Chronic idiopathic constipation (CIC) is a common problem (Ford et al., 2014), and treatment guidelines recommend patients “increase fiber intake” (AAFP, 2010; Wald, 2016). This generic recommendation presumes that dietary fiber and isolated fibers are all efficacious, a misconception not supported by the available clinical evidence (Lambeau & McRorie, 2017; McRorie & McKeown, 2017). Of five epidemiologic studies identified, only one study in women (Dukas, Willett, & Giovannucci, 2003) showed a significant association between a high-fiber diet and a reduced risk of constipation. The most recent study (Markland 2013) assessed the National Health and Nutrition Examination Survey (NHANES) data base and found no association between dietary fiber intake and the incidence of constipation in men or women. Two studies assessed constipation in elderly adults (Campbell, Busby, & Horwath, 1993; Whitehead, Drinkwater, Cheskin, Heller, & Schuster, 1989) and showed no association between dietary fiber intake and the incidence of constipation. The fifth study (Everhart et al., 1989) showed mixed results. This lack of reproducible results led to the following conclusion by the American Gastroenterological Association Technical Review on Constipation (Bharucha, Pemberton, & Locke, 2013): “Constipation was associated with low dietary fiber intake in some, but not other, studies. However, these associations do not necessarily indicate causation. Although it is reasonable to try and modify these risk factors, doing so may not improve bowel function.”
As noted above, epidemiological determinants can assess for statistical associations between diet and incidence of disease, but they lack the control necessary to establish that the fiber component of a diet is actually the cause of an observed beneficial effect. To establish that fiber is causing a benefit, isolated fibers (e.g., fiber supplements) must be assessed in randomized controlled intervention clinical studies. When assessed in well-controlled clinical studies, most isolated fibers failed to produce a laxative effect compared with placebo (McRorie & Chey, 2016). Readily fermented fibers (e.g., inulin, polydextrose, and partially hydrolyzed guar gum) are not different from placebo for effects on stool output or stool consistency, refuting the theory that an increase in biomass provides a laxative effect (McRorie & McKeown, 2017). Both wheat bran and psyllium have been presumed to be efficacious in treating constipation based on data from healthy participants (Slavin, 2008). It is unknown, however, whether laxative data from healthy participants is predictive of efficacy in patients with CIC. Wheat bran is the hard outer layer of the wheat kernel that is removed in the production of refined grains (e.g., flour). Wheat bran is a poorly fermented insoluble fiber, and clinical studies have delivered wheat bran in various forms (e.g., cereal and baked goods) and degree of milling (e.g., large/coarse particles to small/finely ground particles) (McRorie & Fahey, 2015). Psyllium is the outer husk of the Plantago seed, and milling separates the psyllium husk from the seed. Psyllium husk is a predominantly soluble nonfermented gel-forming fiber that can be delivered in several forms (e.g., hydrated powder, capsules, baked goods) (McRorie & Fahey, 2015). The purpose of this review was to compare the laxative efficacy of two isolated fibers (wheat bran and psyllium) in patients with CIC and address enduring misconceptions about fiber and laxation in treatment guidelines.
A comprehensive literature review was conducted with the use of the Scopus, SciFinder, and PubMed scientific databases, limited to the previous 50 years (1968–2018; latest date included, December 31, 2018). The reference section of each publication was also searched by title for studies not identified by database searches. We searched for and included publications that objectively assessed stool output (e.g., grams per day). Key search words included the following: fiber, functional fiber, dietary fiber, wheat, bran, psyllium, ispaghula, Plantago, laxation, laxative, constipation, stool, feces, fecal, water content, soluble, and insoluble. Although the primary focus included studies in a constipated population, studies in healthy participants and patients with uncomplicated diverticular disease were also included for comparison. Randomized studies that assessed stool output versus a concurrent (parallel or crossover) control group (e.g., placebo or diet without added fiber) were the primary focus of this review. Sequential studies (change from baseline) were also included for comparison but identified as to study design.
