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Plant Residue and Bacteria as Bases for Increased Stool Weight Accompanying Consumption of Higher Dietary Fiber Diets

Department of Nutritional Sciences, University of Wisconsin-Madison, Madison, Wisconsin, JAPAN (V.S.H., J.A.M.)
Laboratory of Nutrition, Faculty of Home Economics, Kanto Gakuin Women’s Junior College, Yokohama, JAPAN (S.K.)

Address reprint requests to: J. A. Marlett, Department of Nutritional Sciences, University of Wisconsin-Madison, 1415 Linden Dr., Madison, WI, 53706. E-mail: jmarlett{at}nutrisci.wisc.edu

	ABSTRACT

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Objective: Stool diluting effects of relatively inert material, such as unfermentable dietary fiber, has been proposed as an effect of fiber beneficial to the colon. Stool dilution by increasing bacterial mass may be beneficial or deleterious, depending on bacterial metabolic products. The purpose of this study was to determine the basis for stool weight when two stepwise increases of fiber from all classes of fiber-containing foods were consumed.

Methods: Stool from five men consuming three constant diets containing 15, 30 and 42 g/d of dietary fiber were fractionated into plant material and bacteria and analyzed for neutral and amino sugar content. Fecal nitrogen, fat and ash were measured.

Results: Daily gravimetric yield and sugar content of the plant fraction from stool increased with each fiber addition. Compared to the low fiber diet, the medium fiber diet decreased the concentration of the bacterial mass in wet stool by 11% and the high fiber diet by an additional 32%. The high fiber diet decreased stool fat concentration; the medium and high fiber diets decreased stool nitrogen concentration to the same extent. Apparent digestibility of plant-derived neutral sugars decreased with each fiber addition.

Conclusions: Inherently less fermentable plant material modulates the colon environment in three beneficial ways: it is a relatively unreactive diluent of lumenal contents; it adds mass to promote distal movement of waste; it does not promote a large bacterial mass.

Key words: dietary fiber, microflora, fiber digestibility, stool composition

	INTRODUCTION

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A healthy diet is one that includes a wide variety of foods. Consistent with this dictum is the recommendation that adequate dietary fiber should be obtained through food sources, not supplements or concentrates [1]. Yet much of the information we have about the functions of dietary fiber was obtained by studying the effects of single sources and fiber isolates [1–6]. Most sources of insoluble fiber and fiber provided by mixed-food diets increase stool weight and normalize gastrointestinal transit time and defecation frequency [1,2,7], physiological responses protective against colon diseases, such as diverticulosis and cancer. Although the mechanisms by which diet impacts on the health of the large intestine remain elusive [8–10], it is well established that an energy dense diet which is high in fat and low in dietary fiber is linked with a higher incidence of colorectal cancer [8,9]. Major postulated mechanisms to explain the role of a diet adequate in fiber in colon health emphasize three aspects of the colonic environment [11,12]. Adequate fiber diets add material to the waste stream which would dilute colonic lumenal contents, presumably reducing the concentration of noxious materials. Secondly, additional dietary fiber would modify the physicochemical environment, such as bacterial substrate supply. Finally, the additional mass in the colon that occurs with adequate fiber diets eases distal propulsion of lumenal contents, and this decreased transit time means less time for interaction between the enterocyte and lumenal contents.

The additional stool mass that occurs with larger fiber intakes may be unfermented plant material or more bacteria. Stool moisture concentration is usually not affected by added fiber [1,2]. Limited data indicate the basis of the additional stool is dependent on the source of the supplemental fiber [4–6]. Unfermented plant material would function as a diluent of colon lumenal contents. Additional bacteria also would be a diluent, but a metabolically active one, with the potential to produce compounds either toxic or beneficial to the mucosa. Limited data suggest that bacteria form carcinogenic compounds from lipophilic and nitrogenous materials [12]. While daily excretion of fat and nitrogen is usually increased when more fiber is consumed, fiber’s effect on the concentrations of stool fat and nitrogen is variable, and sometimes the concentrations are reduced [3–6].

