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Position of the American Dietetic Association: Health

Implications of Dietary Fiber

ABSTRACTIt is the position of the American Die-tetic Association that the public shouldconsume adequate amounts of dietaryfiber from a variety of plant foods. Pop-ulations that consume more dietary fi-ber have less chronic disease. In addi-tion, intake of dietary fiber hasbeneficial effects on risk factors for de-

veloping several chronic diseases. Di-

etary Reference Intakes recommendconsumption of 14 g dietary fiber per1,000 kcal, or 25 g for adult women and38 g for adult men, based on epidemio-logic studies showing protectionagainst cardiovascular disease. Appro-priate kinds and amounts of dietary fi-ber for children, the critically ill, andthe very old are unknown. The DietaryReference Intakes for fiber are based onrecommended energy intake, not clini-cal fiber studies. Usual intake of di-etary fiber in the United States is only15 g/day. Although solubility of fiber

was thought to determine physiologicaleffect, more recent studies suggestother properties of fiber, perhaps fer-mentability or viscosity are importantparameters. High-fiber diets providebulk, are more satiating, and have beenlinked to lower body weights. Evidencethat fiber decreases cancer is mixedand further research is needed.Healthy children and adults canachieve adequate dietary fiber intakesby increasing variety in daily food pat-terns. Dietary messages to increaseconsumption of high-fiber foods such aswhole grains, legumes, fruits, and veg-

etables should be broadly supported byfood and nutrition professionals. Con-sumers are also turning to fiber supple-ments and bulk laxatives as additionalfiber sources. Few fiber supplementshave been studied for physiological ef-fectiveness, so the best advice is to con-sume fiber in foods. Look for physiolog-

ical studies of effectiveness beforeselecting functional fibers in dieteticspractice. J Am Diet Assoc. 2008;108:1716-1731.

POSITION STATEMENT

It is the position of The American Di- etetic Association that the public

should consume adequate amounts of dietary fiber from a variety of plant foods.

In 2002, the Institute of Medicinepublished a new set of definitionsfor dietary fiber (1). The new defi-

nition suggested that the term dietary fiber would describe the nondigestiblecarbohydrates and lignin that are in-trinsic and intact in plants, whereas

functional fiber consists of the iso-lated nondigestible carbohydratesthat have beneficial physiological ef-fects in human beings. Total fiberwould then be the sum of dietary fiberand functional fiber. Nondigestiblemeans not digested and absorbed inthe human small intestine. Fiberscan be fermented in the large intes-tine or can pass through the digestivetract unfermented. There is no bio-chemical assay that reflects dietaryfiber or functional fiber nutritionalstatus (eg, blood fiber levels cannot be

measured because fiber is not ab-sorbed). No data are available to de-termine an Estimated Average Re-quirement and thus calculate aRecommended Dietary Allowance fortotal fiber, so an Adequate Intake (AI)was instead developed. The AI for fi-ber is based on the median fiber in-take level observed to achieve thelowest risk of coronary heart disease(CHD). A Tolerable Upper IntakeLevel was not set for dietary fiber orfunctional fiber.

Dietary fiber is part of a plant ma-

trix which is largely intact. Nondi-gestible plant carbohydrates in foodsare usually a mixture of polysaccha-rides that are integral components of the plant cell wall or intercellularstructure. This definition recognizesthat the three-dimensional plant ma-trix is responsible for some of thephysicochemical properties attrib-uted to dietary fiber and that dietaryfiber contains other nutrients nor-mally found in foods, which are im-portant in the potential health ef-

0002-8223/08/10810-0015$34.00/0doi: 10.1016/j.jada.2008.08.007

This American Dietetic Associa-tion (ADA) position paper uses

ADA’s Evidence Analysis Process

and information from ADA’s Evi-dence Analysis Library. The use of an evidence-based approach pro-

vides important added benefits toearlier review methods. The majoradvantage of the approach is themore rigorous standardization of review criteria, which minimizesthe likelihood of reviewer bias andincreases the ease with which dis-parate articles may be compared.

For a detailed description of themethods used in this position paper,access ADA’s Evidence Analysis Pro-cess (www.adaevidencelibrary.com/ category.cfm?cidϭ7&catϭ0).

Conclusion Statements are as-signed a grade by an expert workgroup based on the systematic anal-ysis and evaluation of the support-ingresearch evidence:Grade IϭGood,Grade IIϭFair, Grade IIIϭ Limited,Grade IV ϭExpert Opinion only, andGrade V ϭGrade is not assignable(because there is no evidence to sup-port or refute the conclusion). Evi-dence-based information for thisand other topics can be found at theEvidence Analysis Library (www.

adaevidencelibrary.com ) and sub-scriptions for non-ADA memberscan be purchased at the Evidence

Analys is Library’s on-line store(www.adaevidencelibrary.com/ store.cfm).

1716 Journal of the AMERICAN DIETETIC ASSOCIATION © 2008 by the American Dietetic Association

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fects. Cereal brans, which areobtained by grinding, are anatomicallayers of the grain consisting of intactcells and substantial amounts of starch and protein; they are catego-rized as dietary fiber sources.

Dietary Reference Intakes (DRIs)for total fiber by life stage group areshown in Table 1. The AIs for totalfiber are based on the intake levelobserved to protect against CHDbased on epidemiologic, clinical, andmechanistic data. The reduction of risk of diabetes can be used as a sec-ondary endpoint to support the rec-ommended intake level. The relation-ship of fiber intake to colon cancer isthe subject of ongoing investigation.The DRI development panel sug-gested the recommended intakes of total fiber may also help ameliorateconstipation and diverticular disease,provide fuel for colon cells, reduceblood glucose and lipid levels, andprovide a source of nutrient-rich, low-energy-dense foods that could con-tribute to satiety, although these ben-

efits were not used as the basis for the

AI.There is no AI for fiber for healthy

infants aged 0 to 6 months who arefed human milk because human milkdoes not contain dietary fiber. During the 7- to 12-month age period, solidfood intake becomes more significant,and so dietary fiber intake may in-crease. However, there are no data ondietary fiber intake in this age groupand no theoretical reason to establishan AI. There is also no information toindicate that fiber intake as a func-tion of energy intake differs during

the life cycle.Fiber recommendations for chil-

dren and elderly persons were alsobased on the consumption of 14 g fiberper 1,000 kcal consumed. No pub-lished studies have defined desirablefiber intakes for infants and childrenyounger than age 2 years. Until thereis more information about the effectsof dietary fiber in the very young, arational approach would be to intro-duce a variety of fruits, vegetables,and easily digested cereals as solid

foods are brought into the diet. Al-though based on limited clinical data,a previous fiber recommendation forchildren older than 2 years is to in-crease dietary fiber intake to anamount equal to or greater than theirage plus 5 g/day and to achieve in-

takes of 25 to 35 g/day after age 20years (2).

Little clinical data are available forfiber needs in the elderly. Thus, thefiber AI for older adults is also basedon 14 g/1,000 kcal. As older adultsrequire less dietary energy thanyoung adults, the AI for fiber con-sumption in older adults decreases.

All fiber recommendations need torecognize the importance of adequatefluid intake, and caution should beused when recommending fiber tothose with gastrointestinal diseases,

including constipation.The 2005 US Dietary Guidelines

recommend high-fiber food such aswhole grains and vegetables andfruits, and fiber intake levels of 14g/1,000 kcal (3). MyPyramid also sup-ports this recommendation (4). Nutri-tion Facts labels use 25 g dietary fiberper day for a 2,000 kcal/day diet or 30g/day for a 2,500 kcal/day diet asgoals for American intake.

Dietary fiber intake continues to beless than recommended in the UnitedStates with usual intakes averaging only 15 g per day (1). When askedabout their perceptions of their di-etary fiber intake, 73% of individualswith a mean fiber intake below 20 g/dthink the amount of fiber they con-sume is “about right” (5). Many pop-ular American foods contain little di-etary fiber. Servings of commonlyconsumed grains, fruits, and vegeta-bles contain only 1 to 3 g dietary fiber(6) (Table 2). Major sources of dietaryfiber in the US food supply includegrains and vegetables (7). White flourand white potatoes provide the mostfiber to the diet, about 16% and 9%,respectively, not because they areconcentrated fiber sources, but be-cause they are widely consumed. Le-gumes are very rich in dietary fiber,but because of low consumption onlyprovide about 6% of the fiber in theUS diet. Fruits provide only 10% of the fiber in the overall US diet be-cause of low fruit consumption andthe low amount of fiber in fruits, ex-cept for dried fruits.

