Dietary Fiber

Dietary Fiber

Joanne L. Slavin, PhD, RD

Fiber is not an essential nutrient in the usual way we consider essential nutrients. Tube-fed patients have survived on liquid diets devoid of fiber for many years. Additionally, traditional cold-fish based diets consumed in the Arctic region contained little plant-based material and thus little fiber. Unlike most nutrients, the benefits of fiber are linked to its lack of digestion and absorption in the upper gastrointestinal tract. In the colon, fiber may survive transit and increase stool size directly, or it may be fermented by intestinal bacteria and used for their growth and proliferation. Fermentation of dietary fiber in the colon decreases pH, promotes growth of healthy microbiota, and produces short-chain fatty acids that play a role in disease prevention. Short-chain fatty acids are also absorbed in the colon and are a potential energy source. Therefore fiber is not just an inert substance that travels through the digestive tract, but plays many roles in digestion and absorption. Additionally, dietary fiber intake has been linked to the prevention and management of many diseases.

Definition of Fiber

Crude fiber is a term used to describe the residue of plant food left after sequential extraction with solvent, dilute aqueous acid, and dilute aqueous alkali, as done in the Weende method of proximate analysis developed by William Henneberg and Fredrick Stohmann in 1864. Large fractions of the hemicelluloses, lignan, and cellulose are lost in this process and thus are not included in the crude fiber measurements reported in food composition tables. In contrast to crude fiber, there is no generally agreed upon definition or method of analysis for dietary fiber. Hugh Trowell defined dietary fiber as “the residue derived from plant cell walls that is resistant to hydrolysis by human alimentary enzymes.” As such, Trowell’s definition included the plant cell polysaccharides (cellulose and hemicelluloses) and lignan but did not include other plant cell components (mucilages, storage polysaccharides, and algal polysaccharides) that are not hydrolyzed by human digestive enzymes. Hence Trowell (1978) subsequently redefined dietary fiber as “the plant polysaccharides and lignin which are resistant to hydrolysis by the digestive enzymes of man.” This definition essentially described the same components as the “unavailable carbohydrate” being measured at the time by the methods of Southgate (1969). In the United States, the U.S. Food and Drug Administration (FDA) first addressed dietary fiber in 1987, ruling that the amount of dietary fiber listed on foods or supplements be determined through the use of what is now known as the AOAC (Association of Official Analytical Chemists) Method 985.29 or comparable methods, all of which measure nonstarch polysaccharides, lignin, and some resistant starch in plant foods (AOAC, 2007). If applied to animal foods or whole diets, resistant carbohydrates from animal sources would also be measured by these methods.

Some Current Definitions of Dietary Fiber

Numerous definitions for dietary fiber have subsequently been suggested or adopted by various scientific and regulatory agencies. Some specify a physiological definition of dietary fiber, whereas others rely more on a prescribed analytical method. In 1985 Health and Welfare Canada defined dietary fiber as “the endogenous components of plant material in the diet which are resistant to digestion by enzymes produced by humans” (Health and Welfare Canada, 1985). This definition allows inclusion of water-soluble gums, mucilages, and pectic substances, and nonnutritive fiber-associated substances, such as phytates, cutins, proteins, lectins, and waxes. But it excludes indigestible materials formed during food processing, such as Maillard reaction products. In 1987 the Expert Panel on Dietary Fiber of the Life Sciences Research Office (LSRO, Bethesda, MD) proposed a definition of dietary fiber that included nonstarch polysaccharides and lignin but excluded fiber-associated substances found in the plant cell wall as well as indigestible compounds formed during cooking or processing (Pilch, 1987). Both definitions exclude non–plant derived compounds, such as chitan, chitosan, and chondroitin sulfate, and synthetic carbohydrate polymers. The Codex Alimentarius Commission, the joint food standards program of the United Nations Food and Agriculture Organization (FAO) and the World Health Organization (WHO), adopted an official definition of dietary fiber in 2009.

Dietary fibre means carbohydrate polymers with ten or more monomeric units, which are not hydrolysed by the endogenous enzymes in the small intestine of humans and belong to the following categories:

(Codex Alimentarius Commission, 2009, p. 46)

In 2001 the Institute of Medicine (IOM) Panel on the Definition of Dietary Fiber proposed a new set of working definitions for fiber in the food supply. They distinguished between fiber that occurs naturally in plant foods (lignin, cellulose, beta-glucans, hemicelluloses, pectins, gums, inulin and oligofructose, and resistant starch) and isolated or synthetic fibers that may be added to foods or used as dietary supplements (psyllium, chitin and chitosan, fructooligosaccharides, polydextrose and polyols, and resistant dextrins).

Dietary Fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants.

