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Position of the American Dietetic Association: Health Implications of Dietary

ical studies of effectiveness before should consume adequate amounts of ABSTRACT selecting functional fibers in dietetics dietary fiber from a variety of It is the position of the American Die- practice. . tetic Association that the public should J Am Assoc. 2008;108: consume adequate amounts of dietary 1716-1731. n 2002, the Institute of Medicine fiber from a variety of plant foods. Pop- published a new set of definitions ulations that consume more dietary fi- Ifor dietary fiber (1). The new defi- ber have less chronic disease. In addi- nition suggested that the term dietary tion, intake of dietary fiber has This American Dietetic Associa- fiber would describe the nondigestible beneficial effects on risk factors for de- tion (ADA) position paper uses and that are in- veloping several chronic diseases. Di- ADA’s Evidence Analysis Process trinsic and intact in , whereas etary Reference Intakes recommend and information from ADA’s Evi- functional fiber consists of the iso- consumption of 14 g dietary fiber per dence Analysis Library. The use of lated nondigestible carbohydrates 1,000 kcal, or 25 g for adult women and an evidence-based approach pro- that have beneficial physiological ef- 38 g for adult men, based on epidemio- vides important added benefits to fects in human beings. Total fiber logic studies showing protection earlier review methods. The major would then be the sum of dietary fiber against . Appro- advantage of the approach is the and functional fiber. Nondigestible priate kinds and amounts of dietary fi- more rigorous standardization of means not digested and absorbed in ber for children, the critically ill, and review criteria, which minimizes the human . the very old are unknown. The Dietary the likelihood of reviewer bias and can be fermented in the large intes- Reference Intakes for fiber are based on increases the ease with which dis- tine or can pass through the digestive recommended energy intake, not clini- parate articles may be compared. tract unfermented. There is no bio- For a detailed description of the cal fiber studies. Usual intake of di- chemical assay that reflects dietary methods used in this position paper, etary fiber in the United States is only fiber or functional fiber nutritional access ADA’s Evidence Analysis Pro- 15 g/day. Although of fiber status (eg, blood fiber levels cannot be cess (www.adaevidencelibrary.com/ was thought to determine physiological measured because fiber is not ab- category.cfm?cidϭ7&catϭ0). effect, more recent studies suggest sorbed). No data are available to de- Conclusion Statements are as- other properties of fiber, perhaps fer- termine an Estimated Average Re- signed a grade by an expert work mentability or are important quirement and thus calculate a parameters. High-fiber diets provide group based on the systematic anal- ysis and evaluation of the support- Recommended Dietary Allowance for bulk, are more satiating, and have been total fiber, so an Adequate Intake (AI) linked to lower body weights. Evidence ing research evidence: Grade IϭGood, Grade IIϭFair, Grade IIIϭ Limited, was instead developed. The AI for fi- that fiber decreases is mixed ber is based on the median fiber in- and further research is needed. Grade IVϭExpert Opinion only, and Grade VϭGrade is not assignable take level observed to achieve the Healthy children and adults can lowest risk of coronary heart disease achieve adequate dietary fiber intakes (because there is no evidence to sup- port or refute the conclusion). Evi- (CHD). A Tolerable Upper Intake by increasing variety in daily pat- Level was not set for dietary fiber or terns. Dietary messages to increase dence-based information for this functional fiber. consumption of high-fiber foods such as and other topics can be found at the Dietary fiber is part of a plant ma- whole grains, , , and veg- Evidence Analysis Library (www. trix which is largely intact. Nondi- etables should be broadly supported by adaevidencelibrary.com) and sub- gestible plant carbohydrates in foods food and professionals. Con- scriptions for non-ADA members are usually a mixture of polysaccha- sumers are also turning to fiber supple- can be purchased at the Evidence rides that are integral components of ments and bulk as additional Analysis Library’s on-line store the plant cell wall or intercellular fiber sources. Few fiber supplements (www.adaevidencelibrary.com/ have been studied for physiological ef- store.cfm). structure. This definition recognizes fectiveness, so the best advice is to con- that the three-dimensional plant ma- sume fiber in foods. Look for physiolog- trix is responsible for some of the physicochemical properties attrib- uted to dietary fiber and that dietary POSITION STATEMENT fiber contains other nor- 0002-8223/08/10810-0015$34.00/0 It is the position of The American Di- mally found in foods, which are im- doi: 10.1016/j.jada.2008.08.007 etetic Association that the public portant in the potential health ef-

1716 Journal of the AMERICAN DIETETIC ASSOCIATION © 2008 by the American Dietetic Association foods are brought into the diet. Al- Table 1. Dietary Reference Intakes (DRI) for total fibera by life stage group and DRI values though based on limited clinical data, (g/1,000 kcal/d)b a previous fiber recommendation for Adequate Intakec children older than 2 years is to in- Life stage group Men g/1,000 kcal/d Women g/1,000 kcal/d crease dietary fiber intake to an amount equal to or greater than their 0-6 mo NDd ND ND ND age plus 5 g/day and to achieve in- 7-12 mo ND ND ND ND takes of 25 to 35 g/day after age 20 1-3 y 14 19 14 19 years (2). 4-8 y 14 25 14 25 Little clinical data are available for 9-13 y 14 31 14 26 fiber needs in the elderly. Thus, the 14-18 y 14 38 14 26 fiber AI for older adults is also based 19-30 y 14 38 14 25 on 14 g/1,000 kcal. As older adults 31-50 y 14 38 14 25 require less dietary energy than 51-70 y 14 30 14 21 young adults, the AI for fiber con- Ͼ70 y 14 30 14 21 sumption in older adults decreases. Pregnancy All fiber recommendations need to Ͻ18 y NAe NA 14 29 recognize the importance of adequate 19-50 y NA NA 14 28 fluid intake, and caution should be Lactation used when recommending fiber to Ͻ18 y NA NA 14 29 those with gastrointestinal diseases, 19-50 y NA NA 14 29 including . The 2005 US Dietary Guidelines aTotal fiber is the combination of dietary fiber (the edible, nondigestible and lignin components in plant foods) and functional fiber (isolated, extracted, or synthetic fiber that has proven health benefits). recommend high-fiber food such as bValues are example of the total grams per day of total fiber calculated from g/1,000 kcal multiplied by the median whole grains and and energy intake (kcal/1,000 kcal/day) from the Continuing Survey of Food Intakes by Individuals 1994-1996, 1998. fruits, and fiber intake levels of 14 cIf sufficient scientific evidence is not available to establish an Estimated Average Requirement, and thus calculate a g/1,000 kcal (3). MyPyramid also sup- Recommended Dietary Allowance, an Adequate Intake (AI) is usually developed. For healthy, breastfed infants, the AI is ports this recommendation (4). Nutri- the mean intake. The AI for other life stage and sex groups is believed to cover the needs of all healthy individuals in tion Facts labels use 25 g dietary fiber 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. per day for a 2,000 kcal/day diet or 30 dNDϭnot determined. g/day for a 2,500 kcal/day diet as eNAϭnot applicable. goals for American intake. Dietary fiber intake continues to be less than recommended in the United fects. brans, which are efits were not used as the basis for the States with usual intakes averaging obtained by grinding, are anatomical AI. only 15 g per day (1). When asked layers of the grain consisting of intact There is no AI for fiber for healthy about their perceptions of their di- cells and substantial amounts of infants aged 0 to 6 months who are etary fiber intake, 73% of individuals and protein; they are catego- fed human milk because human milk with a mean fiber intake below 20 g/d rized as dietary fiber sources. does not contain dietary fiber. During think the amount of fiber they con- Dietary Reference Intakes (DRIs) the 7- to 12-month age period, solid sume is “about right” (5). Many pop- for total fiber by life stage group are food intake becomes more significant, ular American foods contain little di- shown in Table 1. The AIs for total and so dietary fiber intake may in- etary fiber. Servings of commonly fiber are based on the intake level crease. However, there are no data on consumed grains, fruits, and vegeta- dietary fiber intake in this age group observed to protect against CHD bles contain only 1 to 3 g dietary fiber and no theoretical reason to establish based on epidemiologic, clinical, and (6)(Table 2). Major sources of dietary an AI. There is also no information to mechanistic data. The reduction of fiber in the US food supply include indicate that fiber intake as a func- risk of can be used as a sec- tion of energy intake differs during grains and vegetables (7). White flour ondary endpoint to support the rec- the life cycle. and white potatoes provide the most ommended intake level. The relation- Fiber recommendations for chil- fiber to the diet, about 16% and 9%, ship of fiber intake to colon cancer is dren and elderly persons were also respectively, not because they are the subject of ongoing investigation. based on the consumption of 14 g fiber concentrated fiber sources, but be- The DRI development panel sug- per 1,000 kcal consumed. No pub- cause they are widely consumed. Le- gested the recommended intakes of lished studies have defined desirable gumes are very rich in dietary fiber, total fiber may also help ameliorate fiber intakes for infants and children but because of low consumption only constipation and diverticular disease, younger than age 2 years. Until there provide about 6% of the fiber in the provide fuel for colon cells, reduce is more information about the effects US diet. Fruits provide only 10% of blood and lipid levels, and of dietary fiber in the very young, a the fiber in the overall US diet be- provide a source of -rich, low- rational approach would be to intro- cause of low consumption and energy-dense foods that could con- duce a variety of fruits, vegetables, the low amount of fiber in fruits, ex- tribute to satiety, although these ben- and easily digested as solid cept for dried fruits.