Table 1. Dietary Reference Intakes (DRI) for total fibera by life stage group and DRI values(g/1,000 kcal/d)b

Life stage group

Adequate Intakec

Men g/1,000 kcal/d Women g/1,000 kcal/d

0-6 mo NDd ND ND ND7-12 mo ND ND ND ND1-3 y 14 19 14 194-8 y 14 25 14 259-13 y 14 31 14 2614-18 y 14 38 14 2619-30 y 14 38 14 2531-50 y 14 38 14 2551-70 y 14 30 14 21Ͼ70 y 14 30 14 21PregnancyϽ18 y NA e NA 14 2919-50 y NA NA 14 28LactationϽ18 y NA NA 14 29

19-50 y NA NA 14 29

aTotal fiber is the combination of dietary fiber (the edible, nondigestible carbohydrate and lignin components in plant

foods) and functional fiber (isolated, extracted, or synthetic fiber that has proven health benefits).b Values are example of the total grams per day of total fiber calculated from g/1,000 kcal multiplied by the median

energy intake (kcal/1,000 kcal/day) from the Continuing Survey of Food Intakes by Individuals 1994-1996, 1998.cIf sufficient scientific evidence is not available to establish an Estimated Average Requirement, and thus calculate a

Recommended Dietary Allowance, an Adequate Intake (AI) is usually developed. For healthy, breastfed infants, the AI is

the mean intake. The AI for other life stage and sex groups is believed to cover the needs of all healthy individuals in

the group, but a lack of data or uncertainty in the data prevents being able to specify with confidence the percentage

of individuals covered by this intake.dNDϭnot determined.eNA ϭnot applicable.

October 2008 ● Journal of the AMERICAN DIETETIC ASSOCIATION 1717



DEFINITION AND SOURCES OF FIBER

A variety of definitions of dietary fiber

exist (8). Some are based primarilyupon analytical methods used to iso-late and quantify dietary fiberwhereas others are physiologicallybased. Dietary fiber is primarily thestorage and cell wall polysaccharidesof plants that cannot be hydrolyzed byhuman digestive enzymes. Lignin,which is a complex molecule of poly-phenylpropane units and presentonly in small amounts in the humandiet, is also usually included as a com-ponent of dietary fiber. For labeling

the dietary fiber content of food prod-ucts within the United States, dietary

fiber is defined as the material iso-lated by analytical methods approvedby the Association of Official Analyt-ical Chemists (AOA C), generally

AOAC Method 985.29 (8). A variety of low molecular carbohydrates such asresistant starch, polydextrose, andnondigestible oligosaccharides in-cluding fructo- and galacto-oligosac-chardies are being developed and in-creasingly used in food processing.Generally these compounds are notcaptured by AOAC Method 985.29.

Other AOAC-accepted methods tomeasure the fiber content of thesenovel fibers have been developed orare currently in development (8).

Although the Institute of Medicinereport recommended that the termssolubl e fiber and insoluble fiber not be

used (1), food labels still may includesoluble and insoluble fiber data. Thewater-soluble fiber is precipitated in amixture of enzymes and ethanol. Di-etary fiber was divided into solubleand insoluble fiber in an attempt toassign physiologic effects to chemicaltypes of fiber. Oat bran, barley bran,and psyllium, mostly soluble fiber,have health claims for their ability tolower blood lipid levels. Wheat branand other more insoluble fibers aretypically linked to laxation. Yet, sci-entific support that soluble fibers

lower blood cholesterol, whereas in-soluble fibers increase stool size, isinconsistent at best.

Resistant starch (the sum of starchand starch-degradation products notdigested in the small intestine) (9)reaches the large intestine and wouldfunction as dietary fiber. Legumes area primary source of resistant starch,with as much as 35% of legume starchescaping digestion (10). Smallamounts of resistant starch are pro-duced by processing and baking of ce-real and grain products. Many newfunctional fibers increasingly being

added to processed foods are resistantstarches. Murphy and colleagues (11)estimated resistant starch intakes inthe United States. A database of re-sistant starch concentrations in foodswas developed from published values.These values were linked to foods re-ported in 24-hour dietary recalls fromparticipants in the 1999-2002 Na-tional Health and Nutrition Exami-nation Surveys to estimate resistantstarch intakes. Americans aged 1year and older were estimated to con-sume approximately 4.9 g resistant

starch per day (range 2.8 to 7.9g/day).

Other functional fibers were reviewed by the DRI committee and arelisted in the Figure. Dietary fiber in-cludes plant nonstarch polysaccha-rides (eg, cellulose, pectin, gums,hemicellulose, ␤-glucans, and fibercontained in oat and wheat bran),plant carbohydrates that are not re-covered by alcohol precipitation (eg,inulin, oligosaccharides, and fruc-tans), lignin, and some resistant

Table 2. Dietary fiber content of commonly consumed fruits, vegetables, grains, and otherfoodsa

Food Serving sizeTotal dietary fiber(g/serving)

Fruits

Prunes, dried 5 prunes 3.0Orange 1 orange 3.1

Apple, large with skin 1 apple 3,7Banana 1 banana 2.8Raisins 1 miniature box (14 g) 0.6Figs, dried 2 figs 4.6Pear 1 pear 4.0Peaches, canned 1 ⁄ 2 c 1.3Strawberries, raw 1 c, sliced 3.8VegetablesBeans, kidney, canned 1 ⁄ 2 c 4.5Peas, split, cooked 1 ⁄ 2 c 8.1Lentils, cooked 1 ⁄ 2 c 7.8Lettuce, iceberg 1 c, shredded 0.8

Peas, green, canned

1

⁄ 2

c 3.5Brussels sprouts 1 ⁄ 2 c 2.0Spinach, cooked 1 ⁄ 2 c 2.2Carrots, raw 1 ⁄ 2 c 1.8Potatoes, boiled 1 ⁄ 2 c 1.6Broccoli, raw 1 ⁄ 2 c 1.3Celery, raw 1 ⁄ 2 c 1.0GrainsWheat bran flakes 3 ⁄ 4 c 4.6Raisin bran 1 c 7.5Shredded wheat 2 biscuits 5.0Rice, brown, cooked 1 c 3.5Bread, white wheat 1 slice 0.6Bread, whole wheat 1 slice 1.9Oatmeal, cooked 3 ⁄ 4 c 3.0

Oat bran muffin 1 muffin 2.6Rye crispbread 1 wafer 1.7Crackers, graham 2 squares 0.4Other

Apple pie 1 piece 1.9Nuts, mixed, dry roast 1 oz 2.6Chocolate cake 1 slice 1.8

Yellow cake 1 slice 0.2

aSource: Adapted from the US Department of Agriculture Nutrient Database for Standard Reference, Release 14

(http://www.nal.usda.gov/fnic/foodcomp/Data/SR14/sr14.html ).

1718 October 2008 Volume 108 Number 10

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starch. Potential functional fibers in-clude isolated, nondigestible plant(eg, resistant starch, pectin, andgums), animal (eg, chitin and chi-

tosan), or commercially produced car-bohydrates (eg, resistant starch, poly-dextrose, inulin, and indigestibledextrins) (1).

BENEFITS OF ADEQUATE FIBER INTAKE

This American Dietetic Association(ADA) position paper uses ADA’s Ev-idence Analysis Process and informa-tion from A DA’s Evidence AnalysisLibrary (12). Four topics were in-cluded in the evidence analysis fordietary fiber: cardiovascular disease,gastrointestinal health and disease,

weight control, and diabetes. The Ev-idence Analysis Library does not in-clude the topic of dietary fiber andcancer.

Cardiovascular Disease

What is the evidence that dietary fiber from whole foods and dietary supple-ments is beneficial in cardiovasculardisease?

Conclusion Statement. Based on currentdata, dietary fiber intake from wholefoods or supplements may lower blood

pressure, improve serum lipid levels,and reduce indicators of inflamma-tion. Benefits may occur with intakesof 12 to 33 g fiber per day from wholefoods or up to 42.5 g fiber per day fromsupplements. Grade II–Fair.

The DRI recommendations for di-etary fiber are based on protectionagainst cardiovascular disease (CVD),so there is consistent and strong datafor this relationship (1). The commit-tee used epidemiologic, cohort studiesthat estimated dietary fiber intake

from food frequencies and followedsubjects prospectively until CVD wasdetected. Dietary fiber intake levelsfound to be protective against CVD

were then used to determine an AI fordietary fiber. Although there wererecommendations for dietary fiber in-take before 2002, there were no offi-cial recommendations until the 2002DRIs (1).

Since the publication of the DRIs,other epidemiologic studies also sup-port that dietary fiber intake protectsagainst CVD. Bazzano and colleagues(13) examined the relationship be-tween total and soluble dietary fiberintake and the risk of CHD and CVDin 9,776 adults who were free of CVDat baseline and who participated in

the National Health and NutritionExamination Survey I EpidemiologicFollow-up Study. A 24-hour dietaryrecall was used to assess dietary in-take. A higher intake of dietary fiber,particularly water-soluble fiber, re-duced risk of CHD.

