Whole-grain cereals contain insoluble fibers, including cellulose, hemicellulose, and lignin. To date, intervention studies examining the effect of cereal fiber supplementation on the risk of type 2 diabetes are limited In a randomized controlled study of 61 adults with metabolic syndrome , the consumption of a diet based on several whole-grain cereal products for 12 weeks had no effect on fasting plasma concentrations of glucose , insulin , or lipids , or on measures of insulin resistance compared with a refined grain-based diet There was only some weak evidence of an effect of whole-grain cereals on postprandial insulin response A recent month, randomized, double-blind , placebo -controlled trial examined the effect of daily supplementation with 15 g of primarily insoluble fiber on glycemic control in adults with impaired glucose tolerance who were counselled to adopt more healthy lifestyle habits.
Insoluble fiber supplementation failed to improve fasting glucose concentration, measures of insulin sensitivity , HbA1c concentration a marker of glycemic control , as well as glucose and insulin responses after a glucose load compared to placebo Although observational data suggest a protective association of cereal fiber against type 2 diabetes, the current yet limited evidence from intervention trials does not support a role for insoluble fiber in glycemic control in individuals at risk of type 2 diabetes.
Fiber-related benefits on glucose homeostasis have been linked to the viscosity of certain soluble fibers e. Apart from fiber, other bioactive compounds in cereal, like magnesium, might contribute to improving glycemic control in people with impaired glucose tolerance see the article on Magnesium Adherence to a Mediterranean-style diet, the composition of which intends to meet the AI for fiber, has been associated with a lower risk of developing type 2 diabetes 52, The term metabolic syndrome refers to a cluster of metabolic disorders that increase the risk of cardiovascular disease and type 2 diabetes mellitus.
Two recent meta-analyses of observational studies have reported an inverse association between total fiber intake and odds of metabolic syndrome in cross-sectional studies but not in prospective cohort studies 55, Numerous observational studies have examined the relationship between consumption of fiber and risk of cancer at various sites.
Total fiber intakes were not associated with the risk of cancer at other sites, i. In addition, further analyses suggested that the consumption of cereal fiber was inversely related to the risk of colorectal, stomach, and liver cancers, and vegetable fiber intake was inversely associated with breast cancer.
Yet, associations between the consumption of specific fiber types were not examined in relation to the risk of endometrial, kidney, or bladder cancer in EPIC participants Pooling information from individual observational studies can be helpful to draw conclusions regarding the potential associations between dietary fiber consumption and cancer risk. Results from the most recent meta-analyses of observational studies are reported in Table 2.
Three most recent meta-analyses of observational studies have reported evidence of an inverse association between fiber intake and risk of colorectal cancer see Table 2 A recent review that included earlier meta-analyses reached a similar conclusion Several mechanisms have been proposed to explain why consuming fiber can have a protective effect against colorectal cancer. First, the presence of insoluble , coarse fiber can increase stool bulk thereby promoting the fecal excretion of carcinogens like nitrosamines Fiber can also reduce exposure of the gut mucosa to carcinogens by shortening transit time Secondly, fiber consumption influences the composition of the gut microbiota.
In vitro studies have shown inulin to selectively stimulate the proliferation of beneficial bacteria while limiting the growth of potentially pathogenic bacteria reviewed in Gut bacterial imbalance dysbiosis has been associated with the incidence of several conditions, including colorectal cancer The major health benefits conferred by the consumption of fiber are thus likely mediated by the bacteria the fiber contributes to feed.
Depending on the physicochemical characteristics of fiber, some fiber, like inulin, can be fermented by colonic bacteria and lead to the formation of short-chain fatty acids, namely acetate, propionate, and butyrate. These short-chain fatty acids have been found to protect against gastrointestinal bacterial pathogens 64 and to display anti-inflammatory and anti-carcinogenic actions 65, A few controlled clinical trials have examined the effect of fiber consumption on the recurrence of colorectal adenomas precancerous polyps , but none have been conducted in the last two decades.
These trials examined the effect of wheat bran fiber , psyllium 72 , and a high-fiber diet 73 on the risk of adenomas in participants with a history of adenomas. A meta-analysis of these trials found no difference between intervention and control groups in the number of participants with at least one new adenomatous polyp during the follow-up period years , regardless of the type of fiber intervention Alterations in gut bacteria composition have also been reported in women with breast cancer and might contribute to increased estrogen metabolism and absorption, resulting in higher circulating estrogen concentrations However, it is not known whether fiber-associated effects on endogenous estrogen concentrations have a clinically significant impact on breast cancer risk 4.
