Dietary fiber or roughage is the indigestible portion of food derived from plants. It has two main components:
- Mechanisms of action
- Physicochemical properties
- Dietary fiber and the upper gastrointestinal tract
- Fiber in the colon
- Dietary fiber and cholesterol metabolism
- Dietary fiber and fecal weight
- Effects of fiber intake
- Table legend
- Dietary fiber and obesity
- Guidelines on fiber intake
- United States
- United Kingdom
- Fiber and calories
- Fiber and fermentation
- Short chain fatty acids
- US FDA approved health claims
Dietary fibers can act by changing the nature of the contents of the gastrointestinal tract and by changing how other nutrients and chemicals are absorbed. Some types of soluble fiber absorb water to become a gelatinous, viscous substance which is fermented by bacteria in the digestive tract. Some types of insoluble fiber have bulking action and are not fermented. Lignin, a major dietary insoluble fiber source, may alter the rate and metabolism of soluble fibers. Other types of insoluble fiber, notably resistant starch, are fully fermented. Some but not all soluble plant fibers block intestinal mucosal adherence and translocation of potentially pathogenic bacteria and may therefore modulate intestinal inflammation, an effect that has been termed contrabiotic.
Chemically, dietary fiber consists of non-starch polysaccharides such as arabinoxylans, cellulose, and many other plant components such as resistant starch, resistant dextrins, inulin, lignin, chitins, pectins, beta-glucans, and oligosaccharides. A novel position has been adopted by the US Department of Agriculture to include functional fibers as isolated fiber sources that may be included in the diet. The term "fiber" is something of a misnomer, since many types of so-called dietary fiber are not actually fibrous.
Food sources of dietary fiber are often divided according to whether they provide (predominantly) soluble or insoluble fiber. Plant foods contain both types of fiber in varying degrees, according to the plant's characteristics.
Advantages of consuming fiber are the production of healthful compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive hygroscopic properties) to increase bulk, soften stool, and shorten transit time through the intestinal tract. A disadvantage of a diet high in fiber is the potential for significant intestinal gas production and bloating.
Originally, fiber was defined to be the components of plants that resist human digestive enzymes, a definition that includes lignin and polysaccharides. The definition was later changed to also include resistant starch, along with inulin and other oligosaccharides.
Official definition of dietary fiber differs a little among different institutions:
Mechanisms of action
Many molecules that are considered to be "dietary fiber" are so because humans lack the necessary enzymes to split the glycosidic bond and they reach the large intestine. Many foods contain varying types of dietary fibers, all of which contribute to health in different ways.
Dietary fibers have three primary mechanisms: bulking, viscosity and fermentation. Different fibers have different effects, suggesting that a variety of dietary fibers contribute to overall health. Some fibers contribute through one primary mechanism. For instance, cellulose and wheat bran provide excellent bulking effects, but are minimally fermented. Alternatively, many dietary fibers can contribute to health through more than one of these mechanisms. For instance, psyllium provides bulking as well as viscosity.
Bulking fibers can be soluble (i.e., psyllium) or insoluble (i.e., cellulose and hemicellulose). They absorb water and can significantly increase stool weight and regularity. Most bulking fibers are not fermented or are minimally fermented throughout the intestinal tract.
Viscous fibers thicken the contents of the intestinal tract and may attenuate the absorption of sugar, reduce sugar response after eating, and reduce lipid absorption (notably shown with cholesterol absorption). Their use in food formulations is often limited to low levels, due to their viscosity and thickening effects. Some viscous fibers may also be partially or fully fermented within the intestinal tract (guar gum, beta-glucan, glucomannan and pectins), but some viscous fibers are minimally or not fermented (modified cellulose such as methylcellulose and psyllium).
Fermentable fibers are consumed by the microbiota within the large intestines, mildly increasing fecal bulk and producing short-chain fatty acids as byproducts with wide-ranging physiological activities (discussion below). Resistant starch, inulin, fructooligosaccharide and galactooligosaccharide are dietary fibers which are fully fermented. These include insoluble as well as soluble fibers. This fermentation impacts the expression of many genes within the large intestine, which impact digestive function and lipid and glucose metabolism, as well as the immune system, inflammation and more.
