirm that these fiber-supplemented formulas confer
any real benefit in the management or prevention of diarrhea or constipation in tube-fed
patients.(1)(6)
Diabetes Mellitus
Next to gastrointestinal diseases, the role of dietary fiber in the treatment of diabetes mellitus has
probably received the most attention. The diet recommended by the American Diabetes
Association contains 40 g of dietary fiber.(7) Experiments have shown that adding purified
soluble dietary fiber in the form of guar gum will decrease the glucose and insulin response to a
meal or a glucose tolerance test.(8)
Many studies have also shown that consumption of equal amounts of carbohydrates in different
foods will cause different elevations in blood glucose levels.(9) This observation has led to the
concept of the glycemic index of foods and may be due, at least in part, to the effects of fiber.
Clinical studies performed in metabolic wards have also demonstrated significant improvements
in glycemic control in diabetic patients who were placed on high-fiber, high-carbohydrate
diets.(10) Such studies have been criticized because the study diets contain very high levels of
dietary fiber (in the range of 40 to 60 g per day), and such diets are unusual in Western countries,
making compliance a significant problem.(9) The long-term benefits of more moderate intakes of
dietary fiber are not as clear.
More research is needed before purified fiber supplements or the glycemic index can be
recommended for use in routine practice. Diets moderately high in fiber (e.g., 30 g per day) tend
to be relatively high in complex carbohydrates and lower in fat than the average American diet.
In line with American Diabetes Association recommendations, it seems prudent to recommend
these moderate dietary changes to patients with diabetes mellitus. The occasional patient who is
very motivated and willing to follow a very high-fiber, plant-based diet may be able to improve
glycemic control and decrease medication requirements.
Hyperlipidemia
When consumed in large amounts, both purified soluble fiber sources (e.g., pectin and psyllium)
and foods high in soluble fiber (e.g., oat bran and beans) lower serum cholesterol
levels.(4)(8)(10)(11) Insoluble, hard fiber sources such as wheat bran have little or no effect on
serum cholesterol. Several mechanisms have been proposed for this effect, including alterations
in bile acid metabolism and changes in fatty acid metabolism by colonic bacteria.(4)
Since the addition of fiber to the diet usually affects other components of the diet, such as
carbohydrate and fat content (which in themselves have significant effects on serum lipid levels),
determining the precise effect of fiber in lipid-lowering programs has been difficult. The addition
of about two-thirds cup to one and one-half cups (dry weight) of oat bran per day will lower
serum low-density lipoprotein cholesterol levels by 10 to 20 percent.(11) In view of these data, it
makes sense ot recommend an increase in dietary soluble fiber as part of a low-fat diet for
patients with hypercholesterolemia. However, it must be remembered that large quantities of
fiber are required for significant effects.
As with any treatment program, response should be monitored carefully. If dietary treatment is
not adequate, drug treatment is recommended for patients with severe hypercholesterolemia.(12)
Obesity
Because dietary fiber can produce a feeling of fullness in the stomach, it has long been suggested
as an aid in weight loss diets. In fact, several clinical trials have demonstrated slightly greater
weight loss during treatment with purified fiber sources compared with placebo. Other studies
have not confirmed this effect, and no long-term studies are available to evaluate this type of
treatment.(13)
As in the treatment of diabetes mellitus and hyperlipidemia, low-fat diets rich in complex
carbohydrates and fiber often are recommended for the treatment of obesity.(14) The exact role
of purified fiber supplements as adjuncts to this dietary approach remains to be determined. It is
clear that fiber has no magical effects in promoting weight loss.(4)
Disease Prevention
Epidemiologic evidence has suggested that dietary fiber may affect the risk for heart disease and
cancer (particularly cancer of the colon). This evidence has been based largely on the
observation that the rates of these diseases are far lower in many developing countries than in
Western Europe and the United States.(15) Diets in developing countries tend to be higher in
dietary fiber than those in the United States. A significant problem in the interpretation of this
observation is that these diets differ in other nutrients as well. Some nutrients, such as fat and
vitamin A, may also affect rates of risk for heart disease and cancer. Numerous clinical
investigations on the effects of dietary fiber support this epidemiologic evidence.(15) As
discussed earlier, trials have shown that increased fiber intake can improve diabetes mellitus and
hyperlipidemia, both important risk factors for heart disease. Studies in animal models have
shown that dietary fiber can inhibit the action of certain carcinogens in the colon.
The weight of evidence supports the contention that dietary factors are important in the etiology
of heart disease and certain types of cancer.(16) More research is needed before definite
conclusions can be reached about the specific role of dietary fiber in these diseases. This
evidence will be difficult to obtain because of the complex interactions of dietary fiber with other
nutrients and the possibility of changes in the action of fiber with purification or processing.
The American Heart Association and the American Cancer Society have recommended diets
with lower fat and higher fiber contents than the typical American diet.(4) These dietary changes
should be recommended to patients while pointing out that there is insufficient evidence to
recommend the routine use of purified fiber supplements.(17)
Side Effects and Adverse Reactions
The most annoying problems associated with increasing dietary fiber intake, either in food or
purified form, are gastrointestinal disturbances. Methane, carbon dioxide and hydrogen gases are
produced by bacterial metabolism of undigested fiber residue in the colon.(1) Increased flatus
and gas pains are reasons commonly given for failure to adhere to high-fiber diets. It is
frequently stated that these symptoms decrease over time, but limited evidence exists to support
this assertion.
Nonetheless, it is appropriate to counsel patients about gas production as they increase their fiber
intake and offer reassurance that this effect may improve over time. A recently introduced
enzyme product that breaks down some indigestible polysaccharides (Beano; AkPharma,
Pleasantville, N.J.) may help patients tolerate the gastrointestinal effects of increased fiber
intake. Patients should also be warned that severe constipation and even colon obstruction may
occur with high-fiber diets if fluid intake is inadequate.