October 2008 ● Journal of the AMERICAN DIETETIC ASSOCIATION 1717 Other AOAC-accepted methods to Table 2. Dietary fiber content of commonly consumed fruits, vegetables, grains, and other a measure the fiber content of these foods novel fibers have been developed or Total dietary fiber are currently in development (8). Food Serving size (g/serving) Although the Institute of Medicine report recommended that the terms Fruits soluble fiber and insoluble fiber not be , dried 5 prunes 3.0 used (1), food labels still may include 1 orange 3.1 soluble and insoluble fiber data. The , large with skin 1 apple 3,7 water-soluble fiber is precipitated in a 1 banana 2.8 mixture of enzymes and ethanol. Di- 1 miniature box (14 g) 0.6 etary fiber was divided into soluble Figs, dried 2 figs 4.6 and insoluble fiber in an attempt to 1 pear 4.0 assign physiologic effects to chemical , canned 1⁄2 c 1.3 types of fiber. , bran, , raw 1 c, sliced 3.8 and , mostly soluble fiber, Vegetables have health claims for their ability to , kidney, canned 1⁄2 c 4.5 lower blood lipid levels. bran , split, cooked 1⁄2 c 8.1 and other more insoluble fibers are , cooked 1⁄2 c 7.8 typically linked to laxation. Yet, sci- , iceberg 1 c, shredded 0.8 entific support that soluble fibers Peas, green, canned 1⁄2 c 3.5 lower blood , whereas in- Brussels sprouts 1⁄2 c 2.0 soluble fibers increase stool size, is , cooked 1⁄2 c 2.2 inconsistent at best. , raw 1⁄2 c 1.8 (the sum of starch Potatoes, boiled 1⁄2 c 1.6 and starch-degradation products not , raw 1⁄2 c 1.3 digested in the small intestine) (9) , raw 1⁄2 c 1.0 reaches the and would Grains function as dietary fiber. Legumes are Wheat bran flakes 3⁄4 c 4.6 a primary source of resistant starch, bran 1 c 7.5 with as much as 35% of starch Shredded wheat 2 biscuits 5.0 escaping digestion (10). Small , brown, cooked 1 c 3.5 amounts of resistant starch are pro- , white wheat 1 slice 0.6 duced by processing and baking of ce- Bread, whole wheat 1 slice 1.9 real and grain products. Many new , cooked 3⁄4 c 3.0 functional fibers increasingly being Oat bran muffin 1 muffin 2.6 added to processed foods are resistant crispbread 1 wafer 1.7 . Murphy and colleagues (11) Crackers, graham 2 squares 0.4 estimated resistant starch intakes in Other the United States. A database of re- 1 piece 1.9 sistant starch concentrations in foods Nuts, mixed, dry roast 1 oz 2.6 was developed from published values. cake 1 slice 1.8 These values were linked to foods re- Yellow cake 1 slice 0.2 ported in 24-hour dietary recalls from participants in the 1999-2002 Na- a Source: Adapted from the US Department of Agriculture Nutrient Database for Standard Reference, Release 14 tional Health and Nutrition Exami- (http://www.nal.usda.gov/fnic/foodcomp/Data/SR14/sr14.html). nation Surveys to estimate resistant starch intakes. Americans aged 1 DEFINITION AND SOURCES OF FIBER the dietary fiber content of food prod- year and older were estimated to con- A variety of definitions of dietary fiber ucts within the United States, dietary sume approximately 4.9 g resistant exist (8). Some are based primarily fiber is defined as the material iso- starch per day (range 2.8 to 7.9 upon analytical methods used to iso- lated by analytical methods approved g/day). late and quantify dietary fiber by the Association of Official Analyt- Other functional fibers were re- whereas others are physiologically ical Chemists (AOAC), generally viewed by the DRI committee and are based. Dietary fiber is primarily the AOAC Method 985.29 (8). A variety of listed in the Figure. Dietary fiber in- storage and cell wall low molecular carbohydrates such as cludes plant nonstarch polysaccha- of plants that cannot be hydrolyzed by resistant starch, , and rides (eg, , , gums, human digestive enzymes. Lignin, nondigestible in- , ␤-, and fiber which is a complex molecule of poly- cluding fructo- and galacto-oligosac- contained in oat and wheat bran), phenylpropane units and present chardies are being developed and in- plant carbohydrates that are not re- only in small amounts in the human creasingly used in food processing. covered by alcohol precipitation (eg, diet, is also usually included as a com- Generally these compounds are not , oligosaccharides, and fruc- ponent of dietary fiber. For labeling captured by AOAC Method 985.29. tans), lignin, and some resistant

1718 October 2008 Volume 108 Number 10 ticenter study among 3,588 men and Dietary women aged 65 years or older and Characteristic fiber free of known CVD at baseline was conducted. During 8.6 years mean fol- Nondigestible animal carbohydrate No a low-up, there were 811 incident CVD Carbohydrates nonrecovered by alcohol precipitation Yes events among 3,588 men and women. Nondigestible mono- and and polyols No Cereal fiber consumption was in- Lignin Yes versely associated with incident CVD Resistant starch Some with 21% lower risk in the highest Intact, naturally occurring food source only Yes quintile of intake, compared with the Resistant to human enzymes Yes lowest quintile. Neither fruit fiber nor Specifies physiological effect No fiber was associated with incident CVD. The authors suggest a Figure. Characteristics of dietary fiber. Source: Adapted from reference 7. Includes inulin, that cereal fiber consumption late in oligosaccharides (three to 10 degrees of polymerization), , polydextrose, methylcellulose, life is associated with lower risk of resistant , and other related compounds. incident CVD, supporting recommen- dations for older adults to increase starch. Potential functional fibers in- from food frequencies and followed consumption of dietary cereal fiber. clude isolated, nondigestible plant subjects prospectively until CVD was intake is also known to (eg, resistant starch, pectin, and detected. Dietary fiber intake levels protect against CVD and Jensen and gums), animal (eg, and chi- found to be protective against CVD colleagues (16) attempted to deter- tosan), or commercially produced car- were then used to determine an AI for mine which parts of the whole grain bohydrates (eg, resistant starch, poly- dietary fiber. Although there were are most important for protection. dextrose, inulin, and indigestible recommendations for dietary fiber in- They measured whole grains, bran, ) (1). take before 2002, there were no offi- and germ intake in a prospective co- cial recommendations until the 2002 hort study of 42,850 male health pro- DRIs (1). fessionals aged 40 to 75 years at base- BENEFITS OF ADEQUATE FIBER INTAKE Since the publication of the DRIs, line in 1986 who were free from CVD. This American Dietetic Association other epidemiologic studies also sup- Whole grain intake and added bran (ADA) position paper uses ADA’s Ev- port that dietary fiber intake protects were protective, while added germ idence Analysis Process and informa- against CVD. Bazzano and colleagues was not. This suggests that whole tion from ADA’s Evidence Analysis (13) examined the relationship be- grains are protective against CVD Library (12). Four topics were in- tween total and soluble dietary fiber and that the bran component of whole cluded in the evidence analysis for intake and the risk of CHD and CVD grains is the important factor in the dietary fiber: cardiovascular disease, in 9,776 adults who were free of CVD protection. gastrointestinal health and disease, at baseline and who participated in Although epidemiologic, prospec- weight control, and diabetes. The Ev- the National Health and Nutrition tive studies are consistent in their idence Analysis Library does not in- Examination Survey I Epidemiologic support that dietary fiber protects clude the topic of dietary fiber and Follow-up Study. A 24-hour dietary against CVD, there is much confusion cancer. recall was used to assess dietary in- about which components of dietary fi- take. A higher intake of dietary fiber, ber are most protective. The DRI com- particularly water-soluble fiber, re- mittee concluded that fiber from cere- Cardiovascular Disease duced risk of CHD. als seems most protective. In What is the evidence that dietary fiber Pereira and colleagues (14) com- addition, certain functional fiber, par- from whole foods and dietary supple- pleted a pooled analysis of cohort ticularly those that are soluble and ments is beneficial in cardiovascular studies of dietary fiber and risk of viscous may alter biomarkers of inter- disease? CHD. Ten prospective cohort studies est in CVD. Several mechanisms have Conclusion Statement. Based on current from the United State and Europe been suggested to explain fiber’s pro- data, dietary fiber intake from whole were used to estimate the association tective properties in CVD. Viscous fi- foods or supplements may lower blood between dietary fiber intake and risk bers lower blood cholesterol levels, pressure, improve serum lipid levels, of CHD. During 6 to 10 years of fol- specifically that fraction transported and reduce indicators of inflamma- low-up, each 10 g/day increment of by low-density lipoproteins (LDL) tion. Benefits may occur with intakes energy-adjusted and measurement (17). Meta-analysis by Brown and col- of 12 to 33 g fiber per day from whole error-corrected total dietary fiber was leagues (18) showed that daily intake foods or up to 42.5 g fiber per day from associated with a 14% decrease in of 2 to 10 g soluble fiber significantly supplements. Grade II–Fair. risk of all coronary events and a 27% lowered serum total cholesterol and The DRI recommendations for di- decrease in risk of coronary death. LDL-cholesterol concentrations. The etary fiber are based on protection Only fiber from cereals and fruits was majority of these studies showed no against cardiovascular disease (CVD), found to be inversely associated with change in high-density lipoprotein so there is consistent and strong data risk of CHD. cholesterol or triacylglycerol concen- for this relationship (1). The commit- The link between fiber intake and trations with soluble fiber. Fibers tee used epidemiologic, cohort studies CVD was also measured in older that lower blood cholesterol levels in- that estimated dietary fiber intake adults (15). A population-based, mul- clude foods such as , barley,

