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Psyllium husks dietary fiber

Psyllium husks dietary fiber

Dietary Fiber and Health

Dietary fiber has definite health benefits, likely to prolong life.


link; Hydration and health

V Dietary fiber

V The effect of dietary fiber in digestion
V The colon
V Colonic Fermentation
V Water-holding capacity (WHC)
V Viscosity and gel formation
V Binding to bile acids

Fiber is an important part of a healthy diet

Harvard School of Public Health  

Dietary fiber a

Dietary fiber [257] (often called roughage) has had many health claims [2365] and is generally accepted as having protective effects b against a range of diseases predominant in Western developed countries [1608] including colorectal cancer, d coronary heart disease [1360], diabetes, obesity, constipation [2682] and diverticular disease [2828]. 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 health benefits of fiber supplements have been reviewed, where some supplements are shown to have little benefit, but others are very useful [2829]. 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. After much debate, there is now an acceptable legal definition of 'dietary fiber' that include carbohydrate polymers with one or more beneficial physiological effects [1678].e


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 diseases. The lowered cholesterol concentrations are associated with highly viscous soluble fibers (e.g., guar gum, psyllium, and β-glucan) rather than soluble, nonviscous, fermentable fibers (e.g., inulin). While the physiological properties of a polysaccharide are difficult to predict from structure alone, they are partly predictable on the basis of physicochemical properties such as fermentation, water-holding capacity, viscosity, and bile acid binding. A model of the effect of dietary fiber on the digestion in the small intestine has been developed [2214]. 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 the 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 [1622] 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 the stool weight, alter the gut transit time, alter the activity of the colonic microflora, influence the 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 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 the colon (inhibition of 7-α-dehydroxylase),
The anti-neoplastic activity of butyrate
Water Holding Capacity The non-fermentable portion of hydrocolloids, for example, CelluloseArabinoxylans, Algal hydrocolloids Increased stool bulk· shorter gut transit times
Viscosity Pectin, Guar, β-Glucans, Psyllium Delayed gastric emptying and slower transit time through small bowel, Glycemic control, cholesterol lowering
Gel Formation Guar, Locust bean gum, Alginate c Reduced rate of nutrient absorption (for example, glucose, bile acids)
Binding of Organic Molecules Hydrocolloids with an extensive hydrophobic surface area, for example, β-Glucans, Arabinoxylans, Methyl-cellulose Binding of bile acids, carcinogens, and mutagens
Large particles irritating the colon Rough wheat bran Stimulate water secretion, Laxative effect
Satiety Modified starch Thick and creamy mouth-feel [2364]

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The digestive tract, adapted from

digestive tract adapted from staff. Blausen gallery 2014. Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762

The effect of dietary fiber in digestion

The path of food in our digestive tract is (see left):

  in → mouth → esophagus → stomach → small intestine (duodenum → jejunum → ileum) → colon → anus → out.


Dietary fiber has effects all the way through although it is not changed (i.e., it remains undigested) until it gets to the colon when it may or may not be fermented dependent on its structure (see below).


Dietary fiber can affects satiety (fullness) by bulking foods, increase its viscosity and gelling in the stomach. These factors slow stomach (gastric) emptying and small intestine transit [2612]


The increased bulk and viscosity together with any gel formation from the dietary fiber delays the hydrolysis and absorption of nutrients (such as amino acids, sugars, and fats) in the small intestine, leading to a more even delivery of nutrients all the way along the length of the small intestine rather than mainly within the jejunum. Thus, more nutrients are delivered to the distal ileum with subsequent stimulation of feedback mechanisms slowing gastric emptying, increasing small bowel transit time and decreasing appetite [2612]. In the presence of sufficient dietary fiber, some nutrients reach the colon, where they are fermented, losing half their calorie content.



