Dietary Fiber and Health ^a

Dietary fiber
Colonic Fermentation
Water-holding capacity (WHC)
Viscosity and gel formation
Binding to bile acids

Dietary fiber

Dietary fiber [257] is generally accepted as having protective effects^b against a range of diseases predominant in Western developed countries including colorectal cancer^d, coronary heart disease [1360], diabetes, obesity, and diverticular disease. They also may help control obesity by increasing satiety and reducing appetite within weight-loss programs [1528]. Some have particularly beneficial effects in treating related conditions such as hypertension in the overweight [1258]. The term 'dietary fiber' is commonly defined as plant material that resists digestion by the secreted enzymes of the human alimentary tract but which may be fermented by microflora in the colon. As such, it includes most hydrocolloids. In spite of much debate, there is unfortunately no acceptable current legal definition of 'dietary fibre' [1157]. Increased fiber consumption has been associated with lowering total serum cholesterol and LDL cholesterol, modifying the glycemic and insulinemic response and protecting the large intestine from disease. While the physiological properties of a polysaccharide are difficult to predict on the basis of structure alone, they are partly predictable on the basis of physicochemical properties such as fermentation, water-holding capacity, viscosity, and bile acid binding. The main components of dietary fiber are non-starch polysaccharides including cellulose, hemicellulose (composed of a variety of heteropolysaccharides including arabinoxylans), β-glucan, and pectins. Another, and often major, component of plant foods that escape absorption and digestion in the small intestine and behaves, at least physiologically, as dietary fiber is resistant starch. Non-digestible oligosaccharides have aroused significant interest in recent years due to their ability to stimulate growth of potentially beneficial bacteria such as Bifidobacteria in the gut as well as other potential health benefits including inhibition of intestinal infection and reduction in cancer risk [1192].

The physiological properties of hydrocolloids are dependent on the site, rate and extent to which they are absorbed or fermented in the intestine. Consumption of hydrocolloids has been found to, increase stool weight, alter gut transit time, alter activity of the colonic microflora, influence appetite, absorb toxins and modify the absorption of fats, sugars, minerals and bile acids. The extent to which specific hydrocolloids exert their physiological effects will be dependent on a complex mixture of structural, chemical and physical properties (summarized below).

**Dietary fiber comparison**
Physicochemical property	Major Dietary Sources	Physiological effect
Fermentation	Resistant starch, β-Glucans, Pectin, Guar	Energy source · increase in biomass Short chain fatty acid production · reduction in pH of colon (inhibition of 7-α-dehydroxylase), anti-neoplastic activity of butyrate
Water Holding Capacity	Non-fermentable portion of hydrocolloids for example, Cellulose, Arabinoxylans, Algal hydrocolloids	Increased stool bulk· shorter gut transit times
Viscosity	Pectin, Guar	Delayed gastric emptying and slower transit time through small bowel
Gel Formation	Guar, Locust bean gum, Alginate ^c	Reduced rate of nutrient absorption (for example, glucose, bile acids)
Binding of Organic Molecules	Hydrocolloids with large hydrophobic surface area for example, β-Glucans, Arabinoxylans, Methyl cellulose	Binding of bile acids, carcinogens and mutagens