Pereira and colleagues (14) com-pleted a pooled analysis of cohortstudies of dietary fiber and risk of CHD. Ten prospective cohort studiesfrom the United State and Europewere used to estimate the associationbetween dietary fiber intake and risk

of CHD. During 6 to 10 years of fol-low-up, each 10 g/day increment of energy-adjusted and measurementerror-corrected total dietary fiber wasassociated with a 14% decrease inrisk of all coronary events and a 27%decrease in risk of coronary death.Only fiber from cereals and fruits wasfound to be inversely associated withrisk of CHD.

The link between fiber intake andCVD was also measured in olderadults (15). A population-based, mul-

ticenter study among 3,588 men andwomen aged 65 years or older andfree of known CVD at baseline wasconducted. During 8.6 years mean fol-low-up, there were 811 incident CVDevents among 3,588 men and women.Cereal fiber consumption was in-

versely associated with incident CVDwith 21% lower risk in the highestquintile of intake, compared with thelowest quintile. Neither fruit fiber nor

vegetable fiber was associated withincident CVD. The authors suggestthat cereal fiber consumption late inlife is associated with lower risk of incident CVD, supporting recommen-dations for older adults to increaseconsumption of dietary cereal fiber.

Whole grain intake is also known toprotect against CVD and Jensen andcolleagues (16) attempted to deter-

mine which parts of the whole grainare most important for protection.They measured whole grains, bran,and germ intake in a prospective co-hort study of 42,850 male health pro-fessionals aged 40 to 75 years at base-line in 1986 who were free from CVD.Whole grain intake and added branwere protective, while added germwas not. This suggests that wholegrains are protective against CVDand that the bran component of wholegrains is the important factor in theprotection.

Although epidemiologic, prospec-

tive studies are consistent in theirsupport that dietary fiber protectsagainst CVD, there is much confusionabout which components of dietary fi-ber are most protective. The DRI com-mittee concluded that fiber from cere-als seems most protective. Inaddition, certain functional fiber, par-ticularly those that are soluble and

viscous may alter biomarkers of inter-est in CVD. Several mechanisms havebeen suggested to explain fiber’s pro-tective properties in CVD. Viscous fi-bers lower blood cholesterol levels,

specifically that fraction transportedby low-density lipoproteins (LDL)(17). Meta-analysis by Brown and col-leagues (18) showed that daily intakeof 2 to 10 g soluble fiber significantlylowered serum total cholesterol andLDL-cholesterol concentrations. Themajority of these studies showed nochange in high-density lipoproteincholesterol or triacylglycerol concen-trations with soluble fiber. Fibersthat lower blood cholesterol levels in-clude foods such as apples, barley,

CharacteristicDietaryfiber

Nondigestible animal carbohydrate NoCarbohydrates nonrecovered by alcohol precipitationa YesNondigestible mono- and disaccharides and polyols NoLignin YesResistant starch SomeIntact, natural ly occurring food source only YesResistant to human enzymes YesSpecifies physiological effect No

Figure. Characteristics of dietary fiber. Source: Adapted from reference 7. aIncludes inulin,oligosaccharides (three to 10 degrees of polymerization), fructans, polydextrose, methylcellulose,resistant maltodextrins, and other related compounds.




beans and other legumes, fruits and vegetables, oatmeal, oat bran, andrice hulls, and purified sources suchas beet fiber, guar gum, karaya gum,konjac mannan, locust bean gum, pec-tin, psyllium seed husk, soy polysac-charide and xanthan gum (17). Three

of these fibers, namely ␤-glucan inoats, ␤-glucan in barley, and psylliumhusk, have been sufficiently studiedfor the US Food and Drug Adminis-tration to authorize a health claimthat foods meeting specific composi-tional requirements and containing 0.75 g to 1.7 g soluble fiber per serv-ing can reduce the risk of heart dis-ease (19,20).

The mechanism by which these fi-ber sources lower blood cholesterollevels has been the focus of many in-

vestigations, and characteristics such

as solubility in water, viscosity, fer-mentability, and the kinds andamounts of protein and tocotrienolshave been explored as possible basisfor this physiological effect of fiber(21). Viscosity is thought to be an im-portant factor in cholesterol lowering,although solubility and molecularweight of fibers also determine cho-lesterol lowering ability. In general,when a soluble fiber that is not vis-cous is evaluated or the fiber istreated to reduce viscosity suffi-ciently, the cholesterol-lowering abil-ity is lost (22,23). As components in

foods are digested and absorbed fromthe small intestine, fiber becomes amajor component in the gut lumen,making the viscosity evident. This

viscosity interferes with bile acid ab-sorption from the ileum (23,24). Inresponse, LDL cholesterol is removedfrom the blood and converted into bileacids by the liver to replace the bileacids lost in the stool. Changes in thecomposition of the biliary bile acidpool accompanying ingestion of some

viscous fibers dampen cholesterolsynthesis (25). Because endogenous

synthesis accounts for about threequarters of the total body cholesterolpool, slowing synthesis (as do statindrugs) could have a favorable influ-ence on blood cholesterol concentra-tions. In fact, studies of a portfoliodiet, including a wide range of foodsknown to lower serum cholesterol (eg,

viscous fiber), reported cholesterol-lowering ability similar to statindrugs (26).

Results of trials with concentrated␤-glucans from oats or barley have

been inconsistent (27). Potential rea-sons for these inconsistencies includelow effectiveness because of process-ing techniques used to isolate ␤-glu-cans, the molecular weight and/or vis-cosity of the ␤-glucans, and thedelivery method of the ␤-glucans.

Higher molecular weight fibers areassociated with increased viscosity.Higher viscosities may be linked togreater reductions in serum choles-terol concentrations and CVD risk,but this relationship is not well estab-lished. Keenan and colleagues (28)reported a 9% to 15% decrease inLDL-cholesterol with a 6-week inter-

vention of low and high molecularweight barley ␤-glucan when given atdoses of 3 and 5 g/day in a parallelstudy of 155 subjects. The high molec-ular weight barley was most effective

in cholesterol lowering, although thedifference was not statistically signif-icant. In contrast, Keogh and col-leagues (29) found no changes in cho-lesterol when 10 g/day isolated barley␤-glucan was fed in a metabolic study.

No differences in effects on bloodlipid levels were found when bothhigh- and low-molecular-weight ␤-glu-can isolated from oats was given tohuman subjects (30). Isolated ␤-glu-can from oats (5 g/day) lowered LDLcholesterol when incorporated into afruit drink (31), but when oats and

barley were compared in a similar de-sign only the lower dose of oat ␤-glu-can (5 g/day) lowered serum lipid lev-els whereas the 10 g/day dose did not(32). In contrast to oats, barley ␤-glu-cans did not lower serum lipid levelsin this study. Concentrated oat ␤-glu-can (6 g/day) lowered serum choles-terol levels in adults with hypercho-lesterolemia (33).

Other Mechanisms Whereby Fiber can Pro-tect Against CVD. Fibers also affectblood pressure and C-reactive protein(CRP), additional biomarkers linked

to risk of CVD. Dietary fiber intakewas inversely associated with CRP inthe National Health and NutritionExamination Survey 1999-2000 (34).Ma and colleagues (35) found similarresults in 524 subjects enrolled in theSeasonal Variation of Blood Choles-terol Levels Study. In an interventionstudy, fiber intake of about 30 g/dayfrom a diet naturally rich in fiber re-duced levels of CRP (36). Results withblood pressure are equally promising.

A meta-analysis of randomized place-

bo-controlled trials found that fiberintake was linked to lower blood pres-sure (37). Reductions in blood pres-sure tended to be larger in older sub-

jects and in populations withhypertension. Whelton and col-leagues (38) also reported that in-

creased intake of dietary fiber re-duced blood pressure in patients withhypertension.

Thus, epidemiologic support thatdietary fiber, especially from grains,protects against CVD is strong enough to use to set standards fordietary guidance on intake of dietaryfiber. These studies find that maxi-mum CVD protection requires intakeof 14 g dietary fiber per 1,000 kcalintake, or 38 g in men and 25 g inwomen based on estimated medianenergy intakes in Americans.

Whether isolated, functional fibersprovide protection against CVD re-mains controversial, although USFood and Drug Administration-ap-proved health claims exist for oats,barley, and pysllium.

Bowel Function

What is the evidence that dietary fiber from whole foods and dietary supple-ments is beneficial in gastrointestinalhealth and disease?

Conclusion Statement. There is a lack of data examining the impact of fiber

from whole foods on outcomes in gas-trointestinal diseases. This may bedue to the complexity and cost of these studies. However, fiber supple-ments may produce benefits in thelaxation of healthy individuals. Moreresearch is needed to clarify dose andtype of fiber in gastrointestinal healthand disease management. Grade

III–Limited.