The human colon

 the human colon

The Colon

The colon is an essential part of our digestive tract and consists of several distinct areas (see right). f It contains the individual's personal symbiotic microbiome, necessary for life and mostly involved in anaerobic fermentation. j It is responsible for the recovery of ingested and secreted water and electrolytes plus the salvage of energy from undigested food by fermentation. The colon has a 'skin' about a mm thick with a high surface area of invaginations ('crypts') but covered with a thick layer highly hydrophilic mucus (≈ 2% w/v of mainly MUC2 g) used as a lubricant and protective layer and associated with high-density water. Every day about 60 g solids and 1600 mL water is ingested with a further about 7 liters of water secreted in the upper digestive tract. The size of the human colon is not well known with its volume (variable but ≈ 0.5 L, ≈ 20% gas; ≈ 4 ˣ 1013 bacteria [2485] similar to the number of human cells in an average body) much smaller than commonly quoted.


Digested food from the small intestine, containing the excess secreted water, enters the ascending colon at the lower right-hand side of the body (on left in the diagram). The ascending colon squeezes in pulses (with the contained material rising and falling) when most of this excess water is being recovered.

3D colon

3D colon

As the material becomes more solid and less liquid its viscosity increases and eventually it passes over the top into the transverse colon. Water continues to be removed along with fermentation products (see below) in the transverse colon and into the descending colon where fecal matter gathers. The sigmoid colon is surrounded by muscle that contracts to expel this matter through the rectum. The food takes about 2 days to travel through the digestive tract with this time spent mostly in the colon. The fecal output (≈ 130 mL, ≈ 19 g bacteria dry weight.) contains only ≈ 11 mL of free water along with ≈ 18 g non-bacterial solids; the remainder is bound water.


typical carcinogenic or co-carcinogenic materials found in the colon

As the material travels through the colon, the organisms present extract energy by rearranging their atoms, generally by removing the more hydrophilic parts such as carbon dioxide and short chain fatty acids. Such an environment tends to keep carcinogenic hydrophobic materials( from the diet or formed by fermentation) away from the live cells of the colon wall, so reducing any cancer risks. Consequently, the molecules that remain are either non-fermentable (e.g., 3,4-benzpyrene and other polycyclic aromatic hydrocarbons from smoked foods and the natural bile acid lithocholic acid) or fermentation products (e.g., heterocyclic aromatic hydrocarbons particularly tryptophan metabolites, as Trp-P-1, from cooked red meat and other high-quality protein, fecapentaene-12: from anaerobic organisms and N-nitroso-dimethylamine from protein and nitrates). Many of these are quite hydrophobic (see right with their relative hydrophobicities h) and associated with cancer. As the fermentation is progressive through the colon, it is not surprising that the highest concentration of such molecules is at the distal end of the colon and this is associated with the higher risk of cancer here, even with the progressively thicker mucus layer (see table below).


Colon compartments
Colon (typical) Right (up) Transverse Left (down) Sigmoid (storage) Rectum (out)
Volume, cm3 350 300 200 80 20
Mucus surface, cm2 380 420 320 200 60
Mucus depth, µm 430 460 540 580 620
Relative cancer risk 6 8 6 26 42


partitioning between the aqueous environments in the colon

Within the colon, there are a number of aqueous environments. The two most important of these are formed by the mucus and dietary fiber. These form separated aqueous compartments (i.e., phases), whether they are completely dissolved or not. Also, dietary fiber can combine with intestinal mucus to produce a denser layer that significantly delays the transport of hydrophobic materials [2561]. The properties of these aqueous phases depend on the structure of the macromolecules and consist of low-density water around the fermented dietary fiber residue and higher density water throughout the mucus. These conditions will encourage the partition of carcinogens and potential carcinogens and protective agents between the phases in such a way as to be potentially either protective or harmful. Further, as the material passes through the colon, the action of microorganisms is likely to change the properties of the phases in a way that may be either beneficial or harmful, depending on the structure.