Functional Fiber consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans.

Total Fiber is the sum of Dietary Fiber and Functional Fiber (IOM, 2001, p. 2)

Neither the Codex nor the IOM definitions have been formally adopted by the FDA in the United States. The European Union adopted a definition similar to the Codex definition as of 2008. The FDA continues to require food labels to report “dietary fiber” based on measurements made with approved AOAC methods that measure nonstarch polysaccharides, lignin, and some resistant starch as a component of total carbohydrate.

Controversies in Defining Dietary Fiber

Fiber definitions and measurement continue to be debated. As can be gleaned from the sampling of definitions just discussed, most definitions suggest that fiber is plant material not digested by mammalian enzymes. But some scientists consider limiting fiber to “plant material” to be too restrictive. Some would include chitosan, which forms the exoskeleton of crustaceans, or certain heat-treated animal proteins that are not readily digestible by mammalian enzymes and thus reach the large intestine relatively intact. Others include “resistant starch” in their definition of fiber. Although it is not truly indigestible by mammalian enzymes, resistant starch may not be digested and absorbed in the small intestine, and thus it can have characteristics similar to those of fiber under certain circumstances. Resistant starches occur in some foods naturally, but resistant starch may also result from manufacturing or food processing. Whether oligosaccharides present in beans and other vegetables (raffinose, stachyose, and verbacose) and the fructan polysaccharides and oligosaccharides in vegetables and fruits, such as onions and artichokes, should be considered dietary fiber is also controversial. The alcohol precipitation steps used in fiber analytical techniques excludes these substances, yet they have some biological effects similar to those of other nondigested polysaccharides.

Another point of discussion is whether dietary fiber has to be intact in the food to be characterized as fiber or whether it can be extracted from food, or even manufactured, and still be called dietary fiber. The basis for this argument is that most of the data describing physiological effects and potential health benefits from fiber were generated using high-fiber foods. Whether the same benefits would come from consuming isolated or manufactured fibers is unknown. Certain isolated fibers, such as oat bran and psyllium, are particularly effective in lowering serum lipids, but most other isolated fibers have not been extensively studied so their physiological effects are largely unknown.

Carbohydrate chemists would prefer a chemical rather than a physiological definition of fiber, as well as a simple, universally accepted, analytical method for dietary fiber to simplify compliance with and enforcement of fiber labeling laws. Methods should be suitable for the analysis of any food or food component to determine how much fiber it contains. However, because the range of fibers in foods varies greatly, it is impossible to find a simple, universally accepted method to measure fiber.

Chemical and Physical Characterization of Dietary Fiber

Properties of dietary fiber depend on the primary and secondary structure of the fiber molecules themselves, as well as on the location of the fiber components within the food and how the food material is prepared or processed.

Components of Dietary Fiber

The botanical categories of fiber are cellulose, hemicelluloses, pectic substances, gums, mucilages, algal polysaccharides, and lignin. With the exception of lignin (a polyphenol), all fibers are complex, nonstarch polysaccharides. They differ from each other in the sugar residues making up the polysaccharide and in the arrangement of the residues. The principal residues in fibers are glucose, galactose, mannose, and certain pentoses. A description of the structure and bonds found in various fiber types and other information about each of the carbohydrate components of dietary fiber is presented in Chapter 4 (see Table 4-2 and Figure 4-13). If animal foods or processed foods containing animal products are analyzed by standard methods for measuring dietary fiber, chitan, chitosans, and glycosaminoglycans will also be included as dietary fiber.

Gums and Mucilages

The terms gum and mucilage are often used interchangeably. In some cases gums and mucilages are distinguished by their function or source in the plant. Gums are usually substances that are secreted in response to injury, as in the collection of gum arabic from cuts in the bark of acacia trees. Mucilages are cell wall components or reserve nonstarch polysaccharides that serve as an energy store for the plant or germinating seed. An example of a mucilage is psyllium, which comes from the seed coat of plantago seeds. Psyllium is obtained by mechanical milling of the outer layer of the seed and is often referred to as psyllium husk. It has a xylan backbone substituted with arabinose and some uronic acids. Psyllium is commonly used as a fiber supplement and laxative (stool softener and bulker).

In most cases the term gum is used generally to refer to all plant polysaccharides that can be hydrated. Water-soluble plant gums are widely used as thickening agents and emulsifiers. Examples of so-named gums are the soluble galactomannans (i.e., mannose polymers with galactose side chains) found in legume seeds. Two of these, guar gum and locust bean (karob) gum, are widely used in ice cream as stabilizers to prevent ice crystal growth. Although actually plant mucilages, these polysaccharides are often called gums. The term mucilage is sometimes used for plant polysaccharides that are highly soluble and yield solutions with reduced viscosity. Seed mucilages such as psyllium and flaxseed gums would be classified as mucilages by this second approach to naming as well as on the basis of their function in the plant. In general, gums can be thought of as tacky, whereas mucilages are slimy.