October 2008 ● Journal of the AMERICAN DIETETIC ASSOCIATION 1719 beans and other legumes, fruits and been inconsistent (27). Potential rea- bo-controlled trials found that fiber vegetables, oatmeal, oat bran, and sons for these inconsistencies include intake was linked to lower blood pres- rice hulls, and purified sources such low effectiveness because of process- sure (37). Reductions in blood pres- as beet fiber, , karaya gum, ing techniques used to isolate ␤-glu- sure tended to be larger in older sub- konjac mannan, locust gum, pec- cans, the molecular weight and/or vis- jects and in populations with tin, psyllium husk, soy polysac- cosity of the ␤-glucans, and the . Whelton and col- charide and (17). Three delivery method of the ␤-glucans. leagues (38) also reported that in- of these fibers, namely ␤- in Higher molecular weight fibers are creased intake of dietary fiber re- , ␤-glucan in barley, and psyllium associated with increased viscosity. duced blood pressure in patients with husk, have been sufficiently studied Higher may be linked to hypertension. for the US Food and Drug Adminis- greater reductions in serum choles- Thus, epidemiologic support that tration to authorize a terol concentrations and CVD risk, dietary fiber, especially from grains, that foods meeting specific composi- but this relationship is not well estab- protects against CVD is strong tional requirements and containing lished. Keenan and colleagues (28) enough to use to set standards for 0.75 g to 1.7 g soluble fiber per serv- reported a 9% to 15% decrease in dietary guidance on intake of dietary ing can reduce the risk of heart dis- LDL-cholesterol with a 6-week inter- fiber. These studies find that maxi- ease (19,20). vention of low and high molecular mum CVD protection requires intake The mechanism by which these fi- weight barley ␤-glucan when given at of 14 g dietary fiber per 1,000 kcal ber sources lower blood cholesterol doses of 3 and 5 g/day in a parallel intake, or 38 g in men and 25 g in levels has been the focus of many in- study of 155 subjects. The high molec- women based on estimated median vestigations, and characteristics such ular weight barley was most effective energy intakes in Americans. as solubility in water, viscosity, fer- in cholesterol lowering, although the Whether isolated, functional fibers mentability, and the kinds and difference was not statistically signif- provide protection against CVD re- amounts of protein and tocotrienols icant. In contrast, Keogh and col- mains controversial, although US have been explored as possible basis leagues (29) found no changes in cho- Food and Drug Administration-ap- for this physiological effect of fiber lesterol when 10 g/day isolated barley proved health claims exist for oats, (21). Viscosity is thought to be an im- ␤-glucan was fed in a metabolic study. barley, and pysllium. portant factor in cholesterol lowering, No differences in effects on blood although solubility and molecular lipid levels were found when both Bowel Function weight of fibers also determine cho- high- and low-molecular-weight ␤-glu- What is the evidence that dietary fiber lesterol lowering ability. In general, can isolated from oats was given to from whole foods and dietary supple- when a soluble fiber that is not vis- human subjects (30). Isolated ␤-glu- cous is evaluated or the fiber is ments is beneficial in gastrointestinal can from oats (5 g/day) lowered LDL health and disease? treated to reduce viscosity suffi- cholesterol when incorporated into a ciently, the cholesterol-lowering abil- fruit drink (31), but when oats and Conclusion Statement. There is a lack of ity is lost (22,23). As components in barley were compared in a similar de- data examining the impact of fiber foods are digested and absorbed from sign only the lower dose of oat ␤-glu- from whole foods on outcomes in gas- the small intestine, fiber becomes a can (5 g/day) lowered serum lipid lev- trointestinal diseases. This may be major component in the gut lumen, due to the complexity and cost of els whereas the 10 g/day dose did not making the viscosity evident. This these studies. However, fiber supple- (32). In contrast to oats, barley ␤-glu- viscosity interferes with ab- ments may produce benefits in the cans did not lower serum lipid levels sorption from the (23,24). In laxation of healthy individuals. More in this study. Concentrated oat ␤-glu- response, LDL cholesterol is removed research is needed to clarify dose and can (6 g/day) lowered serum choles- from the blood and converted into bile type of fiber in gastrointestinal health terol levels in adults with hypercho- acids by the to replace the bile and disease management. Grade acids lost in the stool. Changes in the lesterolemia (33). III–Limited. composition of the biliary bile acid Other Mechanisms Whereby Fiber can Pro- Many fiber sources, including ce- pool accompanying ingestion of some tect Against CVD. Fibers also affect real brans, psyllium seed husk, meth- viscous fibers dampen cholesterol blood pressure and C-reactive protein ylcellulose, and a mixed high-fiber synthesis (25). Because endogenous (CRP), additional biomarkers linked diet, increase stool weight, thereby synthesis accounts for about three to risk of CVD. Dietary fiber intake promoting normal laxation. Stool quarters of the total body cholesterol was inversely associated with CRP in weight continues to increase as fiber pool, slowing synthesis (as do statin the National Health and Nutrition intake increases (39,40), but the drugs) could have a favorable influ- Examination Survey 1999-2000 (34). added fiber tends to normalize defe- ence on blood cholesterol concentra- Ma and colleagues (35) found similar cation frequency to one bowel move- tions. In fact, studies of a portfolio results in 524 subjects enrolled in the ment daily and gastrointestinal tran- diet, including a wide range of foods Seasonal Variation of Blood Choles- sit time to 2 to 4 days. The increase in known to lower serum cholesterol (eg, terol Levels Study. In an intervention stool weight is caused by the presence viscous fiber), reported cholesterol- study, fiber intake of about 30 g/day of the fiber, by the water that the fiber lowering ability similar to statin from a diet naturally rich in fiber re- holds, and by of the fi- drugs (26). duced levels of CRP (36). Results with ber, which increases bacteria in stool. Results of trials with concentrated blood pressure are equally promising. If the fiber is fully and rapidly fer- ␤-glucans from oats or barley have A meta-analysis of randomized place- mented in the large bowel, as are

1720 October 2008 Volume 108 Number 10 most soluble fiber sources, there is no teaching on constipation is based on were fed 15, 30, or 42 g/day diet fiber increase in stool weight (37). It is a myths handed down from one gener- from a mixed diet, there was a signif- common but erroneous belief that the ation to the next. Etiologic factors icant increase in stool weight on all increased stool weight is due primar- thought to be related to constipation, diets. Most of the increased stool ily to water. The moisture content of dietary fiber intake, fluid intake, weight was from undigested dietary human stool is 70% to 75% and this physical activity, drugs, sex hor- fiber, although the midrange of fiber does not change when more fiber is mones, and disease status, have not intake was also associated with an consumed (41). Fiber in the colon is been systematically evaluated for increase in bacterial mass (53). no more effective at holding water in their relationship to constipation. Not just fiber in foods determines the lumen than the other components Clinical is defined as an stool weight. Slavin and colleagues of stool. The one known exception is elevated stool output (Ͼ200 to 250 (54) fed liquid diets containing 0, 30, psyllium seed husk,which does in- g/day); watery, difficult to control and 60 g soy fiber and compared stool crease the concentration of stool wa- bowel movements; and more than weights to those when subjects were ter to about 80% (42). But as more three bowel movements per day (48). consuming their habitual diets. Daily fiber is consumed stool weight does Laxation refers to a slight increase in fecal weight averaged 145 g/day on increase and increased fluid con- the frequency of bowel movements the habitual diets. On the liquid diets sumption should be recommended to and a softer consistency of (49). with added fiber stool weight aver- account for this increase in fecal wa- Other symptoms that are associated aged 67 g/day, 100 g/day, and 150 ter loss. with laxation include increased stool g/day. Estimated fiber intake on the Unlike blood, fecal samples have weight and water content, decreased habitual diet was less than 20 g/day, not been collected and evaluated for a gastrointestinal transit time, loose supporting that other factors in solid large cohort of healthy subjects. Cum- stools, and distention, borbo- foods besides dietary fiber increase mings and colleagues (43) conducted rygmi, abdominal discomfort, and fla- stool weight. a meta-analysis of 11 studies in which tus (50). Carbohydrates