Many fiber sources, including ce-real brans, psyllium seed husk, meth-ylcellulose, and a mixed high-fiberdiet, increase stool weight, thereby

promoting normal laxation. Stoolweight continues to increase as fiberintake increases (39,40), but theadded fiber tends to normalize defe-cation frequency to one bowel move-ment daily and gastrointestinal tran-sit time to 2 to 4 days. The increase instool weight is caused by the presenceof the fiber, by the water that the fiberholds, and by fermentation of the fi-ber, which increases bacteria in stool.If the fiber is fully and rapidly fer-mented in the large bowel, as are




most soluble fiber sources, there is noincrease in stool weight (37). It is acommon but erroneous belief that theincreased stool weight is due primar-ily to water. The moisture content of human stool is 70% to 75% and thisdoes not change when more fiber is

consumed (41). Fiber in the colon isno more effective at holding water inthe lumen than the other componentsof stool. The one known exception ispsyllium seed husk,which does in-crease the concentration of stool wa-ter to about 80% (42). But as morefiber is consumed stool weight doesincrease and increased fluid con-sumption should be recommended toaccount for this increase in fecal wa-ter loss.

Unlike blood, fecal samples havenot been collected and evaluated for a

large cohort of healthy subjects. Cum-mings and colleagues (43) conducteda meta-analysis of 11 studies in whichdaily fecal weight was measured ac-curately in 26 groups of people(Nϭ206) on controlled diets of knownfiber content. Fiber intakes were sig-nificantly related to stool weight(rϭ0.84). Stool weight varied greatlyamong subjects from different coun-tries, ranging from 72 to 470 g/day.Stool weight was inversely related tocolon cancer risk in this study. Spiller(44) suggested that there is a criticalfecal weight of 160 to 200 g/day for

adults, below which colon function be-comes unpredictable and risk of coloncancer increases. Stool weights inhealthy United Kingdom adults aver-aged only 106 g/day (43). It is likelythat average stool weights in theUnited States are also low as Cum-mings and colleagues (43) report thatstool weights in Westernized popula-tions range from 80 to 120 g/day.

Constipation and diarrhea are twoextremes of abnormal bowel function.Constipation is defined as three orfewer spontaneous bowel movements

per week (45). The longer feces re-main in the large intestine, the morewater is absorbed into the intestinalcells, resulting in hard feces and in-creased defecation difficulty. The rec-tum becomes distended, which maycause abdominal discomfort andother adverse symptoms such asheadache, loss of appetite, and nau-sea (46). Leung (47) reviewed the lit-erature on etiology of constipationand found essentially no evidence-based publications. He suggests that

teaching on constipation is based onmyths handed down from one gener-ation to the next. Etiologic factorsthought to be related to constipation,dietary fiber intake, fluid intake,physical activity, drugs, sex hor-mones, and disease status, have not

been systematically evaluated fortheir relationship to constipation.

Clinical diarrhea is defined as anelevated stool output (Ͼ200 to 250g/day); watery, difficult to controlbowel movements; and more thanthree bowel movements per day (48).Laxation refers to a slight increase inthe frequency of bowel movementsand a softer consistency of feces (49).Other symptoms that are associatedwith laxation include increased stoolweight and water content, decreasedgastrointestinal transit time, loose

stools, bloating and distention, borbo-rygmi, abdominal discomfort, and fla-tus (50). Carbohydrates that reachthe large intestine are fermented todifferent degrees, depending on thedegree of polymerization, solubility,and structure of the carbohydrates(51). Fermentation of the carbohy-drates in the large intestine producesgases, which may cause bloating, dis-tention, borborygmi, and flatulence. If the carbohydrates are not fermentedin the large intestine, either becausethe bacteria do not metabolize thecarbohydrates or because intake ex-

ceeds the fermentation capacity of thebacteria, the water remains bound tothe carbohydrates that are elimi-nated in the feces, which increasesfecal bulk, but also may produce awatery stool or diarrhea.

The total amount of poorly digestedcarbohydrates in the diet affects toler-ance. Many foods are natural laxativesbecause they contain indigestible car-bohydrates and other compounds withnatural laxative properties: cabbage,brown bread, oatmeal porridge, fruitswith rough seeds, vegetable acids

(oxalic acid), aloe, rhubarb, cascara,senna, castor oil, honey (fructose), tam-arinds, figs, prunes, raspberries, straw-berries, and stewed apples (52).

Studies have been conducted wherefiber intakes are standardized and fedin addition to controlled diets. Fecalweight increased 5.4 g/g wheat branfiber (mostly insoluble), 4.9 g/g fruitsand vegetables (soluble and insolu-ble), 3 g/g isolated cellulose (insolu-ble), and 1.3 g /g isolated pectin (solu-ble) (Table 3) (43). When subjects

were fed 15, 30, or 42 g/day diet fiberfrom a mixed diet, there was a signif-icant increase in stool weight on alldiets. Most of the increased stoolweight was from undigested dietaryfiber, although the midrange of fiberintake was also associated with an

increase in bacterial mass (53).Not just fiber in foods determines

stool weight. Slavin and colleagues(54) fed liquid diets containing 0, 30,and 60 g soy fiber and compared stoolweights to those when subjects wereconsuming their habitual diets. Dailyfecal weight averaged 145 g/day onthe habitual diets. On the liquid dietswith added fiber stool weight aver-aged 67 g/day, 100 g/day, and 150g/day. Estimated fiber intake on thehabitual diet was less than 20 g/day,supporting that other factors in solid

foods besides dietary fiber increasestool weight.Besides food intake, other factors

also affect stool size. These are oftennoted in studies, but are not wellstudied in research trials. Stress as-sociated with exams or competitioncan speed intestinal transit. Exercisemay speed intestinal transit (55), al-though data on this are conflicting.Bingham and Cummings (56) foundthat on a controlled dietary intake,transit time increased in nine sub-

jects and decreased in five when a9-week exercise program was intro-

duced. Other measures of bowel func-tion, including stool weight or fecalfrequency, were not changed by theexercise program.

Even on rigidly controlled diets of the same composition, there is a large

variation in daily stool weight among subjects. Sex is known to alter colonicf unction (57). Tucker and colleagues(58) examined the predictors of stoolweight when completely controlled di-ets were fed to normal volunteers.They found that personality was abetter predictor of stool weight than

dietary fiber intake, with outgoing subjects more likely to produce higherstool weights.

Weight Control

What is the evidence that dietary fiber from whole foods and dietary supple-ments is beneficial in obesity?

Conclusion Statement. Based on currentdata, dietary fiber intake from wholefoods or supplements may have somebenefit in terms of weight loss and




other health outcomes. Benefits mayoccur with intakes of 20 to 27 g/dayfrom whole foods or up to 20 g fiberper day from supplements. Grade

III–Limited.Heaton (59) proposed that fiber acts

as a physiological obstacle to energyintake by at least three mechanisms:

● fiber displaces available energy andnutrients from the diet;

● fiber increases chewing, which lim-its intake by promoting the secre-tion of saliva and gastric juice, re-sulting in an expansion of thestomach and increased satiety; and

● fiber reduces the absorption effi-ciency of the small intestine.

Human beings may consume a con-stant weight of food and as such, aconstant weight of lower energy (ie,high fiber) food per unit weight maypromote a reduction in weight (60).High-fiber foods have much less en-ergy density compared to high-fatfoods. Thus, high-fiber foods can dis-place other energy sources. The bulk-ing and viscosity properties of dietaryfiber are predominantly responsiblefor influencing satiation and satiety.Fiber-rich foods usually are accompa-nied by increased efforts and/or time

of mastication, which leads to in-creased satiety through a reduction inrate of ingestion.

Intrinsic, hormonal, and colonic ef-fects of dietary fiber decrease food in-take by promoting satiation and/orsatiety (61). Satiation is defined asthe satisfaction of appetite that devel-ops during the course of eating andeventually results in the cessation of eating. Satiety refers to the state inwhich further eating is inhibited andoccurs as a consequence of having

eaten. Dietary fiber also decreasesgastric emptying and/or slows energyand nutrient absorption leading tolower postprandial glucose and lipidlevels. Dietary fiber may also influ-ence fat oxidation and fat storage.

The effects of dietary fiber on hun-

ger, satiety, energy intake, and bodyweight have been reviewed (62). Themajority of studies with controlled en-ergy intake reported an increase inpostmeal satiety and a decrease insubsequent hunger with increased di-etary fiber. With ad libitum energyintake, the average effect of increas-ing dietary fiber across all the studiesindicated that an additional 14 g fiberper day resulted in a 10% decrease inenergy intake and a weight loss of more than 1.9 kg through about 3.8months of intervention. In addition,

the effects of increasing dietary fiberwere reported to be even more im-pressive in individuals with obesity.This group concluded that increasing the population’s mean dietary fiberintake from the current average of about 15 g/day to 25 to 30 g/day wouldbe beneficial and may help reduce theprevalence of obesity.