Finally, a healthy microbiota might also promote the degradation of plant-derived molecules other than fiber, such as lignans, which are precursors of metabolites with anti-estrogenic activities A review of the most recent meta-analyses of observational studies suggests that dietary fiber consumption is inversely associated with the risk of cancer of the esophagus 84, 85 , stomach 86 , pancreas 89, 90 , and ovaries see Table 2 87, The evidence linking fiber intake and esophageal cancer was exclusively based on observations from case-control studies , and for the other cancer sites, the evidence is primarily derived from case-control studies Table 2.
It is important to note that the evidence of an inverse association between fiber intake and risk of ovarian cancer 2 meta-analyses was observed in case-control but not in prospective cohort studies Table 2.
Additionally, there was no evidence of an association between fiber intake and risk of endometrial cancer 1 meta-analysis; 83 , prostate cancer 2 meta-analyses; 91, 92 , or renal cell carcinoma 1 meta-analysis; 93 Table 2. At present, evidence from randomized controlled trials of a causal relationship between fiber intake and risk of any cancer is lacking. Of note, a recent study in a mouse model presenting a dysbiotic microbiota characterized by an increase in fiber- fermenting bacteria showed that the consumption of an obesogenic, high-fat diet enriched with soluble fiber could cause icteric hepatocarcinoma Such findings suggest that a prolonged consumption of fermentable fiber may have detrimental consequences in contexts of dysbiosis.
No such observations were found when insoluble fiber were substituted for soluble fiber Gastrointestinal disorders associated with enteral nutrition prolong the time to recovery.
There was no reduction in incidental diarrhea when the analysis was restricted to studies that used prebiotic fiber i. Subgroup analyses also showed a benefit of fiber in non-critically ill patients but not in critically ill patients A limited number of studies have also examined whether fiber supplementation might help treat established fecal incontinence.
In one placebo -controlled trial, subjects suffering from fecal incontinence were randomized for 32 days to receive fibers with different degrees of fermentability: gum arabic on average, The frequency of fecal incontinence increased with sodium carboxymethylcellulose but decreased with gum arabic and psyllium compared to placebo. Stool consistency and amount did not differ among groups A randomized, cross-over trial in 80 community-dwelling participants with at least one fecal incontinence episode per week found that the reduction in fecal incontinence frequency and severity with psyllium supplementation was equivalent to that observed with antidiarrheal drug loperamide Imodium Several mechanisms have been proposed to explain the potential satiating effect of fibers and the subsequent reduction in food intake.
Adverse effects like gas production and bloating observed with the supplementation of isolated fibers e. However, for many types of fiber examined, results from interventions were mixed, some showing a positive effect on satiety, and others showing no effect. Additionally, there seemed to be no relationship between the physicochemical properties of fiber i. Finally, a positive effect of fiber on satiety was not consistently associated with a reduction in food intake A systematic review of 61 randomized controlled studies examined the effect of different fiber types on body weight This analysis found that dextrins and marine polysaccharides reduced body weight in all the studies, while chitosan, arabinoxylans, and fructans reduced body weight in at least two-thirds of the studies.
For all fiber types combined, however, the average weight reduction was only 0. Psyllium appeared to transiently reduce body weight at 3 and 6 months, yet there was no difference in body weight between psyllium and placebo at the end of the intervention 12 months More research needs to be conducted in order to clarify which types of fiber might play a role in appetite regulation and weight management Several prospective cohort studies have examined dietary fiber intake in relation to all-cause and cause-specific mortality.
A report from the NIH-AARP Diet and Health Study, which followed , older adults for an average of 14 years, found that men and women in the highest versus lowest quintile of dietary fiber intake mean, No associations were found between intakes of either insoluble or soluble fiber and all-cause mortality A meta-analysis of prospective cohort studies published before , which included a total of 1,, participants followed for a mean Another meta-analysis identified 14 prospective cohort studies that examined cereal fiber intake in relation to mortality However, recommendations from the American Diabetes Association and the Academy of Nutrition and Dietetics to people with diabetes are similar to those prescribed for the population as a whole 50 , A meta-analysis of nine randomized controlled trials in a total of 1, participants with type 2 diabetes showed evidence of body weight loss and improvements in glycemic control and blood lipid profile with the consumption of a Mediterranean-style diet compared to a control diet Numerous controlled clinical trials have shown that supplementation with soluble viscous fibers improves markers of glycemic control in people who have type 2 diabetes mellitus.