To compensate for the wide range of fiber doses across studies, the increase in stool output was calculated per gram of fiber consumed: total stool output (grams per day) during the treatment period (with added fiber product), minus total stool output per day during the control period (without added fiber product), divided by the dose of fiber product (grams per day). “Fiber product” refers to the actual dose of wheat bran or psyllium consumed by participants in the clinical studies. This calculation yields the increase in stool output (grams) per gram of fiber product consumed (e.g., 3.0 g/g). Changes in stool consistency can be assessed objectively by stool water content and stool viscosity, and subjectively by the Bristol Stool Scale (McRorie et al., 1998, 2000; McRorie & McKeown, 2017). Stool is predominantly water (e.g., normal/formed stool is ≈74% water), and stool water content is highly correlated with stool consistency (McRorie et al., 1998; McRorie & McKeown, 2017). Relatively small changes in stool water content (e.g., 2%) result in relatively large changes in stool consistency: hard stool has a water content ≤72%; normal/formed stool is ≈74% water; and soft stool is ≈76% water (McRorie et al, 1998; McRorie & McKeown, 2017).
The primary endpoint for this review was the effect of psyllium and wheat bran on stool output in a CIC population. Of the eight studies of patients with CIC, five had concurrent control treatments. Because only one of these five studies involved a head-to-head comparison of psyllium to wheat bran, it was not possible to aggregate all the data for a direct comparison of psyllium and wheat bran. However, the effects of psyllium and wheat bran versus respective controls were each separately estimated from two different aggregate data pooled analyses of the controlled studies. Each pooled analysis was performed using the generic inverse variance method with a random effects model to reflect the heterogeneity in study designs. Calculations were performed with the Comprehensive Meta-Analysis application (version 2.2.064; Biostat Inc., Englewood, NJ). and summarized in forest plots. Eleven of the 36 studies of healthy participants were sequential in design (i.e., no concurrent control), although the remaining 25 studies had concurrent negative controls. To be inclusive of both sequential and controlled studies, a regression analysis was performed wherein the increase in stool output was regressed on the amount of fiber dosed. For sequentially designed studies, the change from baseline (posttreatment minus baseline) in stool output was the dependent variable. For studies with concurrent controls, the posttreatment difference between the fiber and control was the dependent variable. Each study was given equal weight in the analysis. The slope of the regression line was the estimated mean increase in stool output per gram of fiber dosed.
Direct comparisons of the effects of wheat bran and psyllium on stool output and stool water content (stool softening)
Four clinical studies directly compared the effects of wheat bran and psyllium on stool output (Table 1). In the two studies where psyllium and wheat bran were administered at equivalent doses (Ornstein et al., 1997, Stevens et al., 1988), psyllium showed a significantly greater increase in stool output versus wheat bran (p < .05; Table 1). In two additional studies, wheat bran (20 g/day) was dosed at almost three times the dose of psyllium (7 g/day). As discussed in the Methods section, the results were normalized by calculating the grams of stool increase per gram of fiber product consumed (g/g). Results showed that psyllium (5.4 and 4.8 g/g) had a numerically greater effect on stool output than wheat bran (1.8 and 1.1 g/g). When both fibers were dosed at 23 g/day, psyllium increased stool water content by 6%, whereas wheat bran had no effect (Table 1). When wheat bran (20 g/day) was provided at a dose that was almost three times the dose of psyllium (7 g/day), the increase in stool water content for wheat bran (2.1%) was three times the increase observed with psyllium (0.7%; Table 1), a comparable gram-per-gram effect.
Effects of psyllium and wheat bran on stool in patients with chronic idiopathic constipation
Seven publications provided eight clinical assessments of stool output in patients with CIC (Table 2). Three of the psyllium studies and two of the wheat bran studies included concurrent control groups and were assessed by aggregate data pooled analyses to estimate the overall effect compared with a concurrent control. Figure 1A shows the pooled analysis of the effects of psyllium on stool output in patients with CIC. The analysis shows a mean stool output of 4.8 g/g versus control (p < .001). Figure 1B shows the pooled analysis of the effects of wheat bran on stool output in patients with CIC. The analysis showed a mean increase in stool output of 1.4 g/g versus control (p < .001). When the mean stool output data were considered across the two pooled analyses, psyllium provided an increase in stool output that was 3.4 times greater than wheat bran in patients with CIC. A total of five studies assessed the effects of wheat bran (two studies) and psyllium (three studies) on stool water content in patients with CIC (Table 2). Wheat bran (mean + 3.5%) and psyllium (mean 3.0%) both showed increases in stool water content, indicative of a stool-softening effect. Note, however, that the observed stool-softening effects were associated with a wheat bran dose (20 g/day) that was almost twice that of the mean psyllium dose (11 g/day) (Table 2). If calculated on a percent change in stool water content per gram of fiber product consumed, wheat bran showed an increase of 0.175%/g versus 0.273%/g for psyllium.