In the prospective Nurses HealthStudy, women who consumed morefiber weighed less than women whoconsumed less fiber (63). In addition,women in the highest quintile of di-etary fiber intake had a 49% lower

risk of major weight gain. More re-cently, Maskarinec and colleagues(64) reported that plant-based foodsand dietary fiber were most protectiveagainst excess body weight in a largeethnically diverse population. Howarthand colleagues (65) examined the as-sociation of dietary composition vari-ables with body mass index among US adults aged 20 to 59 years in theContinuing Survey of Food Intakes byIndividuals 1994-1996. For women, alow-fiber, high-fat diet was associatedwith the greatest increase in risk of

overweight or obesity compared witha high-fiber, low-fat diet. Davis andcolleagues (66) matched 52 normal-weight women to 52 overweight-obesewomen and found that the normalweight subjects had higher fiber andfruit intake than the subjects withobesity.

Fiber dose is an important consid-eration. Mattes (67) compared a con-trol breakfast bar to a breakfast barcontaining alginate and guar gum(0.6 g fiber vs 4.5 g fiber) in subjects

with obesity. No significant treat-ment effect or cumulative effects of satiety were found with the higherfiber containing bar. In general, largeintakes of fiber are needed to altersatiety. Few studies find any acutechanges in satiety when Ͻ10 g di-

etary fiber are consumed (68).Traditionally, high-fiber foods have

been solid foods. However, some of the newer functional fibers, such asresistant starches and oligosaccha-rides, can be easily added to drinksand may not alter viscosity. Few stud-ies on the satiating effects of drinkssupplemented with these soluble,nonviscous fibers have been pub-lished. Moorhead and colleagues (69)compared test lunches with 200 g whole carrots, blended carrots, or car-rot nutrients. Whole carrots and

blended carrots resulted in signifi-cantly higher satiety. Ad libitum foodintake for the remainder of the daydecreased in this order: carrot nutri-ents, blended carrots, whole carrots.The researchers concluded that bothfiber content and food structure areimportant determinants of satiety. A similar study was conducted using apples, applesauce, and apple juice(with added fiber) as a preload beforea meal (70). Although the three foodscontained the same energy and fiber,subjects ate significantly less lunchwhen consuming the whole apple

compared to the applesauce, apple juice, or no preload. Again, this sug-gests that adding fiber to a beveragemay not necessarily enhance satietyand that solid foods may be more sa-tiating than liquids.

As reviewed by Green and Slavin(68), many studies support that in-creased dietary fiber intake promotessatiety, decreases hunger, and thushelps provide a feeling of fullness.Foods rich in dietary fiber tend tohave a high volume and a low energydensity and should promote satiation

and satiety, and play a role in thecontrol of energy balance. However,research on the effects of differenttypes of fiber on appetite, energy, andfood intake has been inconsistent. Re-sults differ according to the type of fiber, whether it is added as an iso-lated fiber supplement rather thannaturally occurring in food sources.Short-term studies in which fiber isfed to subjects and food and energyintake assessed at subsequent mealssuggest that large amounts of total

Table 3. Average increase in fecal weightper gram fiber fed to men and womena

Fiber typeWeight (g/gfiber fed)

Wheat 5.4

Fruits and vegetables 4.7Gums and mucilages 3.7Cellulose 3.5Oats 3.4Corn 3.3Legumes 2.2Pectin 1.2

aData from reference 43.




fiber are most successful at reducing subsequent energy intake. In addi-tion, more viscous fiber may be moresuccessful in promoting satiety. Long-er-term studies of fiber intake whichexamine the effects of both intrinsicand functional fibers and satiety are

required. Yet there is ample evidencethat increasing consumption of highfiber foods and the addition of viscousfibers to the diet may decrease feel-ings of hunger by inducing satiationand satiety.

Diabetes

What is the evidence that dietary fiber from whole foods and dietary supple-ments is beneficial in diabetes?

Conclusion Statement. Based on the cur-

rent data, diets providing 30 to 50 g fiber per day from whole food sourcesconsistently produce lower serum glu-cose levels compared to a low-fiberdiet. Fiber supplements providing doses of 10 to 29 g/day may have somebenefit in terms of glycemic control.Grade III–Limited.

Although emphasis has beenplaced on specific effects that can bedetected as statistically significantwhen a particular fiber source isconsumed, dietary fiber has manysubtle, less easily quantifiable ef-fects that are beneficial. This is par-

ticularly true for fiber provided byfoods. A fiber-rich meal is processedmore slowly and nutrient absorptionoccurs over a greater time period(71). Further, a diet of foods provid-ing adequate fiber is usually less en-ergy dense and larger in volumethan a low-fiber diet that may limitspontaneous intake of energy (72).This larger mass of food takeslonger to eat and its presence in thestomach may bring a feeling of sati-ety sooner, although this feeling of fullness is short term. A diet of a

wide variety of fiber-containing foods also is usually richer inmicronutrients.

When viscous fibers are isolatedand thereby concentrated, their ef-fects on digestion are frequently eas-ier to detect. When these types of fi-bers are added to a diet, theoretically,the rate of glucose appearance in theblood is slowed and insulin secretionis subsequently reduced. These bene-ficial effects on blood glucose and in-sulin concentrations are most evident

in individuals with diabetes mellitus.In healthy individuals, the rapid in-sulin secretion that causes rapid re-moval of glucose from the blood fre-quently makes it impossible to detecta difference between blood glucoseconcentrations during a test meal

with and without a fiber supplement.Considerable experimental evi-

dence demonstrates that the additionof viscous dietary fibers slow gastricemptying rates, digestion, and the ab-sorption of glucose to benefit immedi-ate postprandial glucose metabolism(73) and long-term glucose control(74,75) in individuals with diabetesmellitus. The long-term ingestion of 50 g dietary fiber per day for 24 weekssignificantly improved glycemic con-trol and reduced the number of hypo-glycemic events in individuals with

type 1 diabetes (76). Some studies of individuals with type 2 (non–insulin-dependent diabetes) suggest thathigh fiber intakes diminish insulindemand (77). Two cohort studiesfound that fiber from cereals, but notfrom fruits and vegetables, had an in-

verse independent relationship withrisk of non–insulin-dependent diabe-tes (78,79).

The mechanisms around how fiberaffects insulin requirements or insu-lin sensitivity are not clear. Gluca-gon-like peptide 1 reduced gastricemptying rates, promoted glucose up-

take and disposal in peripheral tis-sues, enhanced insulin-dependentglucose disposal, inhibited glucagonsecretion, and reduced hepatic glu-cose output in animals and humanbeings (80). These multiple effects of glucagon-like peptide 1 may reducethe amount of insulin required by in-dividuals with impaired glucose me-tabolism when consuming a high-fi-ber diet. As more is learned about thegastrointestinal regulation of food in-take, it is clear that dietary fiber mayplay a role throughout the gastroin-

testinal tract (81).Some soluble fibers increase the

viscosity of the contents of the stom-ach and digestive tract. Higher molec-ular weight fibers increase viscosity.This altered viscosity may be respon-sible for effects on body weight andattenuated glucose and insulin re-sponse because nutrients becometrapped and emptying from the stom-ach is delayed. Few studies have beenpublished on the effectiveness of iso-lated ␤-glucans and glucose and insu-

lin control (82). Poppitt and col-leagues (83) found that a high dose,barley ␤-glucan supplement im-proved glucose control when added toa high-carbohydrate starch food, butnot when added to a high-carbohy-drate beverage. Compared to control,

5 g ␤-glucans from oats significantlylowered postprandial concentrationsof glucose and insulin, while barley␤-glucan did not (84). Barley ␤-glucanreduced plasma glucose and insulinresponses in male subjects (85).

Kaline and colleagues (86) reviewed the importance and effect of dietary fiber in diabetes prevention.They suggest that whole-grain cerealproducts appear especially effectivein the prevention of type 2 diabetesmellitus and suggest a dietary fiberintake of at least 30 g/day for protec-

tion. The Nurses Health Study cohortwas evaluated for the relationshipamong whole grain, bran, and germintake and risk of type 2 diabetes(87). Associations for bran intakewere similar to those for total wholegrain intake, whereas no significantassociation was observed for germ in-take after adjustment for bran. Theyfound that a two serving per day in-crement in whole grain consumptionwas associated with a 21% decreasein risk of type 2 diabetes after adjust-ment for potential confounders andbody mass index.

Cancer

The relationship between cancer anddietary fiber was not included in thedietary fiber Evidence Analysis Li-brary. Some of the studies reviewedin the gastrointestinal health and dis-ease question are relevant to this dis-cussion, but the studies on dietary fi-ber and cancer are inconsistent.