that reach Besides food intake, other factors daily fecal weight was measured ac- the large intestine are fermented to also affect stool size. These are often curately in 26 groups of people different degrees, depending on the noted in studies, but are not well (Nϭ206) on controlled diets of known degree of polymerization, solubility, studied in research trials. Stress as- fiber content. Fiber intakes were sig- and structure of the carbohydrates sociated with exams or competition nificantly related to stool weight (51). Fermentation of the carbohy- can speed intestinal transit. Exercise (rϭ0.84). Stool weight varied greatly drates in the large intestine produces may speed intestinal transit (55), al- among subjects from different coun- gases, which may cause bloating, dis- though data on this are conflicting. tries, ranging from 72 to 470 g/day. tention, borborygmi, and flatulence. If Bingham and Cummings (56) found Stool weight was inversely related to the carbohydrates are not fermented that on a controlled dietary intake, colon cancer risk in this study. Spiller in the large intestine, either because transit time increased in nine sub- (44) suggested that there is a critical the bacteria do not metabolize the jects and decreased in five when a fecal weight of 160 to 200 g/day for carbohydrates or because intake ex- 9-week exercise program was intro- adults, below which colon function be- ceeds the fermentation capacity of the duced. Other measures of bowel func- comes unpredictable and risk of colon bacteria, the water remains bound to tion, including stool weight or fecal cancer increases. Stool weights in the carbohydrates that are elimi- frequency, were not changed by the healthy United Kingdom adults aver- nated in the feces, which increases exercise program. aged only 106 g/day (43). It is likely fecal bulk, but also may produce a Even on rigidly controlled diets of that average stool weights in the watery stool or diarrhea. the same composition, there is a large United States are also low as Cum- The total amount of poorly digested variation in daily stool weight among mings and colleagues (43) report that carbohydrates in the diet affects toler- subjects. Sex is known to alter colonic stool weights in Westernized popula- ance. Many foods are natural laxatives function (57). Tucker and colleagues tions range from 80 to 120 g/day. because they contain indigestible car- (58) examined the predictors of stool Constipation and diarrhea are two bohydrates and other compounds with weight when completely controlled di- extremes of abnormal bowel function. natural properties: , ets were fed to normal volunteers. Constipation is defined as three or brown bread, oatmeal , fruits They found that personality was a fewer spontaneous bowel movements with rough , vegetable acids better predictor of stool weight than per week (45). The longer feces re- (), aloe, , cascara, dietary fiber intake, with outgoing main in the large intestine, the more senna, , (), tam- subjects more likely to produce higher water is absorbed into the intestinal arinds, figs, prunes, , - stool weights. cells, resulting in hard feces and in- , and stewed apples (52). creased difficulty. The rec- Studies have been conducted where tum becomes distended, which may fiber intakes are standardized and fed Weight Control cause abdominal discomfort and in addition to controlled diets. Fecal What is the evidence that dietary fiber other adverse symptoms such as weight increased 5.4 g/g wheat bran from whole foods and dietary supple- headache, loss of appetite, and nau- fiber (mostly insoluble), 4.9 g/g fruits ments is beneficial in ? sea (46). Leung (47) reviewed the lit- and vegetables (soluble and insolu- Conclusion Statement. Based on current erature on etiology of constipation ble), 3 g/g isolated cellulose (insolu- data, dietary fiber intake from whole and found essentially no evidence- ble), and 1.3 g/g isolated pectin (solu- foods or supplements may have some based publications. He suggests that ble) (Table 3)(43). When subjects benefit in terms of weight loss and

October 2008 ● Journal of the AMERICAN DIETETIC ASSOCIATION 1721 eaten. Dietary fiber also decreases with obesity. No significant treat- Table 3. Average increase in fecal weight a gastric emptying and/or slows energy ment effect or cumulative effects of per gram fiber fed to men and women and nutrient absorption leading to satiety were found with the higher Weight (g/g lower postprandial glucose and lipid fiber containing bar. In general, large Fiber type fiber fed) levels. Dietary fiber may also influ- intakes of fiber are needed to alter ence fat oxidation and fat storage. satiety. Few studies find any acute Wheat 5.4 The effects of dietary fiber on hun- changes in satiety when Ͻ10 g di- Fruits and vegetables 4.7 ger, satiety, energy intake, and body etary fiber are consumed (68). Gums and 3.7 weight have been reviewed (62). The Traditionally, high-fiber foods have Cellulose 3.5 majority of studies with controlled en- been solid foods. However, some of Oats 3.4 ergy intake reported an increase in the newer functional fibers, such as Corn 3.3 postmeal satiety and a decrease in resistant starches and oligosaccha- Legumes 2.2 subsequent with increased di- rides, can be easily added to drinks Pectin 1.2 etary fiber. With ad libitum energy and may not alter viscosity. Few stud- intake, the average effect of increas- ies on the satiating effects of drinks a Data from reference 43. ing dietary fiber across all the studies supplemented with these soluble, indicated that an additional 14 g fiber nonviscous fibers have been pub- other health outcomes. Benefits may per day resulted in a 10% decrease in lished. Moorhead and colleagues (69) occur with intakes of 20 to 27 g/day energy intake and a weight loss of compared test lunches with 200 g from whole foods or up to 20 g fiber more than 1.9 kg through about 3.8 whole carrots, blended carrots, or car- per day from supplements. Grade months of intervention. In addition, rot nutrients. Whole carrots and III–Limited. the effects of increasing dietary fiber blended carrots resulted in signifi- Heaton (59) proposed that fiber acts were reported to be even more im- cantly higher satiety. Ad libitum food as a physiological obstacle to energy pressive in individuals with obesity. intake for the remainder of the day intake by at least three mechanisms: This group concluded that increasing decreased in this order: nutri- the population’s mean dietary fiber ents, blended carrots, whole carrots. ● fiber displaces available energy and intake from the current average of The researchers concluded that both nutrients from the diet; about 15 g/day to 25 to 30 g/day would fiber content and food structure are ● fiber increases chewing, which lim- be beneficial and may help reduce the important determinants of satiety. A its intake by promoting the secre- prevalence of obesity. similar study was conducted using tion of saliva and gastric juice, re- In the prospective Nurses Health apples, applesauce, and apple juice sulting in an expansion of the Study, women who consumed more (with added fiber) as a preload before and increased satiety; and fiber weighed less than women who a (70). Although the three foods ● fiber reduces the absorption effi- consumed less fiber (63). In addition, contained the same energy and fiber, ciency of the small intestine. women in the highest quintile of di- subjects ate significantly less lunch etary fiber intake had a 49% lower when consuming the whole apple Human beings may consume a con- risk of major weight gain. More re- compared to the applesauce, apple stant weight of food and as such, a cently, Maskarinec and colleagues juice, or no preload. Again, this sug- constant weight of lower energy (ie, (64) reported that plant-based foods gests that adding fiber to a beverage high fiber) food per unit weight may and dietary fiber were most protective may not necessarily enhance satiety promote a reduction in weight (60). against excess body weight in a large and that solid foods may be more sa- High-fiber foods have much less en- ethnically diverse population. Howarth tiating than liquids. ergy density compared to high-fat and colleagues (65) examined the as- As reviewed by Green and Slavin foods. Thus, high-fiber foods can dis- sociation of dietary composition vari- (68), many studies support that in- place other energy sources. The bulk- ables with body mass index among creased dietary fiber intake promotes ing and viscosity properties of dietary US adults aged 20 to 59 years in the satiety, decreases hunger, and thus fiber are predominantly responsible Continuing Survey of Food Intakes by helps provide a feeling of fullness. for influencing satiation and satiety. Individuals 1994-1996. For women, a Foods rich in dietary fiber tend to Fiber-rich foods usually are accompa- low-fiber, high-fat diet was associated have a high volume and a low energy nied by increased efforts and/or time with the greatest increase in risk of density and should promote satiation of mastication, which leads to in- overweight or obesity compared with and satiety, and play a role in the creased satiety through a reduction in a high-fiber, low-fat diet. Davis and control of energy balance. However, rate of ingestion. colleagues (66) matched 52 normal- research on the effects of different Intrinsic, hormonal, and colonic ef- weight women to 52 overweight-obese types of fiber on appetite, energy, and fects of dietary fiber decrease food in- women and found that the normal food intake has been inconsistent. Re- take by promoting satiation and/or weight subjects had higher fiber and sults differ according to the type of satiety (61). Satiation is defined as fruit intake than the subjects with fiber, whether it is added as an iso- the satisfaction of appetite that devel- obesity. lated fiber supplement rather than ops during the course of eating and Fiber dose is an important consid- naturally occurring in food sources. eventually results in the cessation of eration. Mattes (67) compared a con- Short-term studies in which fiber is eating. Satiety refers to the state in trol breakfast bar to a breakfast bar fed to subjects and food and energy which further eating is inhibited and containing alginate and guar gum intake assessed at subsequent occurs as a consequence of having (0.6 g fiber vs 4.5 g fiber) in subjects suggest that large amounts of total

