HIGH-FIBER DIET

Description
Dietary fiber has demonstrated benefits for health maintenance and disease prevention and is a component of medical nutrition therapy (1).  The diet emphasizes the consumption of dietary fiber from various foods of plant origin, particularly minimally processed fruits, vegetables, legumes, and whole-grain and high-fiber grain products.  A plant-based diet may also provide other nonnutritive components (eg, antioxidants, phytoestrogens) that have implications for health (1).  Dietary fiber intake in the United States continues to be at less than recommended levels, with usual intakes averaging only 14 to 15 g of dietary fiber per day (2).  The American Dietetic Association recommends that healthy adults consume 20 to 35 g of dietary fiber per day.  The American Academy of Pediatrics recommends children 2 and older consume an amount equal to or greater than their age plus 5 g/day (1).  In September 2002 the Institutes of Medicine’s Food and Nutrition Board released for the first time Dietary Reference Intakes (DRIs) for total fiber based on life stage (see Table D-1) (3).  In addition to the DRI’s, the amount and sources of fiber provided should be determined by the nutrition objectives for the specific disease state, as outlined below.

Table D-1:   Dietary Reference Intakes for Total Fiber by Life Stage* (3)

ND-Not Determined
*DRI’s based on 14 g total fiber/1000 calories required for age, gender

General Information
A variety of definitions of dietary fiber exist globally (1).  Some are based primarily on analytical methods used to isolate and quantify dietary fiber, whereas others are physiologically based (1).  Crude fiber is the amount of plant material that remains after treatment with acid or alkali solvents.  It is predominantly a measure of the cellulose content of a food and, as such, significantly underestimates the total dietary fiber found in plant food.  Many older food composition tables report only crude fiber (4).  For labeling purposes in the United States, dietary fiber is defined as the material isolated by analytical methods approved by the Association of Official Chemists.  The Panel on the Definition of Dietary Fiber under the Food and Nutrition Board of the National Academy of Sciences has defined fiber as follows:

    Dietary fiber is the nondigestible component of carbohydrates and lignin naturally found in plant foods (3,5). Functional fiber refers to those fiber sources that are shown to have similar health benefits as dietary fiber, but are isolated or extracted from natural sources or synthetic sources (3,5). Total fiber is the sum of dietary fiber and functional fiber (3,5).  The intent of these proposed definitions is to recognize the physiologic actions of fiber and its demonstrable health effects, and to reduce the emphasis on dietary fiber as a constituent of food requiring quantification (3,5).  There has been a trend to assign specific physiologic effects either to soluble or insoluble fibers (5).  This approach makes it difficult to evaluate the effects of fiber provided by mixed diets (5).  Dietary fiber provided by mixed diets is two-thirds to three-fourths insoluble; however, the exact distribution between soluble and insoluble depends on the method of analysis (5).  In addition, soluble and insoluble fiber foods often have similar benefits vs independent benefits that affect health outcomes. For example, psyllium seed husk (considered an insoluble fiber) and oat bran (soluble fiber source) both increase stool weight and improve laxation as well as lower blood cholesterol levels (1).  It has also been found that not all soluble fibers are hypocholesterolemic agents, but rather only those that are viscous (1).  Based on the review of evidence, the National Academy of Sciences Panel recommends that the terms soluble fibers and insoluble fibers gradually be eliminated and replaced by specific beneficial physiologic effects of fiber (3,5).

Indications
Constipation and normal laxation: Consumption of dietary fiber is a frequent prevention or treatment of constipation. Many fiber sources, including cereal brans, psyllium seed husk, methylcellulose, and a mixed high-fiber diet increase stool weight, thereby promoting normal laxation (6).  The increase in stool weight is caused by the presence of the fiber, by the water that the fiber holds and by partial fermentation of the fiber, which increases the amount of bacteria in stool (7).  The large intestine responds to the larger and softer mass of residue produced by a high-fiber diet by contracting, which moves the contents toward excretion (1).  Fiber in mixed diets, legumes, and whole-grain and high-fiber grain products are particularly effective promoters of normal laxation (1).  A fiber supplement may be needed when food intake is low, as in the case of inactive older adults (1). Common fiber supplements are psyllium seed husk and methylcellulose (1).  Other factors that influence fecal elimination and that should be considered when a dietitian is planning treatment include fluid intake, exercise, stress, and relaxation.