Dietary fibers can change the nature of the contents of the gastrointestinal tract and can change how other nutrients and chemicals are absorbed through bulking and viscosity. Some types of soluble fibers bind to bile acids in the small intestine, making them less likely to re-enter the body; this in turn lowers cholesterol levels in the blood from the actions of cytochrome P450-mediated oxidation of cholesterol.
Insoluble fiber is associated with reduced diabetes risk, but the mechanism by which this occurs is unknown. One type of insoluble dietary fiber, resistant starch has been shown to directly increase insulin sensitivity in healthy people, in type 2 diabetics, and in individuals with insulin resistance, possibly contributing to reduced risk of type 2 diabetes.
Not yet formally proposed as an essential macro-nutrient, dietary fiber is nevertheless regarded as important for the diet, with regulatory authorities in many developed countries recommending increases in fiber intake.
Dietary fiber has distinct physicochemical properties. Most semi-solid foods, fiber and fat are a combination of gel matrices which are hydrated or collapsed with microstructural elements, globules, solutions or encapsulating walls. Fresh fruit and vegetables are cellular materials.
Dietary fiber and the upper gastrointestinal tract
A slowly eaten meal will enter the absorptive phase of the gastrointestinal tract more slowly than a rapidly eaten meal of similar composition. Many of the differences between low and high glycemic foods would disappear if a meal was eaten slowly.
The chemical and physico-chemical nature (lipid, protein, carbohydrate) of the meal will also influence the gastric emptying of the food multiphase system. Fatty foods and hypertonic solutions empty slowly. The movement of food, i.e., chyme, along the gastrointestinal tract is typical of flow in a disperse system. As chyme moves along the gastrointestinal tract, polymer flow and diffusion becomes important.
Following a meal, the stomach and upper gastrointestinal contents consist of
Micelles are colloid-sized clusters of molecules which form in conditions as those above, similar to the critical micelle concentration of detergents. In the upper gastrointestinal tract, these detergents consist of bile acids and di- and monoacyl glycerols which solubilize triacylglycerols and cholesterol.
Two mechanisms bring nutrients into contact with the epithelium:
- intestinal contractions create turbulence; and
- convection currents direct contents from the lumen to the epithelial surface.
The multiple physical phases in the intestinal tract slow the rate of absorption compared to that of the suspension solvent alone.
- Nutrients diffuse through the thin, relatively unstirred layer of fluid adjacent to the epithelium.
- Immobilizing of nutrients and other chemicals within complex polysaccharide molecules affects their release and subsequent absorption from the small intestine, an effect influential on the glycemic index.
- Molecules begin to interact as their concentration increases. During absorption, water must be absorbed at a rate commensurate with the absorption of solutes. The transport of actively and passively absorbed nutrients across epithelium is affected by the unstirred water layer covering the microvillus membrane.
- The presence of mucus or fiber, e.g., pectin or guar, in the unstirred layer may alter the viscosity and solute diffusion coefficient.
Adding viscous polysaccharides to carbohydrate meals can reduce post-prandial blood glucose concentrations. Wheat and maize but not oats modify glucose absorption, the rate being dependent upon the particle size. The reduction in absorption rate with guar gum may be due to the increased resistance by viscous solutions to the convective flows created by intestinal contractions. Dietary fiber interacts with pancreatic and enteric enzymes and their substrates. Human pancreatic enzyme activity is reduced when incubated with most fiber sources. Fiber may affect amylase activity and hence the rate of hydrolysis of starch. The more viscous polysaccharides extend the mouth-to-cecum transit time; guar, tragacanth and pectin being slower than wheat bran.
Fiber in the colon
The colon may be regarded as two organs,
- the right side (cecum and ascending colon), a fermenter. The right side of the colon is involved in nutrient salvage so that dietary fiber, resistant starch, fat and protein are utilized by bacteria and the end-products absorbed for use by the body
- the left side (transverse, descending, and sigmoid colon), affecting continence.