In addition to insufficient exploration of dietary fiber provided by a wide variety of foods, nearly all studies of the effects of fiber involved the evaluation of only two levels of dietary fiber, a low fiber control and a supplemented phase. More information is needed about the effects of higher levels of fiber intake. Young, physically-active individuals and those performing vigorous manual labor would typically ingest 3,000 to 4,000 kcal, which would result in a net consumption of about 35 to 50 g/d of fiber if the recommended level of intake, of 10 to 13 g/1,000 kcal, is followed. The one human study that did examine a second higher level of dietary fiber provided by food observed that stool weight continued to increase, but transit time did not decrease more than that measured during the first level of supplementation [13]. These investigators [13] did not determine what caused stool weight to increase. Available data are insufficient to predict what would happen to fecal composition if the full range of recommended fiber intakes from a mixture of foods was consumed.

In our study, healthy men consumed three constant diets containing low, medium and high amounts of dietary fiber provided by a balanced mixture of food fiber sources to address two issues in the complex association between colon health and diet. First, we tested the hypothesis that dietary fiber provided by a wide variety of foods decreased the concentration of bacteria, nitrogen and fat in stool. Secondly, we proposed that the dilutional effect on colon contents of dietary fiber from food mixtures would be maintained across a range of intakes.

	METHODS

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Human Study
Healthy young men participated in a 96-day human metabolic study that has been described [14]. Briefly, the study consisted of three, 28-day periods during which constant diets were consumed that contained low, medium and high amounts of dietary fiber provided by all classes of fiber-containing foods. There was a 15 d break and a 6 d re-adaptation between the second and third phase of the study. The study had been approved by the College of Agricultural and Life Sciences’ Committee on Research involving Human Beings, University of Wisconsin-Madison. The fecal samples used in this series of analyses were from five individuals randomly selected from the nine men who participated in the study. The mean (±SEM) initial weight, height and age of the subjects were 78.8±7.2 kg, 179±4.5 cm and 23.8±1.6 years, respectively. By adjusting intakes of foods containing little or no dietary fiber, body weights were maintained within 3% of the initial weight throughout the three months.

Diets
The diet during days 1 through 28 consisted of a two-day cycle menu of weighed, low fiber foods, typical of that consumed by the U.S. population [14]. Fiber contents of the two-day cycle menus of diets for the second and third phases (d 29–56 and d 63–90) of the study were increased by using fruits, vegetables and grain products that contained more fiber/serving and by adding legumes and grain brans (Table 1). The diets were designed to contain about 15 g, 30 g and 45 g of fiber daily, and (% of kcal) 50 carbohydrate, 15 protein and 35 fat. The kinds and amounts of protein (14%–15% of energy), carbohydrate (50%–52% of energy) and fat (33%–36% of energy) were similar at the three levels of fiber intake and in diets containing the same amount of fiber, but ranged in energy intake from 3,000 to 3,900 kcal. Intakes of all micronutrients met or exceeded the recommended or safe and adequate intakes.

Fecal Samples and Data Collection
All stools were individually collected, weighed and stored at -20°C within eight hours of defecation [14]. At the end of the study, each stool was homogenized and lyophilized for dry weight determination, chromium analysis and preparation of multi-day composites for analysis [14]. Blending using a Waring Blendor® (Dynamics Corporation of America, New Hartford, CT, 06057) had no effect on subsequent fractionation or analysis of the samples [20]. Excretion of 70% of each dose of chromium, consumed with breakfast at the beginning of weeks 3 and 4, was the measure of whole gut transit time [21]. Data for daily wet and dry stool outputs and defecation frequencies, and stool in the fecal composites were from the last 12 days of each diet period [14].

Fractionation of Fecal Samples
Duplicate aliquots (1–2 g) of the fecal composites were separated into plant, small plant, bacterial and soluble fractions based on differences in particle size and chemical solubility [6]. Briefly, samples were blended (Stomacher®R Lab Blender 400, Tekmar Co., Cincinnati, OH) with an aqueous solution of sodium lauryl sulphate (SLS), filtered through a 150-µm screen, and the residue on the screen was rinsed with water. The supernatant, obtained by centrifugation of the filtrate and rinse from these steps, was lyophilized and designated the soluble fraction. The material on the 150-µm screen was blended and filtered three more times, using SLS in formylsaline instead of water. The final residue on the 150-µm screen was plant material.