A meta-analysis of 28 trials in 1, adults with type 2 diabetes found reductions in HbA1c concentration 20 trials , fasting glucose concentration 28 trials , and insulin resistance 11 trials with soluble viscous fiber supplementation median doses of Another meta-analysis of 35 trials showed that the effect of psyllium varied with baseline fasting glucose concentration: psyllium supplementation had no effect on markers of glycemic control in euglycemic participants but showed a modest benefit in subjects with impaired glucose tolerance , and a greater effect in those with overt type 2 diabetes see also Biological Activities A small randomized uncontrolled trial in 20 healthy participants suggested that supplemental wheat dextrin, which is partially absorbed as sugar in the small intestine , could increase fasting glucose concentration into the prediabetes range after one month of supplementation Only insoluble fibers and soluble viscous fibers that resist bacterial fermentation in the colon have a potential laxative effect see Improving regularity in stool elimination The prevalence of chronic constipation is higher among people with diabetes mellitus , in women during pregnancy and after delivery, or in older people.
The management of constipation in these patients is usually similar to the management in the rest of the population, although the etiology might be different. Bulk-forming laxatives, including psyllium, bran, and methylcellulose, are commonly recommended to improve stool regularity in patients with diabetes mellitus However, there is no evidence that methylcellulose and bran are efficacious in patients with constipation In these patients, psyllium is also recognized to improve glycemic control see Diabetes mellitus.
There is a need for good quality, randomized , double-blind , controlled trials to examine the effect of fiber supplementation in the treatment of constipation in older adults in long-term care or in pregnant women and new mothers , The American College of Gastroenterology recognizes the efficacy of soluble fiber in the treatment of chronic idiopathic constipation.
It also recognizes that the evidence from observational studies is mixed, as constipation is associated with low-fiber diets in some, but not all, studies It recommends a gradual increase in fiber intake, in particular to limit the potential adverse effects associated with the intake of insoluble fiber, i. Irritable bowel syndrome IBS is a functional disorder of the intestines, characterized by episodes of abdominal pain or discomfort associated with altered gut mobility and changes in bowel habits i.
Although the pathophysiology of IBS remains unclear, certain food components have been recognized as a cause for symptoms of IBS. Dietary restriction of highly fermentable , soluble , short-chain carbohydrates , identified as FODMAP Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols and including some dietary fibers e. On the other hand, soluble, poorly fermentable, long-chain carbohydrate fiber types may improve the symptoms related to excessive gas production.
Two meta-analyses of randomized controlled trials and cross-over studies found a beneficial effect of fiber that was limited to soluble fiber, primarily psyllium , Accordingly, the American College of Gastroenterology recognizes that soluble fiber like psyllium can provide overall symptom relief in IBS, while insoluble fiber e.
More research is needed to document the effect of specific soluble fibers, considering physicochemical properties viscosity and fermentability , doses, and duration of supplementation, and to provide stronger recommendations to individuals diagnosed with IBS Future trials should also consider subjects with all of the IBS types, i. Diverticular disease or diverticulosis is a rather common gastrointestinal condition in Western countries characterized by the formation of small pouches diverticula in the colon Complications of diverticulitis include abscesses, fistulas, obstruction, and perforation The etiology of diverticulosis is thought to be multifactorial, involving both genetic and environmental risk factors.
Despite little supporting evidence, it has been proposed that low fiber intakes that characterize Western diets might contribute to increasing the risk of diverticulosis , This low-fiber hypothesis is disputed. In particular, a cross-sectional study of 2, adults found higher odds of diverticula assessed by colonoscopy among participants in the highest versus lowest quartile of fiber intake, measured by food frequency questionnaires A review identified interventions — published over four decades — that examined the effect of dietary or supplemental fiber on the reduction of abdominal pain in patients suffering from symptomatic uncomplicated diverticular disease SUDD , as well as on the risk of acute diverticulitis However, a meta-analysis could not be conducted nor any conclusion provided regarding the efficacy of fiber in the treatment of SUDD due to the very poor quality of the studies and their substantial heterogeneity in terms of study design and quantity and quality of fiber types used Another recent review of the literature focused on the effect of fiber-restricted diets in the management of acute uncomplicated diverticulitis Based on the review of three randomized controlled trials and two observational studies , the authors found a reduced length of hospital stay with non-restricted diets compared to restricted diets, but no difference regarding the incidence of treatment failure i.