The presence of bacteria in the colon produces an ‘organ’ of intense, mainly reductive, metabolic activity, whereas the liver is oxidative. The substrates utilized by the cecum have either passed along the entire intestine or are biliary excretion products. The effects of dietary fiber in the colon are on
- bacterial fermentation of some dietary fibers
- thereby an increase in bacterial mass
- an increase in bacterial enzyme activity
- changes in the water-holding capacity of the fiber residue after fermentation
Enlargement of the cecum is a common finding when some dietary fibers are fed and this is now believed to be normal physiological adjustment. Such an increase may be due to a number of factors, prolonged cecal residence of the fiber, increased bacterial mass, or increased bacterial end-products. Some non-absorbed carbohydrates, e.g. pectin, gum arabic, oligosaccharides and resistant starch, are fermented to short-chain fatty acids (chiefly acetic, propionic and n-butyric), and carbon dioxide, hydrogen and methane. The cecal fermentation of 40–50 g of complex polysaccharides will yield 400–500 mmol total short-chain fatty acids, 240–300 mmol acetate, and 80–100 mmol of both propionate and butyrate. Almost all of these short-chain fatty acids will be absorbed from the colon. This means that fecal short-chain fatty acid estimations do not reflect cecal and colonic fermentation, only the efficiency of absorption, the ability of the fiber residue to sequestrate short-chain fatty acids, and the continued fermentation of fiber around the colon, which presumably will continue until the substrate is exhausted. The production of short-chain fatty acids has several possible actions on the gut mucosa. All of the short-chain fatty acids are readily absorbed by the colonic mucosa, but only acetic acid reaches the systemic circulation in appreciable amounts. Butyric acid appears to be used as a fuel by the colonic mucosa as the preferred energy source for colonic cells.
Dietary fiber and cholesterol metabolism
Dietary fiber may act on each phase of ingestion, digestion, absorption and excretion to affect cholesterol metabolism, such as the following:
- Caloric energy of foods through a bulking effect
- Slowing of gastric emptying time
- A glycemic index type of action on absorption
- A slowing of bile acid absorption in the ileum so bile acids escape through to the cecum
- Altered or increased bile acid metabolism in the cecum
- Indirectly by absorbed short-chain fatty acids, especially propionic acid, resulting from fiber fermentation affecting the cholesterol metabolism in the liver.
- Binding of bile acids to fiber or bacteria in the cecum with increased fecal loss from the entero-hepatic circulation.
An important action of some fibers is to reduce the reabsorption of bile acids in the ileum and hence the amount and type of bile acid and fats reaching the colon. A reduction in the reabsorption of bile acid from the ileum has several direct effects.
- Bile acids may be trapped within the lumen of the ileum either because of a high luminal viscosity or because of binding to a dietary fiber.
- Lignin in fiber adsorbs bile acids, but the unconjugated form of the bile acids are adsorbed more than the conjugated form. In the ileum where bile acids are primarily absorbed the bile acids are predominantly conjugated.
- The enterohepatic circulation of bile acids may be altered and there is an increased flow of bile acids to the cecum, where they are deconjugated and 7alpha-dehydroxylated.
- These water-soluble form, bile acids e.g., deoxycholic and lithocholic are adsorbed to dietary fiber and an increased fecal loss of sterols, dependent in part on the amount and type of fiber.
- A further factor is an increase in the bacterial mass and activity of the ileum as some fibers e.g., pectin are digested by bacteria. The bacterial mass increases and cecal bacterial activity increases.
- The enteric loss of bile acids results in increased synthesis of bile acids from cholesterol which in turn reduces body cholesterol.
The fibers that are most effective in influencing sterol metabolism (e.g. pectin) are fermented in the colon. It is therefore unlikely that the reduction in body cholesterol is due to adsorption to this fermented fiber in the colon.
- There might be alterations in the end-products of bile acid bacterial metabolism or the release of short chain fatty acids which are absorbed from the colon, return to the liver in the portal vein and modulate either the synthesis of cholesterol or its catabolism to bile acids.
- The prime mechanism whereby fiber influences cholesterol metabolism is through bacteria binding bile acids in the colon after the initial deconjugation and dehydroxylation.
- Fermentable fibers e.g., pectin will by virtue of their providing a medium for bacterial growth increase the bacterial mass in the colon. The sequestrated bile acids are then excreted in feces.