Material that passed through the 150-µm screen was then filtered through a 35-µm screen, rinsed, then blended and filtered twice more. The residue on the 35-µm screen was small plant material. Plant material is reported as the sum of the plant and small plant fractions. The filtrates and rinses that passed through the 35-µm screen were centrifuged to recover a pellet, which was the bacterial fraction. Samples were re-centrifuged when the supernatant of the plant, small plant or bacterial fractions was visibly cloudy or floating particulate was on the surface.

Analysis of Fecal Samples
Duplicate analyses were performed using the composited, lyophilized fecal samples. The AOAC method, as modified for analysis of body tissue [22], was used to determine fecal fat content. Fecal nitrogen content was measured by a micro-Kjeldahl method. The residue obtained by heating dry feces 475°C for 24 hours was fecal ash. Stool chromium content was determined by a method [23] adapted from Guncaga et al. [24]. The mean (±SEM) recovery of the 45 doses of chromium administered throughout the study was 97±3%.

The method of Kraus et al. [25], as modified [26], was used to measure the monosaccharide contents of the fecal composites and fecal fractions. Duplicate samples (25 mg of feces, soluble and bacterial fractions, 15 mg of plant and small plant fractions) were acid-hydrolyzed, neutralized, reduced and derivatized to the alditol acetate forms. Derivatized samples were analyzed by gas-liquid chromatography using a flame ionization detector and a fused silica column [26]. Response factors were determined and applied to results to account for hydrolysis and derivatization losses. Sugars are expressed as their anhydrous forms [25].

Apparent Digestibility of Fiber-Derived Sugars
Daily intakes of neutral sugars (glucose, arabinose, xylose, mannose, galactose) in the fiber extracted from the diet composites were the measure of intake. Daily excretions of the same neutral sugars in the plant plus small plant fractions isolated from stool were the measures of unfermented fiber-derived sugars. Apparent digestibility of fiber-derived sugars was calculated as the difference between intake and excretion, expressed as a percentage of intake.

Statistics
All data are expressed as the mean±SEM. Statistical significance was determined by two-way analysis of variance (SAS, release 6.11, 1995; SAS Institute Inc, Cary, NC). Means were compared by the Fisher’s protected least-significant-difference method when significant (p<0.05) differences were identified.

	RESULTS

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Stool Mass and Fecal Proximate Composition
Compared to output during the low fiber diet (94.1±11.1 g/d), stool wet weight increased to 137.9±13.2 g/d (p<0.02) and 182.7±22.3 g/d (p<0.0001) during the medium and high fiber diets, respectively. Stool moisture during the medium fiber diet was 70.2±1.3%, less (p<0.05) than what was measured during the low fiber diet, of 72.5±2.1%, but not different from stool moisture during the high fiber diet, 71.0±2.0%. Because the changes in stool moisture were modest, daily dry output (bar height, Fig. 1) followed changes seen in wet weight. Gastrointestinal transit time decreased by an average of seven hours, with each addition of fiber (low: 64.5±7.6 h, medium: 57.3±10.6 h, high: 50.6±9.2 h), changes that were not significant.

View larger version (82K): [in this window] [in a new window]   Figure 1. Daily excretion of plant (P), bacterial (B) and soluble (S) fractions of stool by men fed constant diets of different fiber contents. Data are mean from five men, expressed on basis of stool dry weight. The pooled SEM of all fractions is 0.71. Each fiber addition increased daily stool plant (p < 0.0001) and soluble (p < 0.02) fractions. Daily output of bacterial mass during the medium fiber phase was greater (p < 0.02) than during the low fiber phase. Significant differences were indentified by two-way analysis of variance.

View larger version (81K): [in this window] [in a new window]   Figure 2. Composition of stool from men fed constant diets of different fiber contents. Data are mean ± SEM from five men. Error bars less than one are not visible on the figure. Stool ash concentration was decreased (p < 0.05) by each fiber addition. Stool fat concentration was less (p < 0.01) during the high fiber phase, compared to the low or medium fiber phase. Concentration of stool nitrogen was less (p < 0.05) during the medium and high fiber phases than during the low fiber phase. Significant differences were identified by two-way analysis of variance.