Algal Polysaccharides

Algal polysaccharides are extracted from algae and represent a diverse group of fibers. Like the plant gums and mucilages, many algal polysaccharides are used by the food industry as thickeners, binders, and emulsifiers. Carrageenans are a family of linear sulfated polysaccharides that are extracted from red algae. Carrageenans are made up of repeating sulfated and nonsulfated galactose and anhydrogalactose units joined by alternating α(1,3) and β(1,4) linkages. Agar is a mixture of agarose, a linear polymer of repeating units of D-galactose and 3,6-anhydro-L-galactose, with some sulfated residues, and a lesser amount of smaller molecules called agaropectins. Agar is extracted from red algae. Alginates are linear polymers of D-mannuronate and its C5 epimer, α-L-glucuronate, linked β(1,4); they are extracted from brown algae.

Chitin, Chitosan, and Glycosaminoglycans

Chitin is the second most abundant polysaccharide in nature after cellulose. Chitin is an unbranched polymer of N-acetyl-D-glucosamine residues linked β(1,4); some of the sugar residues are deacetylated. The mostly deacetylated form of chitin is called chitosan. Chitin is the principal component of the cell walls of fungi (mushrooms), the exoskeletons of arthropods (e.g., crustaceans and insects), the radulas of mollusks (e.g., snails), and the beaks of cephalopods (e.g., squid). Chitan is isolated from the shells of crabs, shrimp, and crawfish, and it is used as an additive to thicken and stabilize foods. Glycosaminoglycans (or mucopolysaccharides) are linear polysaccharide chains of repeating disaccharide units made up of hexose units linked in any combination of β(1,4), β(1,3), and α(1,4). Glycosaminoglycans include chondroitin sulfate, dermatan sulfate, heparan sulfate, heparin, and hyaluronan. They are rich in uronic acid and sulfated residues and thus are highly negatively charged. Most glycosaminoglycans are covalently linked to proteins, forming proteoglycans. Proteoglycans and hyaluronic acid are found in the cellular plasma membranes and extracellular matrices of tissues of higher animals.

Primary Structure

The primary structure of fiber includes the number and sequence of monosaccharide residues in the backbone chain and side chains, any substituents on the monosaccharide residues, and the positions of the bonds linking the residues. The stereochemistry of the glycosidic bonds, as well as the particular sugar units involved in these linkages, affect structure and digestibility. Arrangement of the sugar residues in the polysaccharide is often very important to the physiological effects of the fiber. Branching and substitutions on the primary carbohydrate chain can be major determinants of physical properties. The degree of methoxylation or sulfation of certain fibers also affects the physiological properties of the fiber. For example, if the galacturonic acid residues in pectin are methoxylated, there are no anionic groups available for binding calcium or trapping water. This will not affect the gel-forming properties of pectin as long as there are sufficient unmethoxylated portions of the molecule to form the gel. However, if methoxylation is randomly distributed throughout the molecule, a significant impact on gel-forming ability can occur.

Secondary Structure

In addition to the primary structure of fiber, the molecule’s secondary structure also can affect digestibility. For example, the α(1,4) glucose linkage in starch is readily cleaved by mammalian enzymes. However, modification of the same starch molecule to produce a different three-dimensional organization may render it resistant to human digestive enzymes. In other words, the packing or arrangement of the molecule can restrict access of enzymes to the bonds they normally hydrolyze. This is why starch can become “resistant” to enzymatic hydrolysis and act like dietary fiber. Thus, raw starches, such as raw potato and banana starch, and retrograded starch in cooked and cooled food products are resistant to pancreatic amylase and thus reach the colon relatively intact. This characteristic is the basis behind some arguments that resistant starch should be included in the definition of fiber.

Physical Factors

In addition to the chemistry of the fiber molecules themselves, where fiber components are located within the plant and whether fiber is extracted from the plant and then added to the diet or is eaten as part of the intact plant material may have significant physiological consequences. If the fiber is contained within an intact plant cell, the cell wall must first be disrupted for the physiological effects of the particular fibers to be exerted. Resistance to breakage of the cell wall depends on the structure of the cell wall and its degree of lignification. The number of plant cells per particle ingested (particle size) also may determine the accessibility of the cell wall to digestive enzymes (Slavin, 2003), as may cooking, processing, and mastication of the food (Bjorck et al., 1994).