    Diverticulosis: Diverticulardisease of the colon is thought to occur secondary to increased intracolonic pressure caused by hard, dry fecal material and the increased effort necessary to eliminate this type of stool.  Well-controlled experimental studies confirming the benefits of a high-fiber diet in the prevention and management of diverticular disease are relatively few, with less than conclusive results.  One study found that 90% of patients with diverticular disease remained symptom-free after 5 years on a high-fiber diet (8). This result may be explained by the fact that a high-fiber diet promotes the formation of soft, large stools that are defecated more easily, resulting in lower colonic pressure and less straining during elimination (1). Also, a high-fiber diet may reduce the chances that one of the existing diverticula will burst or become inflamed (1). The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) currently recommend a high-fiber diet of 20 to 35 g of fiber each day for the treatment of diverticular disease (1, 9).  In some cases, mild pain medications may also be considered to help relieve symptoms, however many pain medications can affect emptying of the colon, an undesirable side effect for people with diverticulosis (9). 

    To increase stool bulk, studies suggest increasing theconsumption of whole-grain breads, cereals, and brans.   In cases of diverticulosis, a common practice has been to provide a high intake of fiber but to exclude hulls of nuts and corn and seeds because they may get trapped in one of the diverticula (1).  However, there have been no controlled studies to demonstrate that skins and seeds are harmful (8).  The NIDDK currently recommends that foods such as nuts, popcorn hulls, and sunflower, pumpkin, caraway, and sesame seeds should be avoided (9). The seeds in tomatoes, zucchini, cucumbers, strawberries, and raspberries, as well as poppy seeds, are generally considered harmless (9). 

    Irritable bowel syndrome: Irritable bowel syndrome (IBS) is characterized by intestinal dysfunction of at least 3 months’ duration, during which time diarrhea, diarrhea alternating with constipation, and chronic constipation may be experienced in the absence of any underlying disease states (8).  Abdominal pain, the passage of mucus, and pain alleviation following defecation may also indicate IBS (10). In patients with a strong family history of allergy and hypersensitivity to certain foods, a trial of food elimination and challenge may be justified (8).  A normal diet with moderate to high fiber (20-35g/day) may help relieve symptoms and promote bowel motility (8).  Additional fiber in the form of bulk laxatives (eg, Metamucil) may be an alternative for those who do not want to make the necessary diet changes.

    Cardiovascular disease and hypercholesterolemia:The one characteristic common to all cholesterol-lowering fibers is viscosity (11).  Fibers that lower blood cholesterol include foods such as apples, barley, beans and other legumes, fruits, vegetables, oatmeal, oat bran and rice hull; and purified sources such as beet fiber, guar gum, karaya gum, konjac mannan, locust bean, gum, pectin, psyllium seed husk, soy polysaccharide and xanthan gum (12).  The US Food and Drug Administration (FDA) has studied two fibers, beta glucan in oats and psyllium husk, to authorize a health claim that foods meeting specific compositional requirements and containing 0.75 g of beta glucan or 1.7 g of psyllium husk per serving can reduce the risk of heart disease (13).  The mechanism by which viscous fibers lower cholesterol is that the viscosity interferes with bile acid absorption from the ileum.  In response, low-density lipoprotein (LDL) cholesterol is removed from the blood and converted into bile acids by the liver to replace the bile acids lost in the stool.  Some evidence also indicates that changes in the composition of the bile acid pool accompanying ingestion of some viscous fibers dampen cholesterol synthesis (1,14).  A secondary benefit of a high-fiber diet in treating cardiovascular disease may include lower energy, fat, and simple sugar contents, all of which would be effective treatments for the obesity and hypertriglyceridemia also associated with cardiovascular disease (1,15).