Large-Bowel Cancer. Extensive epide-miologic evidence supports the theorythat dietary fiber may protect against

large-bowel cancer. Epidemiologicstudies that compare colorectal can-cer incidence or mortality ratesamong countries with estimates of national dietary fiber consumptionsuggest that fiber in the diet may pro-tect against colon cancer. Data col-lected from 20 populations in 12 coun-tries showed that average stoolweight varied from 72 to 470 g/dayand was in versely related to coloncancer risk (88). When results of 13case-control studies of colorectal can-




cer rates and dietary practices werepooled, the authors concluded thatthe results provided substantive evi-dence that consumption of fiber-richfoods is inversely related to risks of both colon and rectal cancers (89).The authors estimated that the risk of

colorectal cancer in the US populationcould be reduced by about 31% withan average increase in fiber intakefrom food sources of about 13 g/day.

Three intervention studies do notsupport the protective properties of dietary fiber against colon cancer (90-92). The studies found no significanteffect of high-fiber intakes on the re-currence of colorectal adenomas.Each article describes a well-planneddietary intervention to determinewhether high-fiber food consumptioncould lower colorectal cancer risk, as

measured by a change in colorectaladenomas, a precursor of most large-bowel cancers. Several reasons havebeen given for the failure to demon-strate a benefit. Perhaps the fiber in-terventions were not long enough, thefiber dose was not high enough, recur-rence of adenoma is not an appropri-ate measure of fiber’s effectiveness inpreventing colon cancer, or these in-dividuals had already optimized theirdiets because the fiber intake by thelow-fiber control subjects exceededthat of the American population. Yetincreasing dietary fiber consumption

during 3 years did not alter recur-rence of adenomas. Despite the incon-sistency in the results of fiber andcolon cancer studies, the scientificconsensus is that there is enough ev-idence on the protectiveness of di-etary fiber against colon cancer thathealth professionals should be pro-moting increased consumption of di-etary fiber (93).

Recent follow-up of the Polyp Pre- vention Trial also found no effect of alow-fat, high-fiber, high-fruit and veg-etable diet on adenoma recurrence 8

years after randomization (94). ThePolyp Prevention Trial was a 4-yeartrial and there was some thought thatdifferences with dietary interventionwould take longer to occur. Eventhough the trial had ended, the exper-imental group continued to consumemore fiber in their diet. Still theyfound no effect of the intervention onlater polyp recurrence.

The European Prospective Investi-gation into Cancer and Nutrition is aprospective cohort study comparing

the dietary habits of more than a half-million people in 10 countries withcolorectal cancer incidence (95). Theyfound that people who ate the mostfiber (those with total fiber from foodsources averaging 33 g/day) had a25% lower incidence of colorectal can-

cer than those who ate the least fiber(12 g/day). The investigators esti-mated that populations with low av-erage fiber consumption could reducecolorectal cancer incidence by 40% bydoubling their fiber intake. Dukasand colleagues (96) reported that inthe Nurses’ Health Study, women inthe highest quintile of dietary fiberintake (median intake 20 g/day) wereless likely to experience constipationthan women in the lowest quintile(median intake 7 g/day).

Although dietary fiber intake may

not protect against colorectal cancerin prospective studies, some supportexists for the protective properties of whole-grain intake. Schatzkin andcolleagues (97) investigated the rela-tionship between whole-grain intakeand invasive colorectal cancer in theprospective National Institutes of Health-AARP Diet and Health Study.Total dietary fiber intake was not as-sociated with colorectal cancer riskwhereas whole grain consumptionwas associated with a modest reducedrisk. The association with whole

grain intake was stronger for rectalthan for colon cancer.

Breast Cancer. Limited epidemiologicevidence has been published on fiberintake and breast cancer risk in hu-man beings. Because the fat and fibercontents of the diet are generally in-

versely related, it is difficult to sepa-rate the independent effects of thesenutrients, and most research has fo-cused on the fat and breast cancerhypothesis. International compari-sons show an inverse correlation be-tween breast cancer death rates and

consumption of fiber-rich foods (98). An interesting exception to the high-fat diet hypothesis in breast cancerwas observed in Finland, where in-take of both fat and fiber is high andthe breast cancer mortality rate isconsiderably lower than in the UnitedStates and other Western countrieswhere the typical diet is high in fat(99). The large amount of fiber in therural Finnish diet may modify thebreast cancer risk associated with ahigh-fat diet. A pooled analysis of 12

case-control studies of dietary factorsand risk of breast cancer found thathigh dietary fiber intake was associ-ated with reduced risk of breast can-cer (100). Dietary fiber intake alsohas been linked to lower risk of be-nign proliferati ve epithelial disorders

of the breast (101). Not all studiesfind a relationship between dietaryfiber intake and breast cancer inci-dence, including a US prospective co-hort study (102). A pooled analysis of eight prospective cohort studies of breast cancer found that fruit and

vegetable consumption during adult-hood was not significantly associatedwith reduced breast cancer risk (103).However, a large, case-control studyreported protective effects with highintake of cereals and grains, vegeta-bles and beans (104).

Jain and colleagues (105) found noassociation among total dietary fiber,fiber fractions, and risk of breast can-cer. Still, nutrition differences, in-cluding dietary fiber intake, appear tobe important variables that contrib-ute to the higher rate of breast cancerexperienced by younger African-

American women (106). In addition, adiet high in vegetables, fruits, and fi-ber did not reduce additional breastcancer events or mortality during 7.3years of follow-up in the Women’sHealth Eating and Living random-

ized trial (107). This study was con-ducted among survivors of early stagebreast cancer and the interventiongroup received a telephone counseling program supplemented with cooking classes and newsletters that pro-moted daily targets of five vegetableservings plus 16 oz vegetable juice,three fruits, 30 g fiber, and reducedfat intake. Thus, results on breastcancer and dietary fiber are mixed,with large US prospective studiesfinding little relationship between di-etary fiber intake and breast cancer.In addition, fruit and vegetable in-take does not appear protectiveagainst breast cancer (103).

Other Cancers. Similar to colon andbreast cancer, results with other can-cers are mixed on whether fiber intakeis protective. In general, results of case-control studies are more positive thanresults with prospective trials. Cerealfiber intakes were found to reduce riskof gastric adenocarcinomas in the EP-IC-EURGAST study (108). Banderaand colleagues (109) conducted a meta-




analysis of the association between di-etary fiber and endometrial cancer.They found support from case-controlstudies, but no support for the singleprospective study that had been con-ducted. Preliminary finding from theEuropean Prospective Investigation

into Cancer and Nutrition study showno association between fruit and vege-table consumption and prostate orbreast cancer (110). Although case-con-trol studies show promise for protectionagainst cancer with dietary fiber in-take, prospective cohort studies fail tosee that fiber intakes protects againstcancer, except by perhaps indirectmethods, including obesity protection.

Other Roles for Fiber in Health

As a result of fiber serving as a sub-strate for bacteria in the large bowel,changes in intestinal bacterial popula-tions, especially with the consumptionof large amounts of purified, homoge-nous fibers (eg, fructooligosaccharidesand arabinogalactans) have been re-ported. A prebiotic is “a selectively fer-mented ingredient that allows specificchanges, both in the composition and/oractivity in the gastrointestinal micro-flora that confers benefits upon hostwell being and health” (111). The mostdata for prebiotic activity have beenpublished on inulin, a fructooligosac-charide, although trans-galactooligo-saccharides also meet the criterianeeded for prebiotic classification ac-cording to Roberfroid (111). Acceptedmethods to document whether a fiber isdeemed a prebiotic are still developing;other functional fibers known to alterthe intestinal microflora may eventu-ally be deemed prebiotics.

Fibers have also been found to af-fect mineral absorption, bone mineralcontent, and bone structure (112). Al-though we typically think of dietaryfibers as decreasing mineral absorp-tion, inulin, oligosaccharides, resis-tant starch, and other fibers havebeen found to enhance mineral ab-sorption, particularly for calcium.Most of the supportive trials in hu-man beings have been conducted inadolescents (113) and postmeno-pausal women (114), two groups gen-erally with poor calcium intakes.Whether the prebiotic fibers will en-hance calcium absorption in the gen-eral population remains to be seen.

Other Components in Fiber-ContainingFoods

There is substantial scientific evi-dence that vegetables, fruits, andwhole grains reduce risk of chronicdiseases, including cancer and heartdisease (115,116). In epidemiologic

studies, it is often easier to countservings of whole foods than translateinformation on food frequency ques-tionnaires to nutrient intakes. In ad-dition, recent studies suggest thatwhole foods offer more protectionagainst chronic diseases than dietaryfiber, antioxidants, or other biologi-cally active components in foods.Thus, associations between dietary fi-ber and disease identified through ep-idemiologic studies may actually bereflections of a synergy among dietaryfiber and these associated substances,

or of an effect of only the associatedmaterials. This suggests that the ad-dition of purified dietary fiber to food-stuffs is less likely to be beneficial asopposed to changing American dietsto include whole foods high in dietaryfiber. The concept of synergy among components in whole foods and theattendant overall healthfulness of a

varied diet are important aspects of any dietary counseling.

