WHAT IS DIETARY FIBER? • unavailable carbohydrate from plant cell walls • the plant polysaccharides and lignin which are resistant to hydrolysis by digestive enzymes of man
Should the Definition Be Formal or Based on Analytic Procedures? In the United States, no formal definition of “dietary fiber” Exists. Instead, numerous accepted analytic procedures can be used to determine “dietary fiber”, so the amount of fiber in a product can be reported on the Nutrition Facts Panel on food labels. In Canada, however, a formal definition of dietary fiber does exist, and all procedures to analyze for dietary fiber must conform to the definition.
Should Dietary Fiber Be Solely from Plant Sources? • chitosan or glycosaminoglycans
Should Low-Molecular-Weight Carbohydrates Be Considered Dietary Fiber? • An alcohol precipitation step in most total fiber analyses excludes monosaccharides and disaccharides, oligosaccharides, and fructans • the oligosaccharides in beans (raffinose, stachyose, and verbacose) do not precipitate in alcohol, nor do certain manufactured carbohydrates, such as methylcellulose and polydextrose.
Should Resistant Starch Be Considered Dietary Fiber? • Foods that contain significant amounts of resistant starch include legumes, green bananas, and potatoes that have been cooked and allowed to cool.
Does Fiber Have to Be Intact from Food? It is possible for an isolated fiber to be even more effective than its original food source, but it is also possible that an isolated fiber may lose its efficacy.
Must Fiber Have Specific Health Benefits? • “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. ”
CHEMICAL NATURE OF DIETARY FIBER • nonstarch polysaccharides: cellulose, βglucans, hemicelluloses, pectins, and gums • Lignin, resistant starch?
• Cellulose and β-glucans are polymers of glucose, with β 1 -4 linkages • Hemicelluloses are a diverse groups of fibers containing xylans, galactans, or mannans as their backbone and various side chains such as arabinose and galactose. • Pectins have galacturonic acid residues as their backbone that differ in their degree of methoxylation, which, in turn, affects their solubility and viscosity. • Gums include galactomannans or arabinogalactans and are found in the nonstructural components of plants along with mucilages.
• lignin (a phenylpropane polymer) is considered part of dietary fiber because it is usually tightly bound to polysaccharides in the plant
Analytic Methods to Determine Dietary Fiber • enzymatic/gravimetric methods • enzymatic/chemical methods
enzymatic/gravimetric methods Prosky method • simpler to perform and can be automated, but they lack the specific information on monosaccharide content
enzymatic/chemical methods • Southgate procedure includes the colorimetric analysis of component monosaccharides , overestimate dietary fiber because starch is not completely solubilized in this assay procedure and would thus count as dietary fiber. • The Englyst procedure, separation by gas chromatography, information on the specific monosaccharides in fiber-containing foods • The Uppsala Method of Theander and colleagues measures neutral polysaccharides, uronic acids, and Klason lignin.
EFFECTS OF FIBER ON GASTROINTESTINAL PHYSIOLOGY AND ENERGY “soluble” and “insoluble” dietary fiber are replaced with functional definitions that are physiologically meaningful such as “viscous” and “fermentable”.
Effect of Fiber on the Stomach • The presence of viscous fibers in the stomach can delay the rate of emptying of ingested foods from the stomach into the duodenum by forming a viscous, gel-like consistency • the presence of other food components and the amount or type of fiber consumed are important
Effect of Fiber on the Small Intestine • The gel-like environment in the small intestine produced from viscous fiber has been shown to inhibit the activity of enzymes associated with fat, carbohydrate, and protein digestion • Limited evidence suggests that dietary fiber may be beneficial in reducing the risk of and treatment for duodenal ulcers (by delayed gastric emptying of the liquid phase)
Effect of Fiber on the Large Intestine • Fermentation • Short-Chain Fatty Acids • Laxation
Fermentation • depends on both the physiochemical properties of the fiber and the colonic microflora. • Fiber sources such as oat bran, pectin, and guar are highly fermented, whereas others such as cellulose and wheat bran may be poorly fermented. • In general, fruits and vegetables (rich in hemicelluloses and pectins) contain more fermentable fiber than do cereals (rich in celluloses). • The degree of fermentability affects fecal bulking ability • Although the highly fermentable fibers are not good bulking agents, they produce large amounts of short-chain fatty acids including butyrate, which is considered the primary energy source for the colon and is hypothesized to be protective against colon cancer.
Short-Chain Fatty Acids and the Butyrate Paradox • butyrate may have one effect on cancer cells and a different effect on normal cells because signal transduction processes are different in these two cell types. • Cell culture conditions (e. g. , a buffered p. H) do not apply in vivo, • amounts of butyrate • the timing of an intervention
laxation • 5. 7 g fecal bulk/g of wheat bran fed compared with 1. 3 g/g of pectin in the diet. • reduce intracolonic pressure and to lower the risk for diverticular disease
Contribution of Fiber to Energy • interference with the absorption of energycontaining macronutrients vs primarily shortchain fatty acids = a zero net balance • 1. 5 to 2. 5 kcal/g • 0 kcal/g if it is insoluble and 4 kcal/g if it is soluble
Fiber, Satiety, and Obesity • Dietary fiber, particularly viscous fiber, has been reported to reduce hunger • an additional 14 g/day of fiber is associated with a 10% decrease in energy intake
Fiber and Glucose Intolerance, Insulin Response, and Diabetes Numerous intervention studies demonstrated a beneficial effect of dietary fiber, particularly viscous fibers, on plasma glucose and insulin response in patients with type 2 diabetes
Fiber and Heart Disease • • lowering blood cholesterol, attenuating blood triglyceride levels, decreasing hypertension, normalizing postprandial blood glucose levels
Colon Cancer • However, whether fiber is protective against this disease is still an unresolved issue. • polyp recurrence may not be a good surrogate marker for colon cancer; • the amount of fiber may not have been sufficient; • the timing of the intervention may not have been optimal; • the type of fiber may not have been correct; • confounding dietary factors may have been present.
Breast Cancer • Epidemiologic studies on the relationship between fiber intake and the incidence of breast cancer appear to show a negative association when fiber intakes are very different. • through decreasing serum estrogen concentrations. Fiber can bind directly to unconjugated estrogens, thus interfering with their reabsorption, and fiber can also decrease the numbers of deconjugating bacteria.
Other Cancers • endometrial, ovarian, and prostate cancer
CONSEQUENCES OF OVERCONSUMPTION OF FIBER • Gastrointestinal Distress • Mineral Bioavailability
• most resistant starch is consumed from pasta and potatoes, • most inulin and oligofructose is consumed from wheat and onions. • Isolated soluble fibers, such as inulin, carageenan, and guar gum, are added at low amounts to lowfat foods to create a creamy texture. • Cellulose is ubiquitous and is found in all plants. • β-Glucans are high in oats and barley. • Fruits such as apples and oranges are good sources of pectin.