Physiological Characterization of Dietary Fiber

Fibers are also categorized by their physiological effects. Dietary fiber has conventionally been categorized as soluble or insoluble because of analytical approaches as well as the belief that solubility of fiber was a good predictor of its physiological effects. Although all fibers hold water to some degree, the soluble fibers have a greater holding capacity and may form gels and viscous solutions. In general, the structural fibers (cellulose, lignin, and some hemicelluloses) are insoluble in water, nonviscous, and poorly fermentable and thus contribute to increased stool bulk. In contrast, the gel-forming fibers (gums, mucilages, beta-glucans, algal polysaccharides, most pectins, and the remaining hemicelluloses) are soluble in water, viscous, and fermentable. Of total dietary fiber intake, approximately 20% to 30% is water soluble and 70% to 80% is water insoluble (Marlett and Cheung, 1997).

Although these generalizations are useful, there are many exceptions to these generalizations. Insoluble fiber is also fermented to some extent, and soluble fiber may be nonviscous. Gum arabic, for example, is a soluble fiber that does not form a viscous solution. Furthermore, some fibers, such as psyllium and oat bran, have physiological benefits attributed to both soluble and insoluble fibers. In addition, the approach of assigning physiological properties to soluble or insoluble fiber does not facilitate the evaluation of the effects of the fiber provided by mixed diets. Thus classification of fiber by water solubility has fallen into disfavor. The current trend is to no longer characterize fibers based on solubility, but to characterize them based on their functionality, which depends more on viscosity and fermentability.

Major Physiological Effects of Fiber and Structure–Function Relationships

The role that fiber plays within the upper and lower gastrointestinal tract depends on the fiber’s physical and chemical attributes. The following sections describe the various effects fibers may have within the gastrointestinal tract segments, as well as how the chemical nature of the fiber contributes to these results.

Effects of Fiber in the Stomach and Small Intestine

Effects of fiber in the upper gastrointestinal tract may include effects on gastric emptying, satiety, and absorption of nutrients from the small intestine.

Gastric Emptying and Satiety

The viscosity of polysaccharides and their ability to form gels in the stomach may slow gastric emptying. Therefore gel-forming fibers may further contribute to a feeling of satiety by maintaining a feeling of fullness for a longer period after a meal (IOM, 2002). In contrast, fibers that do not form gels, such as wheat bran and cellulose, have little effect on the rate at which the meal exits from the stomach.

One of the neurological pathways involved in satiety is the feeling of satiety or fullness that is produced by distention or physical fullness of the stomach. Because fibers are resistant to digestion in the stomach, the bulk they add to the diet produces a feeling of fullness. Therefore, even though caloric intake may be similar, distention resulting from an increased fiber intake leads to a greater feeling of satiety (French and Read, 1994). Fiber includes a wide range of compounds, and although fiber generally affects satiety, not all fibers are equally effective in changing satiety (Slavin and Green, 2007). Typically a large dose of fiber is required, such as 10 g or more in a serving of food (an amount not naturally occurring in a single serving of food). Viscous fibers, such as guar gum, oat bran, and psyllium, are generally more effective, although insoluble fibers, such as wheat bran and cellulose, also are known to alter satiety. Willis and associates (2009) compared the satiety response of different fibers by feeding subjects a low-fiber (1.6 g fiber) or one of four high-fiber (8.0 to 9.6 g fiber) muffins at breakfast. The high-fiber muffins contained corn bran, resistant starch, barley beta-glucans, or polydextrose. Muffins containing resistant starch and corn bran had the most positive impact on satiety, whereas muffins containing polydextrose had little effect compared to the low-fiber muffin.

Generally, whole foods that naturally contain fiber are satiating. Flood-Obbagy and Rolls (2008) compared the effect of fruit in different forms on energy intake and satiety at a meal. Results showed that eating an apple reduced lunch energy intake by 15% compared to control. Fullness ratings differed significantly after preload consumption, with apple being the most satiating, followed by applesauce, then apple juice, then the control food. The addition of a pectin fiber to the apple juice did not alter satiety. However, addition of other fibers to drinks has been shown to affect satiety. Pelkman and colleagues (2007) added low doses of a gelling pectin-alginate fiber to drinks and measured satiety. The drinks were consumed twice a day over 7 days, and energy intake at the evening meal was recorded. The 2.8 g dose of pectin alginate caused a 10% decrease in energy intake at the evening meal. Thus the results indicated that high-fiber foods are more satiating and that certain isolated fibers affect satiety whereas others do not. Clinical studies are needed to assess the effectiveness of isolated fibers on satiety, because there are no measures of fiber chemistry (solubility, structure, etc.) that can predict a fiber’s effect on satiety.

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Feb 26, 2017 | Posted by in PHARMACY | Comments Off on Dietary Fiber

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