DISEASE RISK REDUCTION ANDTHERAPEUTIC USES OF FIBER

A lot of what is known about the ben-efits of a higher-fiber diet comes fromepidemiologic studies and DRI recom-mendations for dietary fiber intakeare based on epidemiologic findings.Sometimes there are disparities be-tween epidemiologic and metabolicstudies. One possible source of dis-crepancy is the time of collection of diet information because the foodsupply and food habits change contin-uously. Foods in current databasesmay not be reflective of what was con-sumed more than a decade ago; this is

particularly true for data for dietaryfiber in foods that have been gatheredlargely in the past 15 years. There arenow fewer differences among meth-ods of determination of total dietaryfiber in US foods so that current fiberdatabases are improved over thosethat were available previously andare reasonably useful for epidemio-logic diet studies.

In contrast, the division of total fi-ber between soluble and insoluble re-mains very method dependent. The

proportion of the total fiber that issoluble varies by two- to threefoldacross major methods of analysis,meaning that there is the same extentof variation among the values for in-soluble fiber. Thus, the use of data-bases to differentiate the effects of

soluble vs insoluble fiber with diseasecould produce statistically significantrelationships, when in fact there arenone. Also, the use of isolated, fre-quently single, fiber sources in meta-bolic studies is not representative of amixed, high-fiber diet.

Clinical Uses of Dietary Fiber

Diverticulosis. Movement of materialthrough the colon is stimulated inpart by the presence of residue in thelumen. When chronic insufficient

bulk characteristic of a low-fiber dietoccurs in the colon, the colon respondswith stronger contractions to propelthe smaller mass distally. Thischronic increased force leads to thecreation of diverticula, which are her-niations of the mucosal layer throughweak regions in the colon muscula-ture. Adequate intake of dietary fibermay prevent the formation of diver-ticula by providing bulk in the colonso that less forceful contractions areneeded to propel it. Although few clin-ical studies have been conducted on

dietary fiber and diverticular disease,case-control studies and case studiesreport success with high-fiber intakes(117).

A high-fiber diet is standard ther-apy for diverticular disease of the co-lon (118). Formed diverticula will notbe resolved by a diet adequate in fi-ber, but the bulk provided by such adiet will prevent the formation of ad-ditional diverticula, lower the pres-sure in the lumen, and reduce thechances that one of the existing diver-ticula will burst or become inflamed.

Generally, small seeds or husks thatmay not be fully digested in the uppergastrointestinal tract are eliminatedfrom a high-fiber diet for a patientwith diverticulosis as a precautionagainst having these small pieces of residue become lodged within adiverticulum.

Prevention of diverticular diseasewith dietary fiber is still unclear fromthe limited research. About 10% to25% of individuals with diverticulardisease will develop diverticulitis and




it is not clear if dietary fiber couldprotect against diverticulitis (119).

Irritable Bowel Syndrome (IBS). Gastroi-ntestinal motility has been related topsyche. IBS affects about 20% of adults in the United States and Eu-rope. IBS may disturb gastrointesti-

nal motility and reduce small intesti-nal absorption, resulting in anincrease in water that reaches thelarge intestine and diarrhea if thelarge intestinal lumen cannot absorbthe excess water; other disruptions tomotility may cause constipation. Inaddition to diarrhea and constipation,symptoms of IBS include bloating,straining, urgency, feeling of incom-plete evacuation, and passage of mu-cus (120).

The composition and health of co-lonic microflora affect the fermenta-

tion of carbohydrates. Antibiotictreatments may alter colonic bacte-ria, reducing fermentation and caus-ing diarrhea. In addition, viral or bac-terial infections, common in children,cause secretory diarrhea in which in-creased chloride ions and water aresecreted into the small intestine butnot reabsorbed. Although large dosesof fermentable carbohydrates maycause diarrhea, people may adaptover time, likely because the fermen-tation capacity of the colonic bacteriaincreases.

Individuals with inflammatorybowel disease (eg, Crohn’s diseaseand ulcerative colitis) may experienceexudative diarrhea when nutrient ab-sorption is diminished, which adds tothe increased osmotic load from thepresence of mucus, blood, and proteinfrom an inflamed gastrointestinaltract. Dietary fiber intake may im-prove symptoms of patients with in-flammatory bowel disease.

A recent review (121) suggests thata strong case cannot be made for aprotective effect of dietary fiberagainst colorectal polyp or cancer.

Also, fiber shows inconsistent resultsin chronic constipation, IBS, and di-

verticulosis. Thus, clinically, dietaryfiber should be considered as a ther-apy for bowel syndromes, but not beapplied across the board as theproven therapy.

Role of Fiber in Critical Illness and Use inEnteral Formulas

No recommendations exist for fiberintake in several disease states or for

patients in long-term-care facilities.Two types of enteral formulas thatcontain dietary fiber include blender-ized formulas made from whole foodsand formulas supplemented with pu-rified fiber sources. Purified fibersources used in enteral products in-

clude oat, pea, hydrolyzed guar gum,and sugar beet fibers, as well as oth-ers. Some formulas use a mixture of fiber sources. Many enteral formulasnow contain fructooligosaccharides.Fructooligosaccharides are short-chain oligosaccharides (usually 2 to10 monosaccharide units) that are notdigested in the upper digestive tractand therefore have some of the samephysiologic effects as soluble fiber(122). Fructooligosaccharides are rap-idly fermented by intestinal bacteriathat produce short-chain fatty acids.

Short-chain fatty acids stimulate wa-ter and electrolyte absorption andmay help treat diarrhea. Fructooligo-saccharides are a preferred substratefor bifidobacteria, but are not used bypotentially pathogenic bacteria, thushelping to maintain and restore thebalance of healthful gut flora. Fruc-tooligosaccharides are not isolated bythe standard AOAC fiber method(AOAC Method 985.29), but newmethods to analyze fructooligosaccha-rides content have been developedand accepted by AOAC.

The original rationale for adding di-

etary fiber to enteral formulas was tonormalize bowel function. Dietary fi-ber is usually promoted as a preven-tive against constipation for normalhealthy populations. Enteral formu-las containing fiber are also used inacute-care settings to prevent diar-rhea associated with tube feeding.Bowel function is affected by morethan fiber level, and there is muchindividual variation in the amount of fiber needed for optimal bowel func-tion. Studies on the biologic effects of enteral formulas containing fiber are

few; even less information is availablefrom patients. The addition of soypolysaccharide to an enteral formulasignificantly increased stool weightsof healthy male adults (123), al-though no differences in stool weightor stool frequency were observed inone study when soy polysaccharidewas added to the enteral formula of patients in a long-term-care facility(124). However, in another study of the same population that was 1 yearin length, soy polysaccharide fiber did

significantly increase daily stool fre-quency, weight, and moisture (125).Thus, existing clinical studies do notuniformly support the assertion thatthe addition of dietary fiber to an en-teral formula improves bowel func-tion.

Dietary fiber is thought to normalizebowel function in healthy subjects, andthere is anecdotal evidence of reductionof diarrhea in patients receiving fiber-containing formulas. No convincing data have been published to documentthat fiber-containing enteral formulasprevent diarrhea in tube-fed patients(126). Unfortunately, there are no stan-dard, accepted ways of defining diar-rhea. The reported incidence of diar-rhea in tube-fed patients ranges from2% to 63%. Stool frequency, stool con-sistency, and stool quantity are the

three features of bowel eliminationusually used to define diarrhea. In ad-dition to fiber, oral agents such as sor-bitol and magnesium have been sug-gested as important intake variablesaffecting stool consistency. Dietary fi-ber may improve fecal incontinence.Patients with fecal incontinence whoconsumed dietary fiber as psyllium orgum arabic had significantly fewer in-continent stools than with placebotreatment (127). Improvements in fecalincontinence or stool consistency didnot appear to be related to unfer-

mented dietary fiber.The results of some clinical studies

with dietary fiber have been disap-pointing, although the model pro-posed, that fiber is fermented by an-aerobic intestinal bacteria thatgenerate short-chain fatty acids thatserve as energy sources for colonicmucosal cells, is probably correct(128). To study the physiologic effectsof dietary fiber, especially in a sickpopulation, is extremely difficult.Most studies have been too short,measurements are semiquantitative,

and dietary fiber and short-chainfatty acid levels were frequently notmeasured. It is not clear that resultsfrom in vitro fermentation studieshave direct application in vivo.

Yang and colleagues (129) evalu-ated the effects of dietary fiber as apart of enteral nutrition formula ondiarrhea, infection, and length of hos-pital study. Seven randomized con-trolled trials with 400 patients wereincluded. The supplement of dietaryfiber in enteral nutrition was com-




pared with standard enteral formulain five trials. Combined analysis didnot show a significant reduction inoccurrence of diarrhea. Combinedanalysis of two trials of infection alsodid not show any support that dietaryfiber could decrease infection rate.

Hospital stay was significantly re-duced.