1722 October 2008 Volume 108 Number 10 fiber are most successful at reducing in individuals with diabetes mellitus. lin control (82). Poppitt and col- subsequent energy intake. In addi- In healthy individuals, the rapid in- leagues (83) found that a high dose, tion, more viscous fiber may be more sulin secretion that causes rapid re- barley ␤-glucan supplement im- successful in promoting satiety. Long- moval of glucose from the blood fre- proved glucose control when added to er-term studies of fiber intake which quently makes it impossible to detect a high-carbohydrate starch food, but examine the effects of both intrinsic a difference between blood glucose not when added to a high-carbohy- and functional fibers and satiety are concentrations during a test meal drate beverage. Compared to control, required. Yet there is ample evidence with and without a fiber supplement. 5g␤-glucans from oats significantly that increasing consumption of high Considerable experimental evi- lowered postprandial concentrations fiber foods and the addition of viscous dence demonstrates that the addition of glucose and , while barley fibers to the diet may decrease feel- of viscous dietary fibers slow gastric ␤-glucan did not (84). Barley ␤-glucan ings of hunger by inducing satiation emptying rates, digestion, and the ab- reduced plasma glucose and insulin and satiety. sorption of glucose to benefit immedi- responses in male subjects (85). ate postprandial glucose Kaline and colleagues (86) re- (73) and long-term glucose control viewed the importance and effect of Diabetes (74,75) in individuals with diabetes dietary fiber in diabetes prevention. What is the evidence that dietary fiber mellitus. The long-term ingestion of They suggest that whole-grain cereal from whole foods and dietary supple- 50 g dietary fiber per day for 24 weeks products appear especially effective ments is beneficial in diabetes? significantly improved glycemic con- in the prevention of trol and reduced the number of hypo- mellitus and suggest a dietary fiber Conclusion Statement. Based on the cur- glycemic events in individuals with intake of at least 30 g/day for protec- rent data, diets providing 30 to 50 g type 1 diabetes (76). Some studies of tion. The Nurses Health Study cohort fiber per day from whole food sources individuals with type 2 (non–insulin- was evaluated for the relationship consistently produce lower serum glu- dependent diabetes) suggest that among whole grain, bran, and germ cose levels compared to a low-fiber high fiber intakes diminish insulin intake and risk of type 2 diabetes diet. Fiber supplements providing demand (77). Two cohort studies (87). Associations for bran intake doses of 10 to 29 g/day may have some found that fiber from cereals, but not were similar to those for total whole benefit in terms of glycemic control. from fruits and vegetables, had an in- grain intake, whereas no significant Grade III–Limited. verse independent relationship with association was observed for germ in- Although emphasis has been risk of non–insulin-dependent diabe- take after adjustment for bran. They placed on specific effects that can be tes (78,79). found that a two serving per day in- detected as statistically significant The mechanisms around how fiber crement in whole grain consumption when a particular fiber source is affects insulin requirements or insu- was associated with a 21% decrease consumed, dietary fiber has many lin sensitivity are not clear. Gluca- in risk of type 2 diabetes after adjust- subtle, less easily quantifiable ef- gon-like peptide 1 reduced gastric ment for potential confounders and fects that are beneficial. This is par- emptying rates, promoted glucose up- body mass index. ticularly true for fiber provided by take and disposal in peripheral tis- foods. A fiber-rich meal is processed sues, enhanced insulin-dependent more slowly and nutrient absorption glucose disposal, inhibited glucagon Cancer occurs over a greater time period secretion, and reduced hepatic glu- The relationship between cancer and (71). Further, a diet of foods provid- cose output in animals and human dietary fiber was not included in the ing adequate fiber is usually less en- beings (80). These multiple effects of dietary fiber Evidence Analysis Li- ergy dense and larger in volume glucagon-like peptide 1 may reduce brary. Some of the studies reviewed than a low-fiber diet that may limit the amount of insulin required by in- in the gastrointestinal health and dis- spontaneous intake of energy (72). dividuals with impaired glucose me- ease question are relevant to this dis- This larger mass of food takes tabolism when consuming a high-fi- cussion, but the studies on dietary fi- longer to eat and its presence in the ber diet. As more is learned about the ber and cancer are inconsistent. stomach may bring a feeling of sati- gastrointestinal regulation of food in- Large-Bowel Cancer. Extensive epide- ety sooner, although this feeling of take, it is clear that dietary fiber may miologic evidence supports the theory fullness is short term. A diet of a play a role throughout the gastroin- that dietary fiber may protect against wide variety of fiber-containing testinal tract (81). large-bowel cancer. Epidemiologic foods also is usually richer in Some soluble fibers increase the studies that compare colorectal can- micronutrients. viscosity of the contents of the stom- cer incidence or mortality rates When viscous fibers are isolated ach and digestive tract. Higher molec- among countries with estimates of and thereby concentrated, their ef- ular weight fibers increase viscosity. national dietary fiber consumption fects on digestion are frequently eas- This altered viscosity may be respon- suggest that fiber in the diet may pro- ier to detect. When these types of fi- sible for effects on body weight and tect against colon cancer. Data col- bers are added to a diet, theoretically, attenuated glucose and insulin re- lected from 20 populations in 12 coun- the rate of glucose appearance in the sponse because nutrients become tries showed that average stool blood is slowed and insulin secretion trapped and emptying from the stom- weight varied from 72 to 470 g/day is subsequently reduced. These bene- ach is delayed. Few studies have been and was inversely related to colon ficial effects on blood glucose and in- published on the effectiveness of iso- cancer risk (88). When results of 13 sulin concentrations are most evident lated ␤-glucans and glucose and insu- case-control studies of colorectal can-

October 2008 ● Journal of the AMERICAN DIETETIC ASSOCIATION 1723 cer rates and dietary practices were the dietary habits of more than a half- case-control studies of dietary factors pooled, the authors concluded that million people in 10 countries with and risk of breast cancer found that the results provided substantive evi- incidence (95). They high dietary fiber intake was associ- dence that consumption of fiber-rich found that people who ate the most ated with reduced risk of breast can- foods is inversely related to risks of fiber (those with total fiber from food cer (100). Dietary fiber intake also both colon and rectal (89). sources averaging 33 g/day) had a has been linked to lower risk of be- The authors estimated that the risk of 25% lower incidence of colorectal can- nign proliferative epithelial disorders colorectal cancer in the US population cer than those who ate the least fiber of the breast (101). Not all studies could be reduced by about 31% with (12 g/day). The investigators esti- find a relationship between dietary an average increase in fiber intake mated that populations with low av- fiber intake and breast cancer inci- from food sources of about 13 g/day. erage fiber consumption could reduce dence, including a US prospective co- Three intervention studies do not colorectal cancer incidence by 40% by hort study (102). A pooled analysis of support the protective properties of doubling their fiber intake. Dukas eight prospective cohort studies of dietary fiber against colon cancer (90- and colleagues (96) reported that in breast cancer found that fruit and 92). The studies found no significant the Nurses’ Health Study, women in vegetable consumption during adult- effect of high-fiber intakes on the re- the highest quintile of dietary fiber hood was not significantly associated currence of colorectal adenomas. intake (median intake 20 g/day) were with reduced breast cancer risk (103). Each article describes a well-planned less likely to experience constipation However, a large, case-control study dietary intervention to determine than women in the lowest quintile reported protective effects with high whether high-fiber food consumption (median intake 7 g/day). intake of cereals and grains, vegeta- could lower colorectal cancer risk, as Although dietary fiber intake may bles and beans (104). measured by a change