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Dietary fibers are non-starch polysaccharides in plant food, that is not well digested by humans. Dietary fibers are divided into 2 components, according to their very different and complementary functions in the bowel.
Soluble Fiber-includes compounds like beta glucans and pectin. These give viscosity, bulk and lubrication in the stomach and small intestine(1,2).
Insoluble fiber-includes compounds like cellulose, hemicellulose and lignin. These maintain an open sponge like structure(2). These compounds eliminate from the body without largely modified(1,3).
Various gums, algal polysaccharides, and pectic substances contribute to dietary fiber as well. Fiber can absorb 10-15 times of its own weight in water(4). The difference between soluble and insoluble fibers is due to the chemical properties of the fiber viscosity, water-holding capacity, cation exchange capacity, adsorption of organic materials, and fermentability are now thought to be important to its beneficial physiologic effects(1).
The regulatory classification of fiber is various in different countries. The daily western diet is generally lacking in suffient dietary fibers(1). It is mainly composed of refined grains and highly digestible sources of starch , sugar, various fats, and animal products(1). Children in particular are usually fiber deficient, with daily intakes often under 5 g of fiber daily, also many adults in western countries take 5-10 g of fiber daily, but then recommend amount of dietary fiber is 20-35 g(4). In addition because most fiber in the western diet is derived from cereal grains, the intake of viscous soluble fiber is typically not adequate(1). A ratio of soluble to insoluble fiber in the diet of 1:2 is recommended as ideal. Epidemilogical and experimental evidence point to an important relationship between a lack of fiber intake and ischemic heart disease, stroke atherosclerosis, type 2 diabetes, overweight and obesity, insulin resistance, hypertension, dyslipidemia, as well as gastrointestinal disorders like diverticulosis, irritable bowel disease, colon cancer, and cholelithiasis(1).
Dietary fibers are rich in whole grain, vegetables, some fruits and sea weeds(2,5). The dietary fiber is important in preventing obesity(5). It may prevent cancer, cardiovascular disease and diabetes mellitus(3,5). Diets that are rich in fiber encourage chewing, that slows the processes of eating and increases saliva flow, which also good for dental health. Dietary fiber, the indigestible cell wall component of plant material is play an important role in human diet and health(2). However, there is evidence indicating that these complex carbohydrates directly interact with the food antioxidants and interfere with the adequate distinction of these compounds(2). The metabolism of phenolic compounds from beverages and defiency of intaking dietary fiber practically starts in the lumen of the small intestine and postabsorption modifications also occur in the liver and other organs(2).
The dietary fiber can reduce the bioavailability of macronutrients, especially fat, and some minerals and trace elements in the human diet. Because it was showed in humans that pectin strongly decreased the bioavailability of beta-carotene. In general, the 2 main effects of dietary fiber in the foregut are to prolong gastric emptying time and delay the absorption of nutrients. Both are dependent on the physicochemical form of the fiber, and it is influenced especially on digesta viscosity(2).
Dietary fiber increasing the viscocity of the luminal contents. The chyme may be considered a 2-phase system with a discontinuous particulate phase. Antioxidants trapped within the particles require first to be released into the continuous solution phase before they can be absorbed through the intestinal wall.
2.0 Effect on Gut Physiology
The effects of dietary fiber on gut physiology will depend on the;
-structure and tissue types
-quality of the intracellular compounds
-form in which the food is taken fresh, cooked or processed
-food particle size
A fiber's viscosity, its water-holding capacity, and its fermentability are the main determinator's of fiber's physiologic effects(1).
Most of the feeding and administering dietary fibers, particularly in the soluble gel forms show that dietary fiber changes the absorption of carbohydrate in the small intestine(6).
The dietary fibers produce no energy(4). Incorporating such preparations into carbohydrate-containing meals has been shown to reduce gastric emptying and postprandial hyperglycemia and insulinaemia in both normal and diabetic subjects(6).
A commercially seperated product of the endosperm of the Indian cluster bean, a legume in which the main storage polysaccharides in the form of galactomannans(6). Galactomannans are resistant to hydrolysis by human digestive enzymes. A guar-wheat bread in which 5-15% of the flour was replaced by guar significantly reduced post prandial insulinaemia in non-diabetic people, with a near-linear response(6). The inhibitory effect on enzyme activity may not have very important effects on digestibility in the small intestine due to the large excess of enzyme activity in pancreatic secretions(6).