The concentration of the carcinogens within the mucus towards the rectum coincides with increased cancer prevalence. The presence of dietary fiber generally tends to partition hydrophobic carcinogens away from this mucus and within the partially fermented dietary fiber, so reducing cancer risk. When more unfermented fiber is present, a low-density water environment with greater aqueous volume is produced which will further extract the hydrophobic molecules so causing a further reduction in their concentration within the mucus and hence in contact with the live cells beneath [1806]; the fiber and mucus environments behaving like a 'typical' partitioning aqueous biphasic system. High stool weight and throughput with a high-fiber diet are associated with low colon cancer occurrence whereas low fiber diets and constipation are more associated with increased cancer risk, due to the longer contact time and low water content with consequent lower partitioning within the fecal material.

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Colonic Fermentation

The rate, site, and extent of hydrocolloid fermentation in the gut are dependent on several factors including solubility, chemical structure, availability of other more readily fermentable substrates and the composition of the colonic microflora [1927]. Hydrocolloids, generally being relatively pure additives, are more available than typical dietary fiber, which is complexed with plant material and often 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, seaweed polysaccharides less so, cellulose is only partially broken down, and lignin is essentially an inert material. Resistant starch, which forms a significant 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. Butyrate directs colonic surface cells to use up remaining oxygen through the β-oxidation pathway. Keeping the colon anaerobic offers protection against the growth of pathogenic bacteria, such as Escherichia coli and Salmonella, that can lead to bowel disease [3011]. The SCFA 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 catalyzes 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 beneficially by stimulating the 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 the growth of the more anaerobic Clostridia species.


There are connections between a person's (healthy) gut flora, and that of their mothers when in the womb, and aspects of their general and mental health [1864], but the details of this still have to be established. [Back to Top to top of page]

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. There is a laxative benefit with insoluble fibers that irritate the bowel wall to stimulate water secretion and with gel-forming fiber that retains water so resisting dehydration. 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. Water-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 entirely 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 bulk.


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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) affect 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 affect both the 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 to top of page]

Binding to bile acids

Important bile acids

Important 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 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 give the primary bile acids more hydrophilic character whereas deoxycholic acid and lithocholic acid are more lipophilic in nature.


The effects of pH, the available surface area and the hydrophobicity of the sterol have been shown to affect the adsorption of bile acids to certain hydrocolloids. The adsorption capacity of bile acids to wheat, corn, oat, barley, and rice fibers was favored by an acidic pH environment, extensive 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 hypocholesterolemic 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]. Several 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 to top of page]


a This page mostly 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 significant 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 the 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 reduces the risk of cancer substantially [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]


e Dietary fiber is now defined (in the European Community) as 'carbohydrate polymers with three or more monomeric units, which are neither digested nor absorbed in the human small intestine and belong to the following categories: (1) edible carbohydrate polymers naturally occurring in the food as consumed; (2) edible carbohydrate polymers which have been obtained from food raw material by physical, enzymatic or chemical means and which have a beneficial physiological effect demonstrated by generally accepted scientific evidence; (3) edible synthetic carbohydrate polymers which have a beneficial physiological effect demonstrated by generally accepted scientific evidence.’ [1678]. [Back]


f The central part of the figure is the actual colon from the 'Visible man'; Joseph Paul Jernigan, a 5’ 11’’ 199 pound, slightly overweight American, executed by lethal injection of KCl for murder on August 5, 1993. His last meal was two cheeseburgers, French fries, tossed garden salad, and Thousand Island dressing plus iced tea. His body was frozen and sliced into 1 mm slices, imaged, and put on the WEB. [Back]


g MUC2 is a long thin charged polyanionic molecule (700 × 10 nm radius) that is 78 % glycosylated. It is further sulfated, particularly when found towards the distal end of the colon. (Allen A., Hutton D. A. and Pearson J. P. The Muc2 gene product: a human intestinal mucin. International Journal of Biochemistry & Cell Biology, 30 (1998) 797-780). [Back]


h Hydrophobicities are calculated by the method of Leo A. J. Calculating Log P(oct) from structures. Chemical Reviews, 93 (1993) 1281-1306. [Back]


j Although many physicians and patients believe that a high-fiber diet and frequent bowel movements prevent the development of diverticulosis, recent work indicates that these are associated with greater, rather than lower, prevalence of diverticulosis [1863]. [Back]



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