in colorectal not protect against colorectal cancer Jain and colleagues (105) found no adenomas, a precursor of most large- in prospective studies, some support association among total dietary fiber, bowel cancers. Several reasons have exists for the protective properties of fiber fractions, and risk of breast can- been given for the failure to demon- whole-grain intake. Schatzkin and cer. Still, nutrition differences, in- strate a benefit. Perhaps the fiber in- colleagues (97) investigated the rela- cluding dietary fiber intake, appear to terventions were not long enough, the tionship between whole-grain intake be important variables that contrib- fiber dose was not high enough, recur- and invasive colorectal cancer in the ute to the higher rate of breast cancer rence of adenoma is not an appropri- prospective National Institutes of experienced by younger African- ate measure of fiber’s effectiveness in Health-AARP Diet and Health Study. American women (106). In addition, a preventing colon cancer, or these in- Total dietary fiber intake was not as- diet high in vegetables, fruits, and fi- dividuals had already optimized their sociated with colorectal cancer risk ber did not reduce additional breast diets because the fiber intake by the whereas whole grain consumption cancer events or mortality during 7.3 low-fiber control subjects exceeded was associated with a modest reduced years of follow-up in the Women’s that of the American population. Yet risk. The association with whole Health Eating and Living random- increasing dietary fiber consumption grain intake was stronger for rectal ized trial (107). This study was con- during 3 years did not alter recur- than for colon cancer. ducted among survivors of early stage rence of adenomas. Despite the incon- breast cancer and the intervention sistency in the results of fiber and Breast Cancer. Limited epidemiologic group received a telephone counseling colon cancer studies, the scientific evidence has been published on fiber program supplemented with consensus is that there is enough ev- intake and breast cancer risk in hu- classes and newsletters that pro- idence on the protectiveness of di- man beings. Because the fat and fiber moted daily targets of five vegetable etary fiber against colon cancer that contents of the diet are generally in- servings plus 16 oz vegetable juice, health professionals should be pro- versely related, it is difficult to sepa- moting increased consumption of di- rate the independent effects of these three fruits, 30 g fiber, and reduced etary fiber (93). nutrients, and most research has fo- fat intake. Thus, results on breast Recent follow-up of the Polyp Pre- cused on the fat and breast cancer cancer and dietary fiber are mixed, vention Trial also found no effect of a hypothesis. International compari- with large US prospective studies low-fat, high-fiber, high-fruit and veg- sons show an inverse correlation be- finding little relationship between di- etable diet on adenoma recurrence 8 tween breast cancer death rates and etary fiber intake and breast cancer. years after randomization (94). The consumption of fiber-rich foods (98). In addition, fruit and vegetable in- Polyp Prevention Trial was a 4-year An interesting exception to the high- take does not appear protective trial and there was some thought that fat diet hypothesis in breast cancer against breast cancer (103). differences with dietary intervention was observed in Finland, where in- Other Cancers. Similar to colon and would take longer to occur. Even take of both fat and fiber is high and breast cancer, results with other can- though the trial had ended, the exper- the breast cancer mortality rate is cers are mixed on whether fiber intake imental group continued to consume considerably lower than in the United is protective. In general, results of case- more fiber in their diet. Still they States and other Western countries control studies are more positive than found no effect of the intervention on where the typical diet is high in fat results with prospective trials. Cereal later polyp recurrence. (99). The large amount of fiber in the fiber intakes were found to reduce risk The European Prospective Investi- rural Finnish diet may modify the of gastric adenocarcinomas in the EP- gation into Cancer and Nutrition is a breast cancer risk associated with a IC-EURGAST study (108). Bandera prospective cohort study comparing high-fat diet. A pooled analysis of 12 and colleagues (109) conducted a meta-

1724 October 2008 Volume 108 Number 10 analysis of the association between di- Other Components in Fiber-Containing proportion of the total fiber that is etary fiber and endometrial cancer. Foods soluble varies by two- to threefold They found support from case-control There is substantial scientific evi- across major methods of analysis, studies, but no support for the single dence that vegetables, fruits, and meaning that there is the same extent prospective study that had been con- whole grains reduce risk of chronic of variation among the values for in- ducted. Preliminary finding from the diseases, including cancer and heart soluble fiber. Thus, the use of data- European Prospective Investigation disease (115,116). In epidemiologic bases to differentiate the effects of into Cancer and Nutrition study show studies, it is often easier to count soluble vs insoluble fiber with disease no association between fruit and vege- servings of whole foods than translate could produce statistically significant table consumption and prostate or information on food frequency ques- relationships, when in fact there are breast cancer (110). Although case-con- tionnaires to nutrient intakes. In ad- none. Also, the use of isolated, fre- trol studies show promise for protection dition, recent studies suggest that quently single, fiber sources in meta- against cancer with dietary fiber in- whole foods offer more protection bolic studies is not representative of a take, prospective cohort studies fail to against chronic diseases than dietary mixed, high-fiber diet. see that fiber intakes protects against fiber, antioxidants, or other biologi- cancer, except by perhaps indirect cally active components in foods. methods, including obesity protection. Thus, associations between dietary fi- Clinical Uses of Dietary Fiber ber and disease identified through ep- . Movement of material idemiologic studies may actually be through the colon is stimulated in Other Roles for Fiber in Health reflections of a synergy among dietary part by the presence of residue in the fiber and these associated substances, As a result of fiber serving as a sub- lumen. When chronic insufficient or of an effect of only the associated bulk characteristic of a low-fiber diet strate for bacteria in the large bowel, materials. This suggests that the ad- changes in intestinal bacterial popula- occurs in the colon, the colon responds dition of purified dietary fiber to food- with stronger contractions to propel tions, especially with the consumption stuffs is less likely to be beneficial as of large amounts of purified, homoge- the smaller mass distally. This opposed to changing American diets chronic increased force leads to the nous fibers (eg, to include whole foods high in dietary and arabinogalactans) have been re- creation of diverticula, which are her- fiber. The concept of synergy among niations of the mucosal layer through ported. A is “a selectively fer- components in whole foods and the mented ingredient that allows specific weak regions in the colon muscula- attendant overall healthfulness of a ture. Adequate intake of dietary fiber changes, both in the composition and/or varied diet are important aspects of may prevent the formation of diver- activity in the gastrointestinal micro- any dietary counseling. ticula by providing bulk in the colon flora that confers benefits upon host so that less forceful contractions are well being and health” (111). The most needed to propel it. Although few clin- data for prebiotic activity have been DISEASE RISK REDUCTION AND ical studies have been conducted on published on inulin, a fructooligosac- THERAPEUTIC USES OF FIBER A lot of what is known about the ben- dietary fiber and diverticular disease, charide, although trans-galactooligo- case-control studies and case studies saccharides also meet the criteria efits of a higher-fiber diet comes from epidemiologic studies and DRI recom- report success with high-fiber intakes needed for prebiotic classification ac- (117). cording to Roberfroid (111). Accepted mendations for dietary fiber intake are based on epidemiologic findings. A high-fiber diet is standard ther- methods to document whether a fiber is apy for diverticular disease of the co- deemed a prebiotic are still developing; Sometimes there are disparities be- tween epidemiologic and metabolic lon (118). Formed diverticula will not other functional fibers known to alter be resolved by a diet adequate in fi- the intestinal microflora may eventu- studies. One possible source of dis- crepancy is the time of collection of ber, but the bulk provided by such a ally be deemed prebiotics. diet information because the food diet will prevent the formation of ad- Fibers have also been found to af- supply and food habits change contin- ditional diverticula, lower the pres- fect absorption, bone mineral uously. Foods in current databases sure in the lumen, and reduce the content, and bone structure (112). Al- may not be reflective of what was con- chances that one of the existing diver- though we typically think of dietary sumed more than a decade ago; this is ticula will burst or become inflamed. fibers as decreasing mineral absorp- particularly true for data for dietary Generally, small seeds or husks that tion, inulin, oligosaccharides, resis- fiber in foods that have been gathered may not be fully digested in the upper tant starch, and other fibers have largely in the past 15 years. There are are eliminated been found to enhance mineral ab- now fewer differences among meth- from a high-fiber diet for a patient sorption, particularly for . ods of determination of total dietary with diverticulosis as a precaution Most of the supportive trials in hu- fiber in US foods so that current fiber against having these small pieces of man beings have been conducted in databases are improved over those residue become lodged within a adolescents (113) and postmeno- that were available previously and diverticulum. pausal women (114), two groups gen- are reasonably useful for epidemio- Prevention of diverticular disease erally with poor calcium intakes. logic diet studies. with dietary fiber is still unclear from Whether the prebiotic fibers will en- In contrast, the division of total fi- the limited research. About 10% to hance calcium absorption in the gen- ber between soluble and insoluble re- 25% of individuals with diverticular eral population remains to be seen. mains very method dependent. The disease will develop and