While there appears to be no clinical benefit in restricting fiber intake in subjects with uncomplicated diverticulitis, the quality of the studies was once again deemed to be very low Despite the lack of high-quality evidence regarding the potential benefit of fiber in the management of diverticular disease, many national guidelines recommend the use of high-fiber diets in patients with SUDD and for the prevention of diverticulitis , A limited number of interventions have examined the effect of fiber supplementation in subjects with symptomatic hemorrhoids.
Another randomized controlled trial in 50 patients with hemorrhoidal prolapse grades II-IV and rectal bleeding showed that psyllium supplementation A follow-up for a median 40 months suggested that psyllium supplementation might help halt the progression of hemorrhoidal prolapse and reduce the number of bleeding episodes Fiber is identified as a shortfall nutrient of public health concern in the Dietary Guidelines for Americans Good sources of dietary fiber include legumes , nuts , whole grains , bran products, fruit, and nonstarchy vegetables.
Legumes e. Although refined grains are often perceived as being poor sources of fiber, they can provide as much fiber as either fruit or vegetables when comparable serving sizes are consumed In addition, not all whole grains are good sources of fiber, yet they provide key micronutrients and phytochemicals that contribute to the health benefit associated with whole grain consumption see the article on Whole Grains All plant-based foods contain a mixture of soluble and insoluble fiber Bran flaxseed, oat cereal, legumes, nuts, fruit, and vegetables are good sources of soluble viscous and nonviscous fiber.
Wheat bran, brown rice, barley, cabbage, celery, and whole grains are rich sources of insoluble fiber. The total fiber content of some fiber-rich foods is presented in Table 3. Some strategies for increasing dietary fiber intake include increasing fruit and nonstarchy vegetable intake, increasing intake of legumes, eating whole-grain cereal or oatmeal for breakfast, substituting whole grains for refined grains, and substituting nuts or popcorn for less healthy snacks.
Glucomannan, sometimes called konjac mannan, is classified as a soluble fiber isolated from konjac flour, which is derived from the plant Amorphophallus konjac. Glucomannan is available as powder and in capsules, which should be taken with plenty of liquids 8.
Glucomannan forms gels that are firmer than regular gelatin products e. The FDA has banned gel candies containing glucomannan e. Pectins are readily fermented soluble viscous fibers, most often extracted from citrus peels and apple pulp. Pectins are widely used as gelling agents in food but are also available as dietary supplements without a prescription 8. Inulins and oligofructose, extracted from chicory root or synthesized from sucrose, are used as food additives 9.
Isolated inulin is added to replace fat in products like salad dressing, while sweet-tasting oligofructose is added to products like fruit yogurts and desserts. Inulins and oligofructose are highly fermentable fibers that are also classified as prebiotics because of their ability to stimulate the growth of potentially beneficial Bifidobacteria species in the human colon Encouraging the growth of Bifidobacteria might promote intestinal health by suppressing the growth of pathogenic bacteria known to cause diarrhea or by enhancing the immune response Although a number of dietary supplements containing inulins and oligofructose are marketed as prebiotics, the health benefits of prebiotics have not yet been convincingly demonstrated in humans 11 , Raw guar gum is a viscous, fermentable fiber derived from the Indian guar or cluster bean 4.
It is used as a thickener or emulsifier in many food products. Dietary supplements containing guar gum have been marketed as weight-loss aids, but there is no evidence of their efficacy Unlike guar gum, partially hydrolyzed guar gum is nonviscous and therefore does not exhibit the biological activities of guar gum i.
Psyllium, a viscous, soluble, gel-forming fiber isolated from psyllium seed husks, is available without a prescription in laxatives, ready-to-eat cereal, and dietary supplements 8.
Psyllium is proven to be efficacious to lower serum cholesterol and improve glycemic control see Biological Activities. Because it also normalizes stool form, psyllium is the only fiber recommended by the American College of Gastroenterology to treat chronic constipation and irritable bowel syndrome see Gastrointestinal disorders.
Chitosan is an indigestible glucosamine polymer derived from chitin. Chitosan is available as a dietary supplement without a prescription in the US, being marketed to promote weight loss and lower cholesterol.
A 1-cup serving often provides up to half of the recommended daily intake of fiber. The fact is, the by-product of fiber fermentation is gas. This is why foods high in fermentable fiber can cause flatulence and stomach discomfort.