Effects of psyllium and wheat bran on stool in healthy participants
The effects of psyllium on stool output in healthy participants were assessed in nine published clinical studies (Table 3). The effects of wheat bran on stool output in healthy participants were assessed in 28 published clinical studies (40 assessments, Table 3). Figure 2 contains plots of the effects of psyllium and wheat bran on stool output increase by dose. The slope of the line represents the grams of stool output increase per gram of fiber product dosed (g/g). In healthy participants, psyllium showed a 5.0 g/g increase in stool output, whereas wheat bran showed a 2.9 g/g increase. For psyllium, the increase in stool output was comparable for healthy participants (5.0 g/g) and CIC (4.8 g/g). For wheat bran, the increase in stool output in healthy participants (2.9 g/g) was more than twice the stool output observed in CIC (1.4 g/g). Twenty-six assessments of wheat bran (mean dose 22 g/day) showed a mean increase in stool water content of +1.1%, a modest mean stool-softening effect (Table 3). Note, however, that nine of the 26 assessments of wheat bran in healthy participants showed a decrease in stool water content, indicative of a stool-hardening effect (Table 3). Two studies directly compared the effects of coarse versus fine wheat bran on stool water content and showed that coarse wheat bran increased stool water content (+2.6% and +1.0%, softer stools), whereas fine wheat bran decreased stool water content (−1.1% and −2.0%, harder stools). In contrast to wheat bran, psyllium (mean dose 14.8 g/day) showed an increase in all studies that assessed stool water content (mean + 5.4%), indicative of a stool-softening effect (Table 3).
Constipation is a common complaint, with a global prevalence of 14% and a higher prevalence in females, older adults, and those in lower economic status (Suares & Ford, 2011). Infrequent small/hard/difficult to pass stools are characteristic of CIC, and an increase in fiber intake is often recommended as a first-line therapy (Wald, 2016). For fiber to exert a clinically meaningful laxative effect, it must 1) resist fermentation and arrive intact in stool and 2) significantly increase stool water content, which is the underlying mechanism for both stool-softening and increased stool output (Lambeau & McRorie, 2017). Stool is mostly water. Normal/formed stool is ≈74% water, whereas hard stool is <72% water, and soft stool is ≈76% water (McRorie et al., 1998, McRorie & McKeown, 2017). Thus, a relatively small change in the percentage of stool water content (e.g., plus or minus 2%) can result in a relatively large change in stool consistency (McRorie & McKeown, 2017).
The current review assessed the relative efficacy of the two isolated fibers, psyllium and wheat bran, for increasing stool output in patients with CIC. Pooled analyses showed that both psyllium (Figure 1A) and wheat bran (Figure 1B) were significantly better than a negative control for increasing stool output in patients with CIC (p < .001). Comparing across pooled analyses, psyllium (4.8 g/g) produced a 3.4-fold greater increase in stool output than wheat bran (1.4 g/g). When healthy participants were assessed, psyllium increased stool output by 5.0 g/g (Figure 2), comparable with the 4.8 g/g observed in CIC. In contrast, wheat bran consumption resulted in an increase in stool output of 2.9 g/g in healthy participants (Figure 2) but only 1.4 g/g in CIC patients. The reason for the differences in stool output for wheat bran between healthy volunteers and CIC patients is unclear but suggests that stool output data in healthy participants should not be presumed to be predictive of efficacy in CIC. These results contradict an oft-cited book chapter, which stated that wheat bran yields an increase in stool output of 5.4 g per gram of “fiber” consumed (5.4 g/g) (Cummings, 2001). This statement has been misinterpreted as 5.4 g of stool produced per gram of “wheat bran” consumed (Institute of Medicine 2002; United Kingdom 2008). The calculation in the book chapter was not the dose of wheat bran consumed but rather the percentage of wheat bran estimated to be “fiber” (44% per table footnote) (Cummings, 2001). The correct stool output value for a dose of “wheat bran” in this book chapter is 2.4 g of stool per gram of wheat bran consumed (5.4 g/g × 0.44 = 2.4 g/g) (Cummings, 2001). The correct interpretation of the book chapter data (2.4 g/g in healthy participants) is consistent with the findings of this review for wheat bran in healthy participants (2.9 g/g).