October 2008 ● Journal of the AMERICAN DIETETIC ASSOCIATION 1725 it is not clear if dietary fiber could patients in long-term-care facilities. significantly increase daily stool fre- protect against diverticulitis (119). Two types of enteral formulas that quency, weight, and moisture (125). (IBS). Gastroi- contain dietary fiber include blender- Thus, existing clinical studies do not ntestinal motility has been related to ized formulas made from whole foods uniformly support the assertion that psyche. IBS affects about 20% of and formulas supplemented with pu- the addition of dietary fiber to an en- adults in the United States and Eu- rified fiber sources. Purified fiber teral formula improves bowel func- rope. IBS may disturb gastrointesti- sources used in enteral products in- tion. nal motility and reduce small intesti- clude oat, , hydrolyzed guar gum, Dietary fiber is thought to normalize nal absorption, resulting in an and beet fibers, as well as oth- bowel function in healthy subjects, and increase in water that reaches the ers. Some formulas use a mixture of there is anecdotal evidence of reduction large intestine and diarrhea if the fiber sources. Many enteral formulas of diarrhea in patients receiving fiber- large intestinal lumen cannot absorb now contain fructooligosaccharides. containing formulas. No convincing the excess water; other disruptions to Fructooligosaccharides are short- data have been published to document motility may cause constipation. In chain oligosaccharides (usually 2 to that fiber-containing enteral formulas addition to diarrhea and constipation, 10 units) that are not prevent diarrhea in tube-fed patients symptoms of IBS include bloating, digested in the upper digestive tract (126). Unfortunately, there are no stan- straining, urgency, feeling of incom- and therefore have some of the same dard, accepted ways of defining diar- plete evacuation, and passage of mu- physiologic effects as soluble fiber rhea. The reported incidence of diar- cus (120). (122). Fructooligosaccharides are rap- rhea in tube-fed patients ranges from The composition and health of co- idly fermented by intestinal bacteria 2% to 63%. Stool frequency, stool con- lonic microflora affect the fermenta- that produce short-chain fatty acids. sistency, and stool quantity are the tion of carbohydrates. Antibiotic Short-chain fatty acids stimulate wa- three features of bowel elimination treatments may alter colonic bacte- ter and electrolyte absorption and usually used to define diarrhea. In ad- ria, reducing fermentation and caus- may help treat diarrhea. Fructooligo- dition to fiber, oral agents such as sor- ing diarrhea. In addition, viral or bac- saccharides are a preferred substrate bitol and have been sug- terial infections, common in children, for bifidobacteria, but are not used by gested as important intake variables cause secretory diarrhea in which in- potentially pathogenic bacteria, thus affecting stool consistency. Dietary fi- creased chloride ions and water are helping to maintain and restore the ber may improve . secreted into the small intestine but balance of healthful gut flora. Fruc- Patients with fecal incontinence who tooligosaccharides are not isolated by not reabsorbed. Although large doses consumed dietary fiber as psyllium or the standard AOAC fiber method of fermentable carbohydrates may had significantly fewer in- (AOAC Method 985.29), but new cause diarrhea, people may adapt continent stools than with placebo methods to analyze fructooligosaccha- over time, likely because the fermen- treatment (127). Improvements in fecal rides content have been developed tation capacity of the colonic bacteria incontinence or stool consistency did and accepted by AOAC. increases. not appear to be related to unfer- The original rationale for adding di- Individuals with inflammatory mented dietary fiber. bowel disease (eg, Crohn’s disease etary fiber to enteral formulas was to The results of some clinical studies and ) may experience normalize bowel function. Dietary fi- with dietary fiber have been disap- exudative diarrhea when nutrient ab- ber is usually promoted as a preven- pointing, although the model pro- sorption is diminished, which adds to tive against constipation for normal posed, that fiber is fermented by an- the increased osmotic load from the healthy populations. Enteral formu- aerobic intestinal bacteria that presence of mucus, blood, and protein las containing fiber are also used in generate short-chain fatty acids that from an inflamed gastrointestinal acute-care settings to prevent diar- tract. Dietary fiber intake may im- rhea associated with tube feeding. serve as energy sources for colonic prove symptoms of patients with in- Bowel function is affected by more mucosal cells, is probably correct flammatory bowel disease. than fiber level, and there is much (128). To study the physiologic effects A recent review (121) suggests that individual variation in the amount of of dietary fiber, especially in a sick a strong case cannot be made for a fiber needed for optimal bowel func- population, is extremely difficult. protective effect of dietary fiber tion. Studies on the biologic effects of Most studies have been too short, against or cancer. enteral formulas containing fiber are measurements are semiquantitative, Also, fiber shows inconsistent results few; even less information is available and dietary fiber and short-chain in chronic constipation, IBS, and di- from patients. The addition of soy levels were frequently not verticulosis. Thus, clinically, dietary to an enteral formula measured. It is not clear that results fiber should be considered as a ther- significantly increased stool weights from in vitro fermentation studies apy for bowel syndromes, but not be of healthy male adults (123), al- have direct application in vivo. applied across the board as the though no differences in stool weight Yang and colleagues (129) evalu- proven therapy. or stool frequency were observed in ated the effects of dietary fiber as a one study when soy polysaccharide part of enteral nutrition formula on was added to the enteral formula of diarrhea, infection, and length of hos- Role of Fiber in Critical Illness and Use in patients in a long-term-care facility pital study. Seven randomized con- Enteral Formulas (124). However, in another study of trolled trials with 400 patients were No recommendations exist for fiber the same population that was 1 year included. The supplement of dietary intake in several disease states or for in length, soy polysaccharide fiber did fiber in enteral nutrition was com-

1726 October 2008 Volume 108 Number 10 pared with standard enteral formula POTENTIAL NEGATIVE EFFECTS OF tinal symptoms such as flatulence, in five trials. Combined analysis did DIETARY FIBER bloating, and abdominal discomfort. not show a significant reduction in Potential negative effects of excessive A large intake of sugar alcohols can occurrence of diarrhea. Combined dietary fiber include reduced absorp- cause osmotic diarrhea because water analysis of two trials of infection also tion of , , proteins, follows the undigested and unab- did not show any support that dietary and energy. It is unlikely that healthy sorbed carbohydrates into the large fiber could decrease infection rate. adults who consume dietary fiber in intestine; if time is inadequate for the Hospital stay was significantly re- amounts within the recommended intestinal cells to absorb the excess duced. ranges will have problems with nutri- water, it will be eliminated in the fe- Few studies have been published ent absorption; however, high dietary ces. The dose of dietary fiber or other on the effectiveness of enteral formu- fiber intakes may not be appropriate poorly digested carbohydrate that las supplemented with prebiotics or for children and older because so little will have a laxative effect or contrib- symbiotics. Standards for prebiotics research has been conducted in these ute to other gastrointestinal symp- are in development and attempts populations. toms depends on a number of factors related to the food and the consumer. have been made to limit use of the Generally, dietary fiber in recom- Gastrointestinal symptoms, although term prebiotic unless significant mended amounts is thought to nor- transient, may affect consumers’ per- changes in gut