Few studies have been publishedon the effectiveness of enteral formu-las supplemented with prebiotics orsymbiotics. Standards for prebioticsare in development and attemptshave been made to limit use of theterm prebiotic unless significantchanges in gut microflora have beenshown in vivo. Symbiotics are usuallydefined as the combination of prebiot-ics and probiotics. When a fiber-freeformula was compared to a fiber-con-

taining formula, no differences wereseen in body weight, cholesterol, lym-phocyte count, renal function, or elec-trolyte balance (130). The fiber-con-taining formula did improve albuminand hemoglobin levels and diabetescontrol. The authors suggest the fi-ber-containing formula would be pre-ferred in long-term care.

An enteral formula supplementedwith prebiotic fiber was compared tostandard enteral formula in pa-tients with sever acute pancreatitis(131). Hospital stay was shorterwith the fiber-supplemented for-mula, and there were fewer compli-cations in the patients receiving thefiber-supplemented formula. Whencontinuous infusion of formula wasfed to elderly, hospitalized patients,the addition of fiber to enteral for-mula reduced the rate of diarrhea(132). Thus, overall there is mixedclinical support for inclusion of di-etary fiber in enteral formulas, al-though results with shortening of hospital stay are promising.

Few feeding studies have been con-ducted on whether prebiotics added toenteral formula will alter gut micro-flora in healthy subjects. Whelan andcolleagues (133) conducted a smallstudy (nϭ10) of healthy subjects con-suming enteral formulas with orwithout prebiotic fructooligosacchar-ides. The FOS formula increased bi-fidobacteria and reduced clostridia.The fructooligosaccharides formulaalso increased total short-chain fattyacids in feces.

POTENTIAL NEGATIVE EFFECTS OFDIETARY FIBER

Potential negative effects of excessivedietary fiber include reduced absorp-tion of vitamins, minerals, proteins,and energy. It is unlikely that healthyadults who consume dietary fiber in

amounts within the recommendedranges will have problems with nutri-ent absorption; however, high dietaryfiber intakes may not be appropriatefor children and older because so littleresearch has been conducted in thesepopulations.

Generally, dietary fiber in recom-mended amounts is thought to nor-malize transit time and should helpwhen either constipation or diarrheais present; however, case historieshave reported diarrhea when exces-sive amounts of dietary fiber are con-

sumed (134), so it is difficult to indi- vidualize fiber intake based on bowelfunction measures. Thus, stool consis-tency cannot be used as a benchmarkof appropriate dietary fiber intake.Intestinal obstruction caused by a ce-cal bezoar was reported in a seriouslyill male given fiber-containing tubefeedings and who was also receiving intestinal motility suppressing medi-cations (135). The bezoar resulted inmesenteric hemorrhage.

Esophageal obstruction from a hy-groscopic pharmacobezoar containing

glucomannan has been recently de-scribed (136). This soluble fiber holdswater and forms a highly viscous so-lution when dissolved in water. Glu-comannan has been promoted as adiet aid because it swells in the gas-trointestinal tract, theoretically pro-ducing a feeling of satiety and full-ness. This report described a 37-year-old woman who developed delayedesophageal obstruction after ingest-ing an over-the-counter diet aid con-taining glucomannan. This case illus-trates potential negative effects of

using highly viscous fiber supple-ments in patients with a history of upper gastrointestinal pathologies.

Because fiber is not digested andabsorbed in the small intestine, it canhave a laxative effect and increasethe ease and/or frequency of laxation(137). Fiber is just one low-digestiblecarbohydrate. Sugar alcohols and re-sistant starch are also poorly digestedand absorbed. Thus, all of thesepoorly digested carbohydrates maycause diarrhea and other gastrointes-

tinal symptoms such as flatulence,bloating, and abdominal discomfort.

A large intake of sugar alcohols cancause osmotic diarrhea because waterfollows the undigested and unab-sorbed carbohydrates into the largeintestine; if time is inadequate for the

intestinal cells to absorb the excesswater, it will be eliminated in the fe-ces. The dose of dietary fiber or otherpoorly digested carbohydrate thatwill have a laxative effect or contrib-ute to other gastrointestinal symp-toms depends on a number of factorsrelated to the food and the consumer.Gastrointestinal symptoms, althoughtransient, may affect consumers’ per-ception of well-being and their accep-tance of food choices containing fiberand other resistant carbohydrates.Educational messages to expect some

gastrointestinal symptoms with in-creased dietary fiber consumption areneeded.

Fermentation of dietary fiber orother nondigested carbohydrates byanaerobic bacteria in the large intes-tine produces gas, including hydro-gen, methane, and carbon dioxide,which may be related to complaints of distention or flatulence. When dietaryfiber is increased, fluid intake shouldbe also, and fiber should be increasedgradually to allow the gastrointesti-nal tract time to adapt. Furthermore,normal laxation may be achieved

with smaller amounts of dietary fiber,and the smallest dose that results innormal laxation should be accepted.

Fiber-enriched enteral formulasmay cause blockages in small-borefeeding tubes. This is most problem-atic with gums and other viscous fi-bers. Formulas containing fiber tendto be more expensive than standardformulas, making them a difficultchoice in the absence of compelling clinical data. Few data have beenpublished on the effectiveness of fi-ber-containing formulas in the long-

term setting, and less expensive andmore effective laxation aids are avail-able.

Research-based recommendationsabout which patients are good candi-dates for fiber-containing enteral for-mulas cannot be made at this time.Tube-fed patients with constipationor diarrhea who are known to haveotherwise healthful gastrointestinaltracts could be considered candidatesfor fiber-containing enteral formulas.Because of the potential protective




role of fiber against diverticulosis, co-lon cancer, diabetes, and heart dis-ease, a fiber-enriched enteral formulamay be indicated for patients in long-term enteral feeding. Fiber-contain-ing enteral formulas may work betterfor certain patients, and they should

be used if they produce positive re-sults. Clinicians should be cautious inprescribing fiber-containing enteralproducts. Because of the wide individ-ual variability of responses to dietaryfiber and the potential problems withlarge doses, the smallest dose of di-etary fiber that gives the desired re-sult should always be used.

CONCLUSIONS

Chronic insufficient intake of dietaryfiber represents a challenge for foodand nutrition professionals that can

be met with enthusiastic recommen-dations for a healthful dietary pat-tern. Increased consumption of fruits,

vegetables, legumes, and whole- andhigh-fiber grain products as recom-mended by MyPyramid would bring the majority of the North Americanadult population close to the recom-mended range of dietary fiber of 14g/1,000 kcal. In addition, a higher fi-ber intake provided by foods is likelyto be less calorically dense and lowerin fat and added sugar. The benefitsof such a varied dietary plan cannot

be overemphasized. Many of the dis-eases of public health significance—obesity, cardiovascular disease, andtype 2 diabetes—as well as the lessprevalent, but no less significant dis-eases of colonic diverticulosis andconstipation, can be prevented ortreated by increasing the amountsand varieties of fiber-containing foods. Promotion of such a food planby food and nutrition professionalsand implementation by the adult pop-ulation should increase fiber intakesof children.

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ADA Position adopted by the House of Delegates Leadership Team on

October 18, 1987, and reaffirmed on September 12, 1992; September 6, 1996;

June 22, 2000; and June 11, 2006. This position is in effect until December

31, 2013. ADA authorizes republication of the position, in its entirety,provided full and proper credit is given. Readers may copy and distribute

this article, providing such distribution is not used to indicate an endorse-

ment of product or service. Commercial distribution is not permitted without

the permission of ADA. Requests to use portions of the position must be

directed to ADA headquarters at 800/877-1600, extension 4835, or

[email protected] .

Author: Joanne L. Slavin, PhD, RD (University of Minnesota, St Paul,

MN).

Reviewers: Hope T. Bilyk, MS, RD, (Rosalind Franklin University of

Medicine and Science, North Chicago, IL); Sharon Denny, MS, RD (ADA

Knowledge Center, Chicago, IL); Mary H. Hager, PhD, RD, FADA (ADA

Government Relations, Washington, DC); Martha McMurry, MS, RD (Ore-

gon Health & Science University, Portland, OR); Nancy J. Moriarity, PhD(Quaker/Tropicana/Gatorade, Barrington, IL); Esther Myers, PhD, RD,

FADA (ADA Scientific Affairs, Chicago, IL); Gail Underbakker, MS, RD

(University of Wisconsin Hospital and Clinics, Preventive Cardiology Pro-

gram, Madison, WI); and Linda Van Horn, PhD, RD (Northwestern Univer-

sity, Feinberg School of Medicine, Chicago, IL).

Association Positions Committee Workgroup: Helen W. Lane, PhD, RD (chair);

Moya Peters, MA, RD; and Jon A. Story, PhD (content advisor).

The authors thank the reviewers for their many constructive comments and

suggestions. The reviewers were not asked to endorse this position or the

supporting paper.

mailto:[email protected]



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