Research update: Effectiveness of fiber supplementation for constipation, weight loss, and supporting gastrointestinal function: A narrative review of meta-analyses

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The following article summarizes the research conducted by Dr. Marc McRae, MSc, DC, FACN, DACBN titled, “Effectiveness of fiber supplementation for constipation, weight loss, and supporting gastrointestinal function: A narrative review of meta-analyses,” which was published in the Journal of Chiropractic Medicine in 2020. (7)

Overview

This table summarizes the results of this study.

Background

Prior reviews on fiber supplementation suggest that fiber is beneficial for reducing the incidence of cardiovascular disease (CVD), type 2 diabetes, and colorectal cancer. (8, 9, 10) Other common reasons found for fiber supplementation include managing constipation, gastrointestinal (GI) conditions, and obesity. (2)

The purpose of this study was to determine whether published meta-analyses support the use of fiber supplementation for treating constipation, managing obesity (weight loss), and providing dietary support for GI disorders such as irritable bowel syndrome (IBS) and chronic inflammatory bowel disease (IBD). The benefits of fiber for these conditions are attributed to its ability to support easier stool passing, decreased stool mass in the gut, and a healthier gut flora.

Constipation affects up to 20% of the adult population in North America and negatively impacts quality of life and increases the risk of developing colorectal cancer. (7) Symptoms of constipation include less than three bowel movements per week, incomplete sensation of evacuation, increased stool hardness, abdominal distension and pain.

IBS is a functional gastrointestinal disorder characterized by recurrent episodes of abdominal pain and accompanied by disturbed changes in bowel habits. This syndrome affects 9 to 27% of the adult North American population. (7) A fiber supplement is commonly recommended as the non-pharmacological approach to about 88% of IBS patients. (7)

Dietary fiber is the part of plant foods that is resistant to digestion and absorption in the small intestine.

IBD includes two subcategories: ulcerative colitis and Crohn’s disease. The incidence rate of IBD in the adult North American population is 0.4 to 0.6%. Crohn’s disease occurs most often at the end of the small intestine in patchy regions of affected tissue next to healthy tissue. (14) Ulcerative colitis occurs in the large intestine and rectum and is present in a continuous pattern, generally starting at the rectum and working up towards the colon. The inflammation present with ulcerative colitis is usually present in only the innermost layer of the colonic tissue. (14)

Dietary and functional fibers increase volume of food while reducing the metabolizable energy density of the food and therefore decreasing the patient’s total energy intake. Soluble fibers slow gastric emptying and delay or decrease the nutrient absorption of fats and glucose. This ultimately reduces both the caloric uptake and pancreatic insulin secretion. Soluble fibers can hold large quantities of water thereby increasing stomach distension, which may trigger afferent vagal signals of fullness, leading to satiation. Short-chain fatty acid (SCFA) products of fermentable dietary fibers can increase satiety by stimulating gene protein coupled receptor (GPCR) on the membrane of colonocytes. This promotes the secretion of appetite-suppressing peptides such as cholecystokinin, peptide YY, and glucagon-like peptide 1.

Methods

This study was conducted by a search for meta-analyses via PubMed and CINAHL from January 1, 1980, to July 31, 2019. The search terms included the following: (fiber OR fibre) and (meta-analysis OR systematic review) and (constipation OR irritable bowel syndrome OR inflammatory bowel disease). The search methods excluded research with qualitative data, abstracts, conference proceedings, and gray literature. This restricted the research to peer-reviewed full-length meta-analyses that included quantitative statistical data. It did not include meta-analyses that did not present study-specific summary data with a minimum of three randomized controlled trials.

The following information was scanned and extracted from meta-analyses accepted into review: number of publications included, number of participants, fiber type and daily dose, pooled treatment effects for clinical end-points, and summary relative risks. All meta-analyses were analyzed for disclosure of quality assessment, statistical heterogeneity, and publication bias prior to being included in this study. As this study was a narrative review of meta-analyses, no statistical analyses were utilized.

Results

Outlined below are the results of the meta-analyses. The initial search resulted in 43 articles; however, only 18 meta-analyses were used for this study. The four categories studied for the review included constipation, body weight loss, IBS, and IBD.

Constipation

Two meta-analyses found a significant increase in stool frequency when fiber supplementation was compared with placebo. Yang et al (16) found an odds ratio (OR) of 1.19 for stool frequency indicating an increase in occurrence. Christodoulides et al (3) found a standardized mean difference (SMD) of 0.39 for stool frequency meaning medium magnitude of correlation according to Cohen. (6) For one of these studies, significantly softened stool consistency was found when compared with placebo; however, this was not found in the other meta-analyses discussing constipation.

This inconsistency could be attributed to the use of five different dietary fiber types used by different studies (i.e., psyllium, inulin, beta-glucan, glucomannan, and resistant maltodextrin). This makes it challenging to select which type should be used in patients with constipation because each fiber type varies significantly.

Inulin and beta-fructan significantly increased stool frequency and consistency (along with the reduction in hardness of the stool) when compared with placebo in two of the meta-analyses. Yurrita et al (5) indicated a standardized mean difference (SMD) of 0.69 for stool frequency and 1.07 for stool consistency meaning a medium to high correlation. However, there was significant heterogeneity observed in both of these studies and significant publication bias found in the study observing the use of inulin. When observing constipation in children, glucomannan was found to significantly increase stool frequency; however, no significance was found in stool consistency.