    Diabetes mellitus: Considerable experimental evidence demonstrates that the addition of viscous dietary fibers slows gastric emptying rates, digestion, and the absorption of glucose to benefit immediate postprandial glucose metabolism and long-term glucose control in individuals with diabetes mellitus (16,17). The American Diabetes Association has determined that the consumption of soluble fiber independent of total fiber has limited documented effects on glycemic control in individuals with diabetes (18).  Although large amounts of dietary fiber (>50 g/day) may have beneficial effects on glycemia, insulinemia, and lipemia, it is not known whether such high levels of fiber intake can be maintained long-term (18).  For general health benefits, the daily consumption of dietary fiber is encouraged from food sources such as whole grains, fruits, and vegetables (18).

    Cancer: People who eat a greater amount of fruits and vegetables have about one half the risk of cancer and a lower mortality from cancer (19).  There is convincing evidence that diets high in vegetables and fruits decrease the risk of cancers of the colon, rectum, lung, stomach, mouth, pharynx, and esophagus and probably protect against cancers of the breast, bladder, larynx, and pancreas (20-22).  Despite inconsistencies in the results of fiber and colon cancer studies, the scientific consensus is that there is enough evidence that dietary fiber protects against colon cancer (1).  In one study, authors estimated that the risk of colorectal cancer in the US population could be reduced by about 31% with an average increase in fiber intake from food sources of approximately 13 g/day (23). 

    Use in enteral formulas: Two types of enteral formulas that contain dietary fiber are currently marketed: blended formulas made from whole foods and formulas supplemented with purified fiber sources (eg, oat, pea, hydrolyzed guar gum, and sugar beet fiber) (1).  Dietary fiber added to enteral formulas is thought to aid in normalizing bowel function and reduce the incidence of diarrhea.  To date, there are no conclusive studies that fiber-containing enteral formulas prevent diarrhea in tube-fed patients (1).  A recent addition to enteral formulas is fructo-oligosaccharides (FOSs), which are short-chain oligosaccharides (usually 2 to 10 monosaccharide units). Because they are not digested in the upper digestive tract, FOSs have some of the same physiologic effects as soluble fiber (24).  The FOSs are rapidly fermented by intestinal bacteria that produce short-chain fatty acids, which stimulate water and electrolyte absorption and should aid in the treatment of diarrhea.  Although FOSs are a preferred substrate for Bifidobacteria, they are not used by potentially pathogenic bacteria, thus helping to maintain and restore the balance of healthy gut flora (1).  Currently, FOSs are not isolated by currently accepted methods for dietary fiber so they cannot technically be called dietary fiber (5).  The newly proposed definitions of dietary fiber, if implemented, should allow a label claim for FOSs as an added fiber (5).

Contraindications
Diverticulitis:A high-fiber diet is contraindicated when inflammation has caused the narrowing or blockage of the intestinal lumen or during acute diverticulitis (8,9).

    Infants and children:The American Academy of Pediatrics does not encourage the addition of high-fiber foods to the diets of infants younger than 1 year old.  High-fiber foods are filling but contain few calories, potentially causing reduced energy intakes in infants, whose stomach capacities are naturally small. It is recommended that children over 2 years of age increase their fiber intake to an amount equal to or greater than their age plus 5 g/day (1).

    Phytobezoar formation:Phytobezoars are masses of vegetable matter that become trapped in the stomach.  Individuals who experience decreased gastric motility or emptying, such as diabetic gastroparesis, or those who have undergone surgical procedures for stomach cancer or peptic ulcer disease may be susceptible (25,26).  These individuals should be advised to avoid the following foods implicated in phytobezoar formation: apples, berries, brussels sprouts, coconuts, figs, green beans, oranges, persimmons, and potato peels (8,27).