microflora have been malize transit time and should help when either constipation or diarrhea ception of well-being and their accep- shown in vivo. Symbiotics are usually tance of food choices containing fiber defined as the combination of prebiot- is present; however, case histories have reported diarrhea when exces- and other resistant carbohydrates. ics and . When a fiber-free Educational messages to expect some sive amounts of dietary fiber are con- formula was compared to a fiber-con- gastrointestinal symptoms with in- sumed (134), so it is difficult to indi- taining formula, no differences were creased dietary fiber consumption are vidualize fiber intake based on bowel seen in body weight, cholesterol, lym- needed. function measures. Thus, stool consis- phocyte count, renal function, or elec- Fermentation of dietary fiber or tency cannot be used as a benchmark trolyte balance (130). The fiber-con- other nondigested carbohydrates by of appropriate dietary fiber intake. taining formula did improve albumin anaerobic bacteria in the large intes- Intestinal obstruction caused by a ce- and hemoglobin levels and diabetes tine produces gas, including hydro- cal bezoar was reported in a seriously control. The authors suggest the fi- gen, methane, and carbon dioxide, ber-containing formula would be pre- ill male given fiber-containing tube which may be related to complaints of ferred in long-term care. feedings and who was also receiving distention or flatulence. When dietary An enteral formula supplemented intestinal motility suppressing medi- fiber is increased, fluid intake should with prebiotic fiber was compared to cations (135). The bezoar resulted in be also, and fiber should be increased mesenteric hemorrhage. standard enteral formula in pa- gradually to allow the gastrointesti- Esophageal obstruction from a hy- tients with sever acute pancreatitis nal tract time to adapt. Furthermore, groscopic pharmacobezoar containing (131). Hospital stay was shorter normal laxation may be achieved has been recently de- with the fiber-supplemented for- with smaller amounts of dietary fiber, scribed (136). This soluble fiber holds mula, and there were fewer compli- and the smallest dose that results in water and forms a highly viscous so- cations in the patients receiving the normal laxation should be accepted. lution when dissolved in water. Glu- Fiber-enriched enteral formulas fiber-supplemented formula. When comannan has been promoted as a continuous infusion of formula was may cause blockages in small-bore diet aid because it swells in the gas- feeding tubes. This is most problem- fed to elderly, hospitalized patients, trointestinal tract, theoretically pro- the addition of fiber to enteral for- atic with gums and other viscous fi- ducing a feeling of satiety and full- bers. Formulas containing fiber tend mula reduced the rate of diarrhea ness. This report described a 37-year- (132). Thus, overall there is mixed to be more expensive than standard old woman who developed delayed formulas, making them a difficult clinical support for inclusion of di- esophageal obstruction after ingest- choice in the absence of compelling etary fiber in enteral formulas, al- ing an over-the-counter diet aid con- clinical data. Few data have been though results with shortening of taining glucomannan. This case illus- published on the effectiveness of fi- hospital stay are promising. trates potential negative effects of ber-containing formulas in the long- Few feeding studies have been con- using highly viscous fiber supple- term setting, and less expensive and ducted on whether prebiotics added to ments in patients with a history of more effective laxation aids are avail- enteral formula will alter gut micro- upper gastrointestinal pathologies. able. flora in healthy subjects. Whelan and Because fiber is not digested and Research-based recommendations colleagues (133) conducted a small absorbed in the small intestine, it can about which patients are good candi- study (nϭ10) of healthy subjects con- have a laxative effect and increase dates for fiber-containing enteral for- suming enteral formulas with or the ease and/or frequency of laxation mulas cannot be made at this time. without prebiotic fructooligosacchar- (137). Fiber is just one low-digestible Tube-fed patients with constipation ides. The FOS formula increased bi- carbohydrate. Sugar alcohols and re- or diarrhea who are known to have fidobacteria and reduced clostridia. sistant starch are also poorly digested otherwise healthful gastrointestinal The fructooligosaccharides formula and absorbed. Thus, all of these tracts could be considered candidates also increased total short-chain fatty poorly digested carbohydrates may for fiber-containing enteral formulas. acids in feces. cause diarrhea and other gastrointes- Because of the potential protective

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1730 October 2008 Volume 108 Number 10 121. Tan K, Seow-Choen F. Fiber and colorectal diseases: Separating fact from fiction. World J Gastroenterol. 2007;13:4161-4167. 122. Roberfroid M, Slavin JL. Nondigestible oli- gosaccharides. Crit Rev Food Science Nutr. 2000;40:461-480. 123. Slavin JL, Nelson NL, McNamara EA, Cashmere K. Bowel function of healthy men consuming liquid diets with and with- out dietary fiber. JPEN J Parenter Enteral Nutr. 1985;9:317-321. 124. Fischer M, Adkins W, Hall L, Marlett JA. The effects of dietary fibre in a on bowel function of mentally retarded individ- uals. J Ment Defic Res. 1985;29:373-381. 125. Liebl BH, Fischer MH, Van Calcar SC, Marlett JA. Dietary fiber and long-term large bowel response in enterally nour- ished nonambulatory profoundly retarded youth. JPEN J Parenter Enteral Nutr. 1990;14:371-375. 126. Bliss DZ, Guenter PA, Settle RG. Defining and reporting diarrhea in tube-fed pa- tients: What a mess! Am J Clin Nutr. 1992; 55:753-759. 127. Bliss DZ, Jung HJ, Savik K, Lowry A, Le- Moine M, Jensen L, Werner C, Schaffer K. Supplementation with dietary fiber im- proves fecal incontinence. Nurs Res. 2001; 50:203-213. 128. Scheppach WM, Bartram HP. Experimen- tal evidence for and clinical implications of fiber and artificial enteral nutrition. Nutri- tion. 1993;9:399-405. 129. Yang G, Wu XT, Zhou Y, Want YL. Appli- cation of dietary fiber in clinical enteral nutrition: A meta-analysis of randomized controlled trials. World J Gastroenterol. 2005;11:3935-3938. 130. Kagansky M, Rimon E. Is there a differ- ence in metabolic outcome between differ- ADA Position adopted by the House of Delegates Leadership Team on ent enteral formulas? J Parenter Enteral Nutr. 2007;31:320-323. October 18, 1987, and reaffirmed on September 12, 1992; September 6, 1996; 131. Karakan T, Ergun M, Dogan I, Cindoruk June 22, 2000; and June 11, 2006. This position is in effect until December M, Unal S. Comparison of early enteral 31, 2013. ADA authorizes republication of the position, in its entirety, nutrition in severe acute pancreatitis with prebiotic fiber supplementation vs stan- provided full and proper credit is given. Readers may copy and distribute dard enteral solution: A prospective ran- this article, providing such distribution is not used to indicate an endorse- domized double-blind study World J Gas- troenterol. 2007;13:2733-2737. ment of product or service. Commercial distribution is not permitted without 132. Shimoni Z, Averbuch Y, Shir E, Gottshalk the permission of ADA. Requests to use portions of the position must be T, Kfir D, Niven M, Moshkowitz M, Fromm directed to ADA headquarters at 800/877-1600, extension 4835, or P. The addition of fiber and the use of con- tinuous infusion decrease the incidence of [email protected]. diarrhea in elderly tube-fed patients in Author: Joanne L. Slavin, PhD, RD (University of Minnesota, St Paul, medical wards of a general regional hospi- MN). tal: A controlled clinical trial. J Clin Gas- troenterol. 2007;41:901-905. Reviewers: Hope T. Bilyk, MS, RD, (Rosalind Franklin University of 133. Whelan K, Judd PA, Preedy VR, Simmer- Medicine and Science, North Chicago, IL); Sharon Denny, MS, RD (ADA ing R, Jann A, Taylor MA. Fructooligosac- charides and fiber partially prevent the al- Knowledge Center, Chicago, IL); Mary H. Hager, PhD, RD, FADA (ADA terations in fecal and short- Government Relations, Washington, DC); Martha McMurry, MS, RD (Ore- chain fatty acid concentrations caused by gon Health & Science University, Portland, OR); Nancy J. Moriarity, PhD standard enteral formula in healthy hu- mans. J Nutr. 2005;135:1896-1902. (Quaker/Tropicana/, Barrington, IL); Esther Myers, PhD, RD, 134. Saibil F. Diarrhea due to fiber overload. FADA (ADA Scientific Affairs, Chicago, IL); Gail Underbakker, MS, RD N Engl J Med. 1989;320:599. (University of Wisconsin Hospital and Clinics, Preventive Cardiology Pro- 135. Cooper SG, Tracey EJ. Small bowel ob- struction caused by oat bran bezoar. gram, Madison, WI); and Linda Van Horn, PhD, RD (Northwestern Univer- N Engl J Med. 1989;320:1148-1149. sity, Feinberg School of Medicine, Chicago, IL). 136. Vanderbeek PB, Fasano C, O’Malley G, Hornstein J. Esophageal obstruction from Association Positions Committee Workgroup: Helen W. Lane, PhD, RD (chair); a hygroscopic pharmacobezoar containing Moya Peters, MA, RD; and Jon A. Story, PhD (content advisor). glucomannan. Clin Toxicol (Phila). 2007; The authors thank the reviewers for their many constructive comments and 45:80-82. 137. Grabitske HA, Slavin JL. Low-digestible suggestions. The reviewers were not asked to endorse this position or the carbohydrates in practice. J Am Diet Assoc. supporting paper. 2008;108:1677-1681.

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