- Other fibers, e.g., gum arabic, act as stabilizers and cause a significant decrease in serum cholesterol without increasing fecal bile acid excretion.
Dietary fiber and fecal weight
Feces consist of plasticine-like material, made up of water, bacteria, lipids, sterols, mucus and fiber.
- Feces are 75% water; bacteria make a large contribution to the dry weight, the residue being unfermented fiber and excreted compounds.
- Fecal output may vary over a range of between 20 and 280 g over 24 hours. The amount of feces egested a day varies for any one individual over a period of time.
- Of dietary constituents, only dietary fiber increases fecal weight.
Water is distributed in the colon in three ways:
- Free water which can be absorbed from the colon.
- Water that is incorporated into bacterial mass.
- Water that is bound by fiber.
Fecal weight is dictated by:
- the holding of water by the residual dietary fiber after fermentation.
- the bacterial mass.
- There may also be an added osmotic effect of products of bacterial fermentation on fecal mass.
Wheat bran is minimally fermented and binds water and when added to the diet increases fecal weight in a predictable linear manner and decreases intestinal transit time. The particle size of the fiber is all-important, coarse wheat bran being more effective than fine wheat bran. The greater the water-holding capacity of the bran, the greater the effect on fecal weight. For most healthy individuals, an increase in wet fecal weight, depending on the particle size of the bran, is generally of the order of 3–5 g/g fiber. The fermentation of some fibers results in an increase in the bacterial content and possibly fecal weight. Other fibers, e.g. pectin, are fermented and have no effect on stool weight.
Effects of fiber intake
Research has shown that fiber may benefit health in several different ways. Lignin and probably related materials that are resistant to enzymatic degradation, diminish the nutritional value of foods.
Color coding of table entries:
Fiber does not bind to minerals and vitamins and therefore does not restrict their absorption, but rather evidence exists that fermentable fiber sources improve absorption of minerals, especially calcium. Some plant foods can reduce the absorption of minerals and vitamins like calcium, zinc, vitamin C, and magnesium, but this is caused by the presence of phytate (which is also thought to have important health benefits), not by fiber.
An experiment designed with a large sample and conducted by NIH-AARP Diet and Health Study studied the correlation between fiber intake and colorectal cancer. The analytic cohort consisted of 291 988 men and 197 623 women aged 50–71 y. Diet was assessed with a self-administered food-frequency questionnaire at baseline in 1995-1996; 2974 incident colorectal cancer cases were identified during 5 y of follow-up. The result was that total fiber intake was not associated with colorectal cancer. But on the other hand, the analyses of fiber from different food sources showed that fiber from grains was associated with a lower risk of colorectal cancer.
Although many researchers believe that dietary fiber intake reduces risk of colon cancer, one study conducted by researchers at the Harvard School of Medicine of over 88,000 women did not show a statistically significant relationship between higher fiber consumption and lower rates of colorectal cancer or adenomas. Similarly, a 2010 study of 58,279 men found no relationship between dietary fiber and colorectal cancer.
Dietary fiber and obesity
Dietary fiber has many functions in diet, one of which may be to aid in energy intake control and reduced risk for development of obesity. The role of dietary fiber in energy intake regulation and obesity development is related to its unique physical and chemical properties that aid in early signals of satiation and enhanced or prolonged signals of satiety. Early signals of satiation may be induced through cephalic- and gastric-phase responses related to the bulking effects of dietary fiber on energy density and palatability, whereas the viscosity-producing effects of certain fibers may enhance satiety through intestinal-phase events related to modified gastrointestinal function and subsequent delay in fat absorption. In general, fiber-rich diets, whether achieved through fiber supplementation or incorporation of high fiber foods into meals, have a reduced energy density compared with high fat diets. This is related to fiber’s ability to add bulk and weight to the diet. There are also indications that women may be more sensitive to dietary manipulation with fiber than men. The relationship of body weight status and fiber effect on energy intake suggests that obese individuals may be more likely to reduce food intake with dietary fiber inclusion.