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Colonic Fermentation

The rate, site and extent of hydrocolloid fermentation in the gut is dependent on a number of factors including solubility, chemical structure, availability of other more readily fermentable substrates and the composition of the colonic microflora. Hydrocolloids, generally being relatively pure additives, are more available than typical dietary fiber, which is complexed with plant material and normally consumed as whole grains. Water-soluble hydrocolloids are more readily available and will be fermented earlier in the colon, than insoluble hydrocolloids, so long as suitable microbial enzymes are available. Terminal residues are fermented first, and hydrocolloids containing α-L-arabinofuranose or α-D-galacturonic acid residues are generally more susceptible to fermentation. Of the major components of dietary fiber, xylans, pectins and gums are significantly fermented in the gut, cellulose is only partly broken down and lignin is essentially an inert material. Resistant starch, which forms a major part of the substrate available for colonic fermentation, is completely degraded in the large bowel and probably has a significant role in the protection systems associated with carbohydrate fermentation. Insoluble fibers are difficult to digest because a two-phase reaction is involved, but insoluble particles (for example, resistant starch) may provide a surface for the growth of bacterial microcolonies in fermentative processes. Exactly where and how quickly fermentation takes place is important. Some soluble fibers become partially insoluble as a result of hydrolysis by gut enzymes, but conformational 'persistence' may prevent the fiber from precipitating while in the colon. There is a time lag before precipitation and, if there is insufficient time for the fiber to precipitate, it persists in solution. The main end products of colonic fermentation are the short chain fatty acids (SCFA) acetic, propionic and butyric and the gases carbon dioxide, hydrogen and methane. SCFA are an important energy source for anaerobic bacteria and may play a role in the prevention of colorectal cancer. They also represent a significant salvage of energy for the body, recovering about half the energy that would have been available had the hydrocolloid been digested and absorbed in the small intestine. Production of SCFA lowers the intestinal pH resulting in enzymatic inhibition of the 7-α-dehydroxylase, that catalyses secondary bile acid formation, and the further reduction of secondary bile acid concentrations due to precipitation. Lower colonic pH may also change the composition of the gut flora to one less prone to produce carcinogens. Butyrate has been proposed to have a direct role in colorectal cancer prevention, due to its ability to inhibit colon carcinoma cell growth in vivo. Specific hydrocolloids may vary in both the composition and total concentration of SCFA produced during colonic fermentation. In a recent study hydrolyzed guar gum produced the highest levels of total SCFA compared with other dietary fiber sources when incubated with human fecal microflora, while cellulose produced significantly higher levels of propionate. Arabinoxylan oligosaccharides have been proposed to alter the bacterial composition of the colon in a beneficial way by stimulating growth of bacteria from the genera Bifidobacteria in preference to the potentially more harmful anaerobic bacteria such as Clostridium spp. Similarly, galactomannans are readily fermented by the beneficial organisms Bifidobacteria and Lactobacilli. Resistant starch, however, has been reported to stimulate growth of the more anaerobic Clostridia species. [Back to Top ]

Water-holding capacity (WHC)

As the primary function of hydrocolloids is to retain water, they have an important effect on stool bulking, and consequently on gut transit times as a high water-holding capacity forces the material through the gut faster. Increased stool weight can cause dilution of the intraluminal contents limiting the exposure of the gut to secondary bile acids and other toxins and potential carcinogens. The water holding capacity (WHC) of a hydrocolloid is related to the primary chemical structure, the hydrophobic/hydrophilic balance and the particle size. Watter-insoluble fibers, such as cellulose, although having relatively low water content, retain water in pores and energy is required to remove it. WHC increases with particle size, due to the greater number of pores and voids in the sponge-like matrix. Conversely pectins and guar gum are almost completely fermented in the gut and, despite high initial WHC, have little effect on transit time. The stiffness of a hydrocolloid network may be important in delaying the escape of gas (by preventing gas bubbles coalescing), which will reduce stool density and increase stool bulking. [Back to Top ]

Viscosity and gel formation

Hydrocolloids may be categorized as either water-soluble (for example, pectins and guar) or water-insoluble (cellulose), though this labeling is not always helpful in predicting their physiological effects. Soluble hydrocolloids are noted for their effect on the stomach and the small intestine whereas insoluble fibers are noted for their effect on the large intestine, though some hydrocolloids (for example, arabinoxylans) have an effect on both. The extent of pre-processing, including cooking, affects the rate of solubilization in the intestine, as does particle size (smaller particles have larger exposed surface areas for their weight). Also the type and regularity of branching present affects both solubility and the extent of the exposed hydrophobic surface. These factors contribute to the hydrodynamic volume of a polysaccharide and hence its viscosity. The entanglement of polymer chains increases viscosity, which depends on about the fifth power of concentration. Above the critical concentration (C*, see rheology), even if fermentation reduces the size of a polysaccharide molecule fourfold, the viscosity will increase eight-fold when half the water is removed in the ascending colon. The high viscosity associated with hydrocolloids is often given as the cause of their effect on glucose and lipid metabolism (that is, reducing hyperglycemia); the consequent high viscosity of the intestinal contents reducing the rate of absorption of fat, glucose, and nutrients so allowing their absorption along a greater length of the small intestine and flattening the absorption profile. [Back to Top ]