Furthermore, fermentable fiber is digested by the friendly bacteria in the gut and used for energy. This can lead to various beneficial effects on health. Some types of soluble fibers form a thick gel when they blend with water, known as viscous fibers. This slows down the digestion and absorption of nutrients, resulting in a prolonged feeling of fullness and reduced appetite 16 , 17 , A review of 44 studies on fiber treatments found that only viscous fibers reduced food intake and caused weight loss Viscous fibers include glucomannan, beta-glucans, pectins, guar gum, and psyllium.
Good whole-food sources include legumes, asparagus, Brussels sprouts, oats and flax seeds. Bottom Line: Viscous fibers form a gel-like substance that sits in the gut, besides leading to enhanced feelings of fullness, reduced appetite and weight loss. They are long chains of glucose molecules, found in potatoes , grains and many other foods. Some starch is actually resistant to digestion so that it passes through the digestive system unchanged.
This type of starch is called resistant starch , and it functions like soluble, fermentable fiber in the gut Resistant starch has numerous powerful health benefits. It improves digestive health, enhances insulin sensitivity, lowers blood sugar levels and significantly reduces appetite 21 , 22 , 23 , 24 , There are several good food sources of resistant starch, including green bananas , in addition to various legumes, cashews, and raw oats. A more detailed list can be found here.
Bottom Line: Resistant starch is a type of starch that escapes digestion. It functions like soluble, fermentable fiber, and has numerous health benefits. Fructan is a term used to describe a small chain of fructose molecules. Oligofructose and inulin are the two main fructan varieties in the diet. However, most people are only eating around half of that, or grams per day 1 , 2. Fiber is mostly found in plant foods, including vegetables, fruits, legumes, whole grains, nuts and seeds for more details, here is a list of 22 high-fiber foods.
Fiber is formally classified into two main types 3 :. However, there is a major problem with classifying fiber in this way. It tells us absolutely nothing about their health effects. A popular alternative method is to classify fiber based on its solubility soluble vs insoluble , viscosity viscous vs non-viscous and fermentability fermentable vs non-fermentable.
Then there is yet another class of nutrients called resistant starches, which are often classified as dietary fibers. Fibers are indigestible carbohydrates found naturally in plant foods.
They are often classified as either dietary found naturally or functional added to foods. Soluble fibers include gums, pectins, psyllium, beta-glucans and others. Insoluble fibers include lignin and cellulose. Fiber is often categorized based on its ability to dissolve in water. Soluble fiber has various benefits for metabolic health, while insoluble fiber functions mostly as a bulking agent.
An estimated trillion live bacteria reside in the human gut , mainly in the large intestine 6. These bacteria are actually crucial for optimal health in humans. They play various roles related to weight management, blood sugar control, immunity, brain function and mental health 7 , 8 , 9 , 10 , 11 , This increases the number and balance of friendly gut bacteria, which also produce short-chain fatty acids with powerful health benefits Most fermentable fibers are soluble, but there are also some insoluble fibers that can function in this way.
The best whole-food sources of fermentable fibers are beans and legumes. A 1-cup serving often provides up to half of the recommended daily intake of fiber. All that being said, one of the by-products of fiber fermentation is gas. This is why foods high in fermentable fiber can cause flatulence and stomach discomfort, especially if people are not used to eating a lot of fiber. Fermentable fiber is digested and used as fuel by the friendly bacteria in the gut.
This can lead to various beneficial effects on health. Some types of soluble fibers form a thick gel when they blend with water. These are known as viscous fibers.
This slows down the digestion and absorption of nutrients, resulting in a prolonged feeling of fullness and reduced appetite 16 , 17 , A review of 44 studies on fiber treatments found that only viscous fibers reduced food intake and caused weight loss Viscous fibers include glucomannan, beta-glucans, pectins, guar gum and psyllium.
Good whole-food sources include legumes, asparagus, Brussels sprouts, oats and flax seeds. Viscous fibers form a gel-like substance that sits in the gut, leading to enhanced feelings of fullness, reduced appetite and weight loss. Registered dietitian Nancy D.
Berkoff, writing in The Vegetarian Times, advises eating a wide variety of whole foods including fresh produce and grains to be sure you're getting enough fiber. Viscous fiber is found exclusively in plant-based foods -- animal-based foods like meat or dairy products do not contain either type of fiber.
Rich sources of viscous fiber include asparagus, Brussels sprouts, sweet potatoes, turnips, apricots, mangoes, oranges, legumes, barley and oat bran. While oatmeal is typically considered one of the best ways for you to increase your viscous fiber intake, oat bran and several types of beans contain more per serving.
Black beans contain 2. If you're trying to include more viscous fiber in your daily diet, be sure to start slow.
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