Guidelines on fiber intake
Current recommendations from the United States National Academy of Sciences, Institute of Medicine, suggest that adults should consume 20–35 grams of dietary fiber per day, but the average American's daily intake of dietary fiber is only 12–18 grams.
The AND (Academy of Nutrition and Dietetics, previously ADA) recommends a minimum of 20–35 g/day for a healthy adult depending on calorie intake (e.g., a 2000 Cal/8400 kJ diet should include 25 g of fiber per day). The AND's recommendation for children is that intake should equal age in years plus 5 g/day (e.g., a 4 year old should consume 9 g/day). No guidelines have yet been established for the elderly or very ill. Patients with current constipation, vomiting, and abdominal pain should see a physician. Certain bulking agents are not commonly recommended with the prescription of opioids because the slow transit time mixed with larger stools may lead to severe constipation, pain, or obstruction.
The British Nutrition Foundation has recommended a minimum fiber intake of 18 g/day for healthy adults.
On average, North Americans consume less than 50% of the dietary fiber levels recommended for good health. In the preferred food choices of today's youth, this value may be as low as 20%, a factor considered by experts as contributing to the obesity levels seen in many developed countries.
The actual fiber intake gaps of different age groups of Americans are shown in the graph from USDA:
Recognizing the growing scientific evidence for physiological benefits of increased fiber intake, regulatory agencies such as the Food and Drug Administration (FDA) of the United States have given approvals to food products making health claims for fiber.
In clinical trials to date, these fiber sources were shown to significantly reduce blood cholesterol levels, an important factor for general cardiovascular health, and to lower risk of onset for some types of cancer.
Viscous fiber sources gaining FDA approval are:
Other examples of bulking fiber sources used in functional foods and supplements include cellulose, guar gum and xanthan gum.
Other examples of fermentable fiber sources (from plant foods or biotechnology) used in functional foods and supplements include resistant starch, inulin, fructans, fructooligosaccharides (FOS), and oligo- or polysaccharides, and resistant dextrins, which may be partially or fully fermented.
Consistent intake of fermentable fiber through foods like berries and other fresh fruit, vegetables, whole grains, seeds, and nuts is now known to reduce risk of some of the world’s most prevalent diseases—obesity, diabetes, high blood cholesterol, cardiovascular disease, and numerous gastrointestinal disorders. In this last category are constipation, inflammatory bowel disease, ulcerative colitis, hemorrhoids, Crohn's disease, diverticulitis, and colon cancer—all disorders of the intestinal tract where fermentable fiber can provide healthful benefits.
Insufficient fiber in the diet can complicate defecation. Low-fiber feces are dehydrated and hardened, making them difficult to evacuate—defining constipation and possibly leading to development of hemorrhoids or anal fissures.
In 2014, the International Scientific Association for Probiotics and Prebiotics submitted a petition to the Food and Drug Administration expanding the physiological effects of fiber consumption to the bulleted list below.
In June 2007, the British Nutrition Foundation issued a statement to define dietary fiber more concisely and list the potential health benefits established to date. Statement: ‘Dietary fibre’ has been used as a collective term for a complex mixture of substances with different chemical and physical properties which exert different types of physiological effects.
The use of certain analytical methods to quantify dietary fiber by nature of its indigestibility results in many other indigestible components being isolated along with the carbohydrate components of dietary fiber. These components include resistant starches and oligosaccharides along with other substances that exist within the plant cell structure and contribute to the material that passes through the digestive tract. Such components are likely to have physiological effects.
Yet, some differentiation has to be made between these indigestible plant components and other partially digested material, such as protein, that appears in the large bowel. Thus, it is better to classify fiber as a group of compounds with different physiological characteristics, rather than to be constrained by defining it chemically (end quote).
Diets naturally high in fiber can be considered to bring about several main physiological consequences:
Fiber is defined by its physiological impact, with many heterogenous types of fibers. Some fibers may primarily impact one of these benefits (i.e., cellulose increases fecal bulking and prevents constipation), but many fibers impact more than one of these benefits (i.e., resistant starch increases bulking, increases colonic fermentation, positively modulates colonic microflora as well as increases satiety and insulin sensitivity). The beneficial effects of high fiber diets are the summation of the effects of the different types of fiber present in the diet and also other components of such diets.