Binding to bile acids

In vitro binding of bile acids by certain components of dietary fiber has been well documented. Bile acids are a group of related amphiphilic steroids, possessing both a hydrophilic and a hydrophobic face. The primary bile acids, cholic acid and chenodeoxycholic acid, are synthesized from cholesterol in the liver and released into the bile conjugated to glycine or taurine in order to solubilize fats and cholesterol for uptake in the small intestine. In the colon, bile acids that are not absorbed and recycled by the enterohepatic circulation are mostly deconjugated and 7-dehydroxylated to give the secondary bile acids, deoxycholic and lithocholic acid. Conjugation and the presence of hydroxyl groups gives the primary bile acids more hydrophilic character whereas deoxycholic acid and lithocholic acid are more lipophilic in nature. The effects of pH, available surface area and hydrophobicity of the sterol on the adsorption of bile acids to certain hydrocolloids has been shown. The adsorption capacity of bile acids to wheat, corn, oat, barley and rice fibers was favored by an acidic pH environment, large hydrophobic surface area and greater hydrophobicity of the bile acid. Adsorption of bile acids by dietary fiber is one of the proposed mechanisms for the hypocholesteremic effect of dietary fiber. Increased fecal excretion of bile acids leads to the increased metabolism of cholesterol in the liver, thus lowering serum cholesterol levels. Evidence in human subjects suggests that soluble fibers such as pectin, psyllium, guar and oat bran are effective serum cholesterol-lowering agents [1360]. Bile acids have been implicated in the etiology of colon cancer [229]. A number of studies have shown that colorectal cancer patients have higher levels of secondary bile acids both entering and leaving the colon. There is a strong relationship between cereal fiber consumption and the prevention of colorectal cancer. Many of the most effective dietary fibers contain arabinoxylans. Cereal fibers are particularly effective at binding or partitioning putative co-carcinogens such as fecapentaenes, heterocyclic amines and secondary bile acids into the matrix of the fiber, thus reducing their cytotoxic effect. These carcinogens/promoters are then carried out of the body by undigested insoluble fiber thus lowering their effective concentrations in the intestinal tract. The exact nature of the interaction between bile acids and dietary fiber is unclear but likely to be hydrophobic in nature.

Several properties allow a polysaccharide to bind bile acids. A high degree of order on the surface affects binding as bile acids are stiff molecules and require an ordered static surface with which to bind. A large hydrophobic surface also promotes binding. In addition, the presence of soluble fiber may influence how much bile acid binds to the insoluble fiber that is present. Low-density water near the surface of a polysaccharide increases the likelihood of bile acids binding to hydrophobic areas, and low-density water is created mainly by stiff-type molecules. There, may be aqueous biphasic effects, and the presence of lignin and other hydrophobic molecules is also important. Lithocholic acid (a major secondary bile acid) exists in chains when it is crystalline; it persists in solution and may 'sit' on ordered arabinoxylan structures. The length of bile acids is equivalent to that of three carbohydrate residues, and consequently, molecules with three carbohydrates in an ordered structure should bind more readily. [Back to Top ]

Footnotes

^a This page originated from reference [228] which contains many original references. [Back]

^b It should not be forgotten that health-promoting vitamins, minerals and antioxidants are also present in the plant materials that form natural dietary fiber but are not covered by the definition of dietary fiber [257]. They may well be responsible for some of the beneficial effects of fiber noted in clinical trials [300]. [Back]

^c Alginate has been shown to possess a range of useful cardiovascular and gastrointestinal properties [885 ]. [Back]

^d Colorectal cancer is a major cause of cancer death in the developed world. Although it is clear that hereditary factors, lifestyle and diet all contribute to the disease, there is currently a dispute over whether dietary fiber is beneficial or not. Fiber from grain, cereals and fruit have been shown to reduce the risk of colorectal adenoma (benign tumors that are known to be associated with the development of cancerous tumors) [923 ]. A major European study of 519,978 individuals showed that in populations with low dietary fiber intake, doubling the total fiber substantially reduces the risk of cancer [924 ]. However, although a pooled analysis of 725,628 individuals from 13 studies did show this association, it failed to show the association when other risk factors were taken into account except at very low dietary fiber intake [925 ]. High dietary fiber intake is still recommended for its (additional) beneficial effects on heart disease and diabetes. [Back]