Because fiber substances can bind divalent cations, it has been feared that highfiber diets could
lead to deficiencies of calcium and zinc, among other minerals. Current evidence suggests that
moderate fiber intake will not cause nutrient deficiencies. Authorities suggest caution with the
use of purified fiber supplements and recommend use of fibers from a variety of food sources.(4)
Recent reports have documented allergic reactions to psyllium seed products in some
individuals,(18) commonly workers who are exposed during the manufacture of these products.
However, cases of allergy due to ingestion of psyllium supplements indicates the need for
physician awareness of this rare complication. Reported reactions have included wheezing,
urticaria and gastrointestinal disturbances.
Final Comment
Although fiber is not a panacea, an increase in dietary fiber can safely be recommended to
patients with constipation and other gastrointestinal disorders, diabetes, hyperlipidemia and
obesity. All patients may be able to help prevent cancer and heart disease by increasing their
dietary fiber intake. With the exception of treatment of constipation, purified fiber supplements
are not generally recommended.
How to Increase the Amount of Fiber in Your Diet
Many foods contain dietary fiber (the part of food that resists digestion by the body). Eating
foods that are high in fiber can help relieve some problems with constipation, hemorrhoids,
diverticulosis and irritable bowel syndrome. Dietary fiber may help lower your cholesterol. It
may also help prevent heart disease, diabetes and certain types of cancer. You might try the
following ideas to increase the fiber in your diet:
* Eat at least five servings of fruits and vegetables each day. Fruits and vegetables that are high
in fiber include:
Apples
Berries
Figs
Oranges
Pears
Prunes
Broccoli
Brussels sprouts
Carrots
Cauliflower
Lettuce
Potatoes
* Replace white bread with whole-grain breads and cereals. Eat brown rice instead of white rice.
Examples of these foods include:
Bran muffins
Brown rice
Oatmeal
Popcorn
Multiple-grain cereals, cooked or dry
100% Whole-wheat bread
* Eat dry bran cereal for breakfast. Check labels on the packages for the amounts of dietary fiber
in each brand. Some cereals may have less fiber than you think.
* Add 1/4 cup of wheat bran (miller's bran) to foods such as cooked cereal or apple-sauce or
meat loaf.
* Eat cooked beans each week.
Many people notice bloating, cramping or gas when they add fiber to their diet. Making small
changes in your diet over a period of time can help prevent this. Start with one of the changes
listed above, then wait several days to a week before making another. If one change doesn't seem
to work for you, try a different one.
It's important to drink more fluids when you increase the amount of fiber you eat. If you don't
already drink over six glasses of liquid a day, drink at least two more glasses of water a day
when you increase your fiber intake.
This information provides a general overview on dietary fiber and may not apply to everyone.
Talk to your family doctor to find out if this information applies to you and to get more
information on this subject.
REFERENCES
(1.)Silk DB. Fibre and enteral nutrition. Gut 1989; 30:246-64.
(2.)Spiller GA. Beyond dietary fiber. Am J Clin Nutr 1991; 54:615-7.
(3.)Marshall JB. Chronic constipation in adults. How far should evaluation and treatment go?
Postgrad Med 1990; 88:49-51,54,57-9,63.
(4.)Council on Scientific Affairs. Dietary fiber and health. JAMA 1989; 262:542-6.
(5.)Ertan A. Colonic diverticulitis. Recognizing and managing its presentations and
complications. Postgrad Med 1990; 88:67-72,77.
(6.)Frankenfield DC, Beyer PL. Dietary fiber and bowel function in tube-fed patients. J Am Diet
Assoc 1991; 91:590-6,599.
(7.)American Diabetes Association. Nutritional recommendations and principles for individuals
with diabetes mellitus: 1986. Diabetes Care 1987; 10:126-32.
(8.)Hockaday TD. Fibre in the management of diabetes. 1. Natural fibre useful as part of total
dietary prescription. BMJ 1990; 300:1334-6.
(9.)Tattersall R, Mansell P. Fibre in the management of diabetes. 2. Benefits of fibre itself are
uncertain. BMJ 1990; 300:1336-7.
(10.)Anderson JW, Smith BM, Geil PB. High-fiber diet for diabetes. Safe and effective
treatment. Postgrad Med 1990; 88:157-61,164,167-8.
(11.)Nuovo J. Use of dietary fiber to lower cholesterol. Am Fam Physician 1989; 39(4):137-40.
(12.)National Cholesterol Education Program Expert Panel. Report of the National Cholesterol
Education Program Expert Panel on Detection, Evaluation, and Treatment of High Blood
Cholesterol in Adults. The Expert Panel. Arch Intern Med 1988; 148:36-69.
(13.)Smith U. Dietary fibre, diabetes and obesity. Int J Obes 1987; 11(Suppl 1):27-31.
(14.)Bray GA, Gray DS. Obesity. Part II--Treatment. West J Med 1988; 149:555-71.
(15.)Heaton KW. Dietary fibre. BMJ 1990; 300:1479-80.
(16.)National Research Council (U.S.). Committee on Diet and Health. Diet and health:
implications for reducing chronic disease risk. Committee on Diet and Health, Food and
Nutrition Board, Commission on Life Sciences, National Research Council. Washington, D.C.:
National Academy Press, 1989.
(17.)Ferguson EF Jr, McKibben BT. Preventing colorectal cancer. South Med J 1990; 83:1295-9.
(18.)James JM, Cooke SK, Barnett A, Sampson HA. Anaphylactic reactions to a psylliumcontaining cereal. J Allergy Clin Immunol 1991; 88(3 Pt 1):402-8.