Defining fiber physiologically allows recognition of indigestible carbohydrates with structures and physiological properties similar to those of naturally occurring dietary fibers.
Fiber and calories
Fiber contributes less energy—usually measured in kilojoules (kJ) or kilocalories (kcal), also known as dietary Calories (Cal)—than sugars and starches because it cannot be fully absorbed by the body. Sugars and starches provide 17 kJ/g (4.1 kcal/g), and the human body has specific enzymes to break them down into glucose, fructose, and galactose, which can then be absorbed by the body. The human body lacks enzymes to break down fiber. Some types of insoluble fiber do not change inside the body, so the body cannot absorb it and it provides no energy (i.e., cellulose). Fermentable fiber is partially fermented, with the degree of fermentability varying with the type of fiber, and contributes some energy when broken down and absorbed by the body. Dietitians have not reached a consensus on how much energy is actually absorbed, but some approximate 8 kJ/g (1.9 kcal/g). Regardless of the type of fiber, the body absorbs less than 17 kJ/g (4.1 kcal/g), which can create inconsistencies for actual product nutrition labels. In some countries fiber is not listed on nutrition labels and is considered to provide no energy. In other countries all fiber must be listed and is simplistically considered to provide 17 kJ/g (4.1 kcal/g) (because chemically fiber is a type of carbohydrate and other carbohydrates provide that amount of energy). In the US, soluble fiber must be counted as 4 kcal/g (17 kJ/g), but insoluble fiber may be (and usually is) treated as not providing energy and not mentioned on the label.
Fiber and fermentation
The American Association of Cereal Chemists has defined soluble fiber this way: "the edible parts of plants or similar carbohydrates resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine." In this definition:
As an example of fermentation, shorter-chain carbohydrates (a type of fiber found in legumes) cannot be digested, but are changed via fermentation in the colon into short-chain fatty acids and gases (which are typically expelled as flatulence).
According to a 2002 journal article, fiber compounds with partial or low fermentability include:
fiber compounds with high fermentability include:
Short-chain fatty acids
When fermentable fiber is fermented, short-chain fatty acids (SCFA) are produced. SCFAs are involved in numerous physiological processes promoting health, including:
SCFAs that are absorbed by the colonic mucosa pass through the colonic wall into the portal circulation (supplying the liver), and the liver transports them into the general circulatory system.
Overall, SCFAs affect major regulatory systems, such as blood glucose and lipid levels, the colonic environment, and intestinal immune functions.
The major SCFAs in humans are butyrate, propionate, and acetate, where butyrate is the major energy source for colonocytes, propionate is destined for uptake by the liver, and acetate enters the peripheral circulation to be metabolized by peripheral tissues.
U.S. FDA-approved health claims
The United States FDA allows producers of foods containing 1.7 g per serving of psyllium husk soluble fiber or 0.75 g of oat or barley soluble fiber as beta-glucans to claim that reduced risk of heart disease can result from their regular consumption.
The FDA statement template for making this claim is: Soluble fiber from foods such as [name of soluble fiber source, and, if desired, name of food product], as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of [name of food product] supplies __ grams of the [necessary daily dietary intake for the benefit] soluble fiber from [name of soluble fiber source] necessary per day to have this effect.
Eligible sources of soluble fiber providing beta-glucan include:
The allowed label may state that diets low in saturated fat and cholesterol and that include soluble fiber from certain of the above foods "may" or "might" reduce the risk of heart disease.
As discussed in FDA regulation 21 CFR 101.81, the daily dietary intake levels of soluble fiber from sources listed above associated with reduced risk of coronary heart disease are:
Soluble fiber from consuming grains is included in other allowed health claims for lowering risk of some types of cancer and heart disease by consuming fruit and vegetables (21 CFR 101.76, 101.77, and 101.78).
A study of 388,000 adults ages 50 to 71 for nine years found that the highest consumers of fiber were 22% less likely to die over this period. In addition to lower risk of death from heart disease, adequate consumption of fiber-containing foods, especially grains, was also associated with reduced incidence of infectious and respiratory illnesses, and, particularly among males, reduced risk of cancer-related death.