Hemorrhoids (swollen and enlarged veins appearing around the rectum and anus) (4) are a complication of prolonged constipation and affect 4 to 5% of the adult North American population. Fiber supplementation analysis showed an overall improvement of 47% reduction in the risk of persistent symptomatic hemorrhoids and a 50% reduction in reported bleeding when compared to placebo. With fiber supplementation and constipation, significant publication bias was observed and the results may be an overestimate of the true therapeutic effect.

Body weight loss

One meta-analysis observed a significant reduction in body weight of 5.5 lbs with fiber supplementation. The meta-analyses used nine different fiber types: flaxseed mucilage, fructooligosaccharide, galactomannan, soluble dextrin, galactooligosaccharide, oligofructose, glucomannan, mannooligosaccharide and pectin, and pectin. Again, these all have various properties, so the relative efficacy between fiber types is uncertain.

With chitosan supplementation, a significant reduction in body weight loss of almost 4 lbs was observed in another analysis. Chitosan effectively chelates luminal free fatty acids, thereby reducing their absorption and promoting their fecal excretion. This reduces total caloric intake derived from free fatty acids.

With glucomannan supplementation, weight loss of 1.7 lbs was observed in one meta-analysis. However, another meta-analysis of longer treatment duration found weight loss of only 0.5 lbs which found that glucomannan intake does not generate statistically significant weight loss.

Guar gum showed no weight loss and the risks outweigh the benefits including symptoms such as flatulence, diarrhea, abdominal pain, and cramps.

Irritable bowel syndrome

Clinical status improved significantly by 27% in patients with IBS using fiber supplementation in one meta-analysis. Consideration should be given to the different effects of soluble and insoluble fiber for patients with IBS. Soluble fiber was found beneficial to the Global Assessment of IBS symptoms, which improved by 49% in another analysis. Insoluble fiber was found to be not more effective than placebo, and some patients reported worsened symptoms (more pain than placebo). Patients suffering from IBS should incorporate more soluble fiber in their diet and monitor insoluble fiber intake while trying to reduce the overall intake.

Inflammatory bowel disease

When looking at the highest amount of dietary fiber intake, meta-analyses found a significant reduction in incidence of Crohn’s disease by 56% and a nonsignificant reduction of ulcerative colitis by 20%. For every 10 g per day of dietary fiber, the incidence of Crohn’s disease decreased by 13%. This is most likely due to the anti-inflammatory effect of the dietary fiber, which significantly reduces the level of C-reactive protein in the blood (a protein made by the liver directly indicative of inflammation). (12)

Critical analysis

When reviewing all of the meta-analyses collected for this study, it was concluded that only the water-soluble type of fiber, such as psyllium or beta-glucan, should be recommended to reduce GI symptoms.

Specific to constipation, dietary and fiber supplementation may promote significant increases in stool frequency and reductions of persistent hemorrhoid symptoms. However, according to the author, there were problems with the heterogeneity in the studies, including different physicochemical properties of the different fibers, dosages, treatment durations, and participant characteristics. There were also issues with publication bias, which undermines support of findings and could contribute to an overestimate of the therapeutic effect.

Specifically for managing obesity, the fiber types studied were not all beneficial such as glucomannan and guar gum. Combining different fiber types failed to prove consistent benefits in either constipation or weight management. The common theme throughout this article was the combination of various fiber types made the studies difficult to interpret which type was truly beneficial for the condition being studied. Future studies should focus on single-fiber type trials with the most beneficial types, including psyllium, beta-glucan, inulin, or chitosan.

Both dietary and functional fiber supplementation may be recommended for patients with constipation. Water-insoluble and non-fermentable fibers directly increase luminal bulk, resulting in decreased gut transit time to promote laxation. Water-soluble fibers have high water-holding capacity leading to bulky/soft stools that are easier to pass. Fermentable fibers may promote laxation by increasing the stool bulk and weight by increasing microbial biomass growth. Clinical trials show that fermentable fibers have no effect on stool output or stool softening. This challenges the concept that increasing microbial biomass provides a regulatory benefit; (11) instead, they may be consumed by large intestine bacteria entirely.

Water-soluble fiber is found to be most beneficial for treating various gastrointestinal issues. Examples of soluble fiber include oat bran, nuts, seeds, lentils, and peas. (1) Insoluble fibers should be avoided due to the risk of increasing abdominal issues. Examples of insoluble fiber include wheat bran, vegetables, and whole grains. (1)

This table is demonstrating which conditions benefit from either a specific type of fiber (e.g., inulin) or a general type of fiber (i.e., soluble or fermentable).

The author of this study discussed how the quality of this review is directly related to quality of included meta-analyses. The author believes this could be dependent on design and reporting quality of individual meta-analyses and studies used to conduct said meta-analyses. Confounding factors include that people who consume high dietary fiber intakes tend to have other healthy behaviors such as being more physically active, consuming more fruit and vegetables, (13) consuming less saturated fat and processed meats, and avoiding smoking and excessive alcohol intake. These factors should be considered for future studies.

Additional research should include large, well-designed randomized controlled studies to identify specific types of fibers and doses regarding clinical end-points such as constipation, obesity management, or symptoms of IBS. Future studies should also follow larger groups of patients for longer durations of time. As the water-soluble fiber types were found to be most beneficial in the meta-analyses reviewed for this study, these should be studied individually to determine the more precise therapeutic effects.

The bottom line

The mean dietary fiber intake in the United States is 15 g per day. However, the recommended dietary fiber intake in the United States is 25 to 30 g per day. (15) The mean is significantly lower than the recommended amount. As a result, this may explain the increasing incidence of GI issues and diseases in the United States today. An increase in fiber supplementation for the general population could also contribute to reducing the risk of obesity in the United States.