Nutritional Adequacy
The diet can be planned to meet the Dietary Reference Intakes (DRIs) as outlined in the Statement on Nutritional Adequacy.

How to Order the Diet
Order as “High-Fiber Diet.”

Planning the Diet

• Promote food intake patterns consistent with the Dietary Guidelines that encourage a wide variety of plant foods to achieve fiber intakes.  Emphasize fruits, vegetables, and whole-grain breads and cereals.
• Foods made from whole-grain flours are substituted for those made with refined flours and starches.
• People who experience difficulty in chewing fruits and vegetables may increase fiber in the diet by consuming one or more servings daily of a high-fiber cereal, such as bran; substituting whole-wheat bread for white bread; and consuming soft or cooked fruits and vegetables. 
• If unprocessed bran is consumed, it must be served thoroughly moistened and mixed with food and be incorporated gradually into the diet.  One tablespoon of bran contains 4.5 g of dietary fiber.  To incorporate bran into the diet, begin with 1 tsp a day and gradually increase in divided doses, as tolerated, to 4 to 6 tbsp a day (8).  Three tablespoons of bran, consumed daily in divided doses, is adequate to promote normal bowel functioning.
• High-fiber foods should be added to the diet gradually.  An increase in fiber consumption may initially generate bloating and flatulence. Patients should be advised that these conditions may occur but will generally subside as the digestive system adjusts to increased fiber consumption.  
• Fiber gathers water in the colon, hence its stool-bulking property.  For this reason, a high-fiber diet should also include a liberal intake of fluids, of at least 64 oz of extra fluid per day (8). Consuming increased amounts of fiber without increasing fluid consumption can lead to the formation of hard, dry stools that are difficult to eliminate.