View larger version (62K): [in this window] [in a new window]   Figure 3. Excretion of total sugars in fractions of feces from men fed constant diets of different fiber contents. Data are mean from five men, expressed on basis of stool dry weight. The pooled SEM of all fractions sugar data is 0.13. More sugars were excreted in plant (P) (p < 0.001) and soluble (S) (p < 0.05) fractions with each fiber addition. Daily output of bacterial (B) sugars increased (p < 0.01) only with the first fiber adddition. Significant differences were identified by two-way analysis of variance.

View larger version (122K): [in this window] [in a new window]   Figure 4. Apparent digestibility of neutral sugars isolated from stool as plant material in men fed constant diets of different fiber contents. Data are the mean from five men, expressed on basis of stool dry weight. The pooled SEM of all digestibility data is 1.2. Abbreviations: ara = arabinose, xyl = xylose, man = mannose, gal = galactose, glc = glucose. Each fiber addition decreased digestibility (p < 0.001) of total neutral sugars and of each individual neutral sugar except for mannose. Significant differences were identified by two-way anaylsis of variance.

The effectiveness of the fractionation of stools was evaluated by tracking the distribution of three key sugars, muramic acid, a sugar found only in bacteria [27], arabinose and xylose, found primarily but not exclusively in plant material [25,28]. Most of the muramic acid in stool was recovered in the bacterial fraction, 89% during the low fiber diet, 86% during the medium fiber diet and 83% during the high fiber diet. The remainder of the muramic acid was in the soluble fraction and probably represents bacterial exopolysaccharides released from the bacterial surface during fractionation. Only traces of muramic acid (1%) were detected in the plant fractions. 54% of the arabinose and 63% of the xylose in stool were in the plant or small plant fraction extracted from the low fiber stools, whereas most (83%–88%) of these two sugars was in the two plant fractions isolated from the medium and high fiber stools. The soluble fraction contained 1%–5% of fecal arabinose and xylose. The remainder of these two sugars was in the bacterial fraction.

	DISCUSSION

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Our results indicate that doubling dietary fiber intake using all types of fiber-containing foods had little effect on the proportion of bacteria and plant material in stool. When low fiber constant diets are consumed, as in this study or in previous experiments [4–6], the majority (55%) of dry stool is bacteria. The concentration of bacteria during the medium fiber phase of our study was only slightly lower, 47% of dry stool. Plant material accounted for 15% of stool dry weight during the medium fiber phase, which also is similar to the 5% to 15% of dry weight of daily stool recovered as plant material when low fiber constant diets are consumed [4–6]. Doubling fiber intake by supplementing a metabolic diet with cabbage [4] or oats [6] also had little effect on the proportion of daily dry stool output that was plant material (48%–56%) or bacteria (10%–16%).

We observed that a threefold increase in fiber intake was needed when a cross section of fiber-containing foods was used before a substantial decrease in the stool concentration of bacteria occurred. The proportion of dry stool that was bacteria decreased to 33% of stool mass with the second addition of fiber and the proportion of plant material increased to 27%. Other investigators have shown that if even more of the highest fiber intake is from whole cereal grains, relative to fiber from fruits, vegetables and legumes, bacterial mass is depressed even further so that it was 28% of dry stool, whereas plant material represented 40% of stool [5].

However, data from two other studies suggest that the tripling of fiber intake may not be necessary to decrease stool bacterial concentration [4,6]. Rather, the source of the additional fiber may be important. Doubling fiber intake produced shifts in stool composition comparable to what we observed during the high fiber diet phase if whole wheat and wheat bran were the major sources of additional fiber [4,6]. In these two studies [4,6] the fiber intake, but not composition, was similar to the amount we used in the medium fiber phase of our study; the major difference in the fiber sources was in the fermentability. Wheat fiber is less completely fermented [4,6,29] than fiber from oats or fruits and vegetables [5,6,30]. Although our objective was to maintain the distribution of fiber sources among the classes of fiber-containing foods in all three phases of the study, we found that it was necessary to use concentrated grain sources (wheat, oat and corn brans) in the high fiber phase of the study to maintain energy and macronutrient contents the same across the three diets [14]. Thus, the diet in the high fiber phase contained a larger proportion of fiber from grains than did the low or medium fiber phase of the study. The response in xylose digestibility also supports the available evidence that it was the grain-derived fiber that was incompletely fermented. Xylose is a major constituent in corn bran [31], and its apparent digestibility decreased during the high fiber period much more than the other fiber-derived sugars, including arabinose which is part of the arabinoxylan and glucose present as cellulose in wheat bran [31]. We do not know why the digestibility of mannose increased as fiber intake increased in this study. However, mannose was only a small component of the fiber-derived neutral sugars, accounting for 1%, 3% and 5% of these sugars in the low, medium and high fiber diets, respectively.