Both coarse wheat bran and psyllium increase stool output and stool water content, albeit by two different mechanisms: 1) insoluble coarse wheat bran particles mechanically irritate the gut mucosa, stimulating secretion of water and mucous, potentially a protective mechanism to prevent mucosal damage; and 2) psyllium forms a nonfermented gel that holds water/resists dehydration throughout the large bowel (McRorie & McKeown, 2017). The mechanism of action for coarse wheat bran was elucidated by comparing the effects of coarse and fine wheat bran to inert plastic particles cut to match the size and shape of the wheat bran particles (Tomlin & Read 1988, Lewis & Heaton, 1999). Note that plastic particles are not fermented and have no water-holding capacity, so any effect would be mechanical in nature. The studies showed that large/coarse wheat bran and plastic particles had a significant effect on stool output, whereas fine/smooth particles did not. Finely ground wheat bran particles do not stimulate secretion of water and mucous but rather add only to the dry mass of stool. This decreases stool water content, a stool-hardening effect that can be constipating (McRorie & McKeown, 2017). Note that the degree of milling (coarse versus fine particles) is not provided on insoluble fiber labels, leaving in question whether a given wheat bran product will be beneficial or detrimental in CIC. Observed results with fine wheat bran might also be relevant to other fiber supplements that are known to reduce rather than increase stool water content, leading to harder stools (e.g., wheat dextrin; Benefiber) (van den Heuval et al., 2004; van den Heuval et al., 2005).
In the second mechanism, nonfermented psyllium forms a soluble gel with high water-holding capacity that resists dehydration throughout the large intestine and arrives intact in stool, forming soft/bulky stools (McRorie & McKeown, 2017). In addition to the laxative efficacy described in this review, psyllium has also been shown to be more effective than docusate sodium for softening hard stools (McRorie et al., 1998). In a randomized study of 170 CIC patients with hard stools (mean stool water content 71%), psyllium (5.1 g bid) increased mean stool water content to 74% (normal/formed stool) by dosing day 3, and the effect was sustained throughout the 2-week treatment period (week 2: mean 74%). In contrast, stool water content in the docusate sodium (100 mg bid) treatment group remained below 72% (hard stool) throughout the study (McRorie et al., 1998).
Misconceptions about the laxative effects of dietary fiber and isolated fibers can be found in treatment guidelines and patient pages, which provide a generic recommendation to “increase fiber intake” and specifically recommend isolated fibers with no clinical evidence of efficacy in CIC (AAFP, 2010; Wald, 2016). Most isolated fibers have no laxative effect, and at least two (fine wheat bran and wheat dextrin) can be constipating (Lambeau & McRorie 2017). Even isolated fibers marketed as over-the-counter (OTC) bulk fiber laxatives may lack clinical evidence of efficacy in CIC. The Food and Drug Administration established the OTC laxative monograph (21 CFR 334) as a mechanism to regulate products that were already being marketed to consumers (e.g., bulk fiber laxatives). A panel recommended inclusion of specific isolated fibers in the bulk laxative monograph, but there was no requirement for a new drug application or evidence of clinical efficacy in patients with CIC. A literature search failed to identify any randomized, placebo-controlled clinical studies in patients with CIC for methylcellulose (chemically treated wood pulp) or calcium polycarbophil (synthetic). This lack of clinical data is consistent with the conclusion by the American College of Gastroenterology Chronic Constipation Task Force (Ford et al., 2014): “Psyllium (e.g., Metamucil, Konsyl) increases stool frequency in patients with CC (Grade B recommendation). There are insufficient data to make a recommendation about the efficacy of calcium polycarbophil (e.g., Perdiem Fiber Therapy, Fibercon), methylcellulose (e.g., Citrucel), and bran in patients with CC (Grade B recommendation).” Treatment guidelines for CIC should be based on reproducible clinical evidence of efficacy in patients with CIC.
In summary, both psyllium and coarse wheat bran increase stool output and stool water content in healthy volunteers, but studies in healthy volunteers may not be predictive of a beneficial effect in patients with CIC. In patients with CIC, multiple well-controlled clinical studies showed that psyllium provided reproducible evidence of a significant increase in both stool output and stool water content, resulting in bulky/soft stools. In contrast, wheat bran provided a minimal effect on stool output and an inconsistent effect on stool water content: coarse wheat bran increased stool water content, a stool-softening effect, but fine wheat bran decreased stool water content, a stool-hardening effect. Compared with wheat bran, psyllium was 3.4 times more effective for increasing stool output in patients with CIC. Treatment guidelines for CIC should make specific, evidence-based recommendations as it pertains to fiber. To do otherwise takes the risk of perpetuating myth and misunderstanding, and depriving patients of an effective therapy for CIC.
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