References

1. Position of The American Dietetic Association: health implications of dietary fiber  J Am Diet Assoc. 2002;102:993-1000.
2. Alaimo K, McDowell MA, Briefel RR, Bischof AM, Caughman CR, Loria CM, Johnson CL.  Dietary Intake of Vitamins, Minerals, and Fiber of Persons Ages 2 Months and over in the United States: Third National Health and Nutrition Examination Survey. Phase I, 1988-1991. Hyattsville, Md: National Center for Health Statistics; 1992. Advance Data from Vital and Health Statistics, No. 258.
3. Institute of Medicine’s Food and Nutrition Board.  Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids.  National Academy of Sciences, 2002: 265-334; preprint at http://www.nap.edu.books/0309085373/html/index.html, accessed September 16, 2002.
4. Slavin JL.  Dietary fiber: classification, chemical analyses, and food sources.  J Am Diet Assoc.  1987;87:1164-1171.
5. Dietary Reference Intakes Proposed Definition of Dietary Fiber.  Washington DC:  National Academy Press; 2001:1-64.
6. Cummings JH.  The effect of dietary fiber on fecal weight and composition.  In: Spiller GA, ed.  CRC Handbook of Dietary Fiber in Human Nutrition.  2nd ed.  Boca Raton, Fla: CRC Press; 1993:263-349.
7. Kurasawa S, Haack VS, Marlett JA.  Plant residue and bacteria as bases for increased stool weight accompanying consumption of higher dietary fiber diets.  J Am Coll Nutr.  2000;19:426-433.
8. Beyer PL. Medical nutrition therapy for lower gastrointestinal tract disorders. In: Mahan KL, Escott-Stump S, eds.  Krause’s Food, Nutrition and Diet Therapy. 10th ed. Philadelphia, Pa: WB Saunders; 2000:666-694.
9. National Institute of Diabetes and Digestive and Kidney Diseases.  Diverticulosis and Diverticulitis.  NIH pub. No. 03-1163.  Bethesda, Md: National Digestive Diseases Information Clearinghouse; September 2003.  Available at http://digestive.niddk.nih.gove/ddiseases. Accessed February 9, 2004.
10. Lynn RB, Friedman LS.  Irritable bowel syndrome.  N Engl J Med.  1993;329:1945.
11. Marlett JA.  Sites and mechanisms for the hypocholesterolemic actions of soluble dietary fiber sources.  In: Kritchevsky D, Bonfield C, eds.  Dietary Fiber in Health and Disease.  New York, NY: Plenum Press;  1997:109-121.
12. Marlett JA.  Dietary fiber and cardiovascular disease.  In: Cho SS, Dreher ML, eds.  Handbook of Dietary Fiber.  New York, NY: Marcel Dekker Inc; 2001:17-30.
13. US Food and Drug Administration.  Health claims: soluble fiber from certain foods and risk of heart disease.  Code of Federal Regulations.  2001;21:101.81.
14. Marlett JA, Hosig KB, Vollendorf NW, Shinnick FL, Haack VS, Story JA.  Mechanism of serum cholesterol reduction by oat bran.  Hepatology.  1994;20:1450-1457.
15. AHA Dietary Guidelines Revision 2000: a statement for healthcare professionals from the Nutrition Committee of the American Heart Association.  Circulation.  2000;102:2284-2299.
16. Anderson JW, Allgood LD, Turner J, Oeltgen PR, Daggy BP.  Effects of psyllium on glucose and serum lipid responses in men with type 2 diabetes and hypercholesterolemia.  Am J Clin Nutr.  1999;70:466-473.
17. Chandalia M, Garg A, Lutjohann D, von Bergmann K, Grundy SM, Brinkley LJ.  Beneficial effects of high dietary fiber intake in patients with type 2 diabetes mellitus.  N Engl J Med.  2000;342:1392-1398.
18. Franz MJ, Bantle JP, Beebe CA, Brunzell JD, Chiasson J-L, Abhimanyu G, Holzmeister LA, Hoogwerf B, Mayer-Davis E, Mooradian AD, Purnell JA, Wheeler M.  Evidence-based nutrition principles and recommendations for the treatment and prevention of diabetes and related complications.  Diabetes Care.  2002;25:148-198.
19. Craig WJ. Phytochemicals: guardians of our health. J Am Diet Assoc. 1997;97(suppl 2):S199-S204.
20. Position of The American Dietetic Association:  functional foods.  J Am Diet Assoc.  1999;99:1278-1285.
21. Food, Nutrition and the Prevention of Cancer: A Global Perspective. Washington, DC: American Institute for Cancer Research; 1997.
22. Moving Towards a Plant-based Diet: Menus and Recipes for Cancer Prevention. Washington, DC: American Institute for Cancer Research; 1998.
23. Howe GR, Benito E, Castelleto R, Cornee J, Esteve J, Gallagher RP, Iscovich JM, Deng-ao J, Kaaks R, Kune GA.  Dietary intake of fiber and decreased risk of cancers of the colon and rectum: evidence from the combined analysis of 13 case-control studies.  J Natl Cancer Inst.  1992;84:1887-1896.
24. Roberfroid M, Slavin JL.  Nondigestible oligosaccharides.  Crit Rev Food Science Nutr.  2000;40:461-480.
25. Parrish CR, Yoshida C.  Nutrition Intervention for the Patient With Gastroparesis: An Update.  Practical Gastroenterology. 2005;29(8):29.
26. Sanders MK.  Bezoars: From Mystical Charms to Medical and Nutritional Management.  Practical Gastroenterology.  2004;28(1):37.
27. Emerson AP.  Foods high in fiber and phytobezoar formation.  J Am Diet Assoc.  1987;87:1675.

DIETARY FIBER CONTENT OF FOODS

Dietary Fiber Content of Common Foods

Dietary Fiber Content of Common Foods (Cont.)

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