The transit time, stool output and defecation frequency responses of the five subjects in this study were not different from those of the nine subjects who participated in the original experiment [14], suggesting that the five subjects randomly selected for this detailed analysis were representative of the larger group. Five of the nine subjects were used for this study, because the fractionation procedure is laborious and requires considerable technical skill, and we learned previously that this number of subjects was sufficient to detect significant differences [6].

The high fiber diet we studied may have provided more nitrogenous and lipophilic substrates for the microflora. Microflora are estimated to be 6% to 10% nitrogen [32,33]. If the nitrogenous materials are assumed to contain 16% nitrogen, 50% to 85% of the increase in fecal nitrogen, which occurred when the medium fiber period is compared to the low fiber period, represents bacterial protein. However, when the high fiber fecal nitrogen excretion is compared to that of the low fiber period, only 10% to 25% of the observed increase could be attributed to bacteria. Bacteria are estimated to be 10% to 20% fat [34], suggesting that bacterially-derived lipid could account for approximately 15% to 30% of the increases in fecal lipid when the medium or high fiber diets are compared to the excretion during the low fiber period. Both lipid and nitrogenous compounds have been implicated as precursors to cancer-promoting compounds [1,12].

It has been argued that some dietary fibers promote a healthy colon environment, because more butyrate is produced during fermentation of the fiber [8,35]. Many of the beneficial effects of butyrate were detected in vitro and have not been reproduced during in vivo experiments [35,36]. Lupton [36] recently concluded after a careful review of the in vivo and in vitro data that wheat fiber was beneficial because it was a diluent, not because more butyrate was produced. Our results are consistent with this conclusion.

Our findings, in conjunction with previous studies [4–6], suggest that less completely fermented fiber sources are most effective at decreasing fecal bacterial concentrations. Cereal grains and brans, such as wheat, rye and corn, promote a healthy colonic lumen because the less fermentable fiber not only acts as a diluent of colon contents but also does not support a large increase in bacterial growth. Presumably even smaller bacterial masses would result in the colon when essentially nonfermentable materials, e.g. oat hulls, methylcellulose, as well as isolated wood pulp cellulose, compared to the incompletely fermented fibers, are consumed. The hypothesis that a smaller colonic bacterial mass is healthy is also consistent with findings from animal models of colon carcinogenesis. Supplements of less completely fermented fiber sources, such as wheat bran, or essentially nonfermentable fibers, such as wood pulp cellulose, have reduced incidence of experimental cancer [1,37]. In contrast, other isolated concentrates, e.g. pectins and gums, are usually rapidly and completely fermented; this produces a large bacterial mass in the proximal colon that has no substrate other than other bacteria for the subsequent one to two days it is moving through the colon. These rapidly and completely fermented fibers do not increase stool weight, nor do they lower the incidence of experimentally induced colon cancer [38].

Typically, the average consumer increases fiber intake by adding cereal sources, not all food groups containing fiber, to their daily intakes. Thus, their higher fiber diets would be equivalent in amounts to our medium fiber test diet. We propose such diets would be healthy for three reasons. First, they contain adequate amounts of dietary fiber. Second, they do not produce high concentrations of metabolically active bacteria in the colon. Third, the colon also would likely have less potentially harmful microbial substrates containing nitrogen or lipid.

	ACKNOWLEDGMENTS

 
This research was supported by National Institutes of Health Grant CA46339, the College of Agricultural and Life Sciences, University of Wisconsin-Madison, and Kanto Gakuin Women’s Junior College, Yokohama, Japan. One of the authors, Shin’ichi Kurasawa, was supported by a sabbatical fellowship from The Promotion and Mutual Aid Corporation for Private Schools of Japan. The authors appreciate the statistical expertise of Peter Crump, Senior Information Processing Consultant, University of Wisconsin-Madison.

Received January 31, 2000. Revised May 4, 2000. Accepted May 4, 2000.

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