Due to the high water-holding capacity of fiber(1,6), its presence in the fluid surrounding the villi increases the viscosity of the digesta, resulting in a thickening of the rate-limiting unstirred layer adjacent to the small intestinal mucosa(6). So the activity of membrane-bound enzymes of the brush border may be reduced. Release of gastrointestinal hormones [eg:-GIP] may be slowed(6). There may also be long-term functional and mophological adaptations of the absorptive surfaces(6). All these changes may occur after eating some types of entire food, not only processed forms of fiber like pectin(6).
2.1 Chemical Interactions of Dietary Fiber
Chemical interactions between polar groups from polyphenols and fiber polysaccharides may occur, but this has not been well studied(2). The presence of dietary fiber in vegetables and fruits may explain the lower bio-availability of carotenoids from plant foods. The interactions of carotenoids and specific components of dietary fiber are not clear. The bio-accessibility of caretenoids is interrupted probably because of micelles formation, which is necessary for the absorption of lipophilic substances, due to the disturbance by polysaccharides(2). It is suggested that fiber interferes with micelles formation by partitioning bile salts and fat in the gel phase of dietary fiber(2).
There is some evidence to suggest that the activity of the pancreatic enzymes, amylase, lipase, trypsin and chymotrypsin can be changed in vitro by incubating with dietary fiber polymers(6).
General human bio-absorption of phenolic compounds contained in food rich in dietary fiber(2). Polyphenols bound to dietary fiber need to be hydrolyzed by enzymes in the upper area of the intestine, otherwise these compounds will not be bio-available for absorption in the human intestine(2). The dietary fiber acts as an entrapping matrix and restricts the diffusion of the enzymes to their substrates. Most of the polyphenols bound to dietary fiber may end in the large intestine(2). They are digestible enzymes do not release completely polyphenols associated to dietary fiber, this suggests that this important fraction of polyphenols is not bio-available in the gut and only after colonic bacterial fermentation could be absorbed(2).
Viscosity as related to dietary fiber refers to the ability of some polysaccharides to form gels when mixed with fluids resulting from physical traps and hydrophilic interactions among the polysaccharide constituents within the fluid or solution(1).
2.2 Effects on carbohydrates.
The mode of action of insoluble dietary fiber is not well understood. It may interfere with enzyme access to carbohydrate substrates(6). Hypoglycaemia can occur in patients with chronic alcoholism, and in normal subjects after prolonged fasting(4,6). In both these situations the cause is probably suppression of gluconeogenesis. Postprandial hypoglycaemia or diarrhoea, due to rapid gastric emptying of hyperosmolar components of a meal into the upper small intestine. In both these conditions, the presence of dietary fiber might prevent the rebound hypoglycaemia(6).
Incorporating pectins and fiber of leguminous seeds probably get their effect on blood levels of glucose and insulin and the glycaemic index by slowing the gastric emptying and intraluminal digestion(6). The rate of change of pH may be slowed, so prolonging the time before the optimum pH for amylase is reached(6).
2.3 The Dietary Fibers and Diabetes Mellitus.
The soluble fiber reduces glucose in diabetics(9). The soluble fiber reduces postprandial glucose concentration and insulinemic responses after a single meal in both normal and diabetic subjects. But the effect is dependent on viscosity than on solubility(7).
A replacement of 9-10% flour with guar is hope to give long-term dietary control of diabetics(6). Other dietary fiber preparations [eg:- pectin] have same effects on glucose absorption(6).
According to the recent studies, showed that is mainly the intake of insoluble cereal dietary fiber and whole grains that is consistantly related with reduced risk of type 2 diabetes mellitus(DM). The American Diabetes Association says that diabetic patients consume 14 g/1000 kcal/day of fiber because a high amount of fiber is needed to improve glycemic control. This amount is between 2 or 3 times higher than that consumed by people in many developed countries(7). High carbohydrate, high fiber diets are effective in both insulin-dependent and non-insulin dependent diabetics. So it is very important to improve diabetic control. And this diabetic control is independent of the amount of carbohydrate in the diet(6). Low total dietary fiber was involved with increased risk of diabetes after adjustment for total calorie intake and potential confounders. This increased risk was seen independently for both low cereal and low vegetable fiber intake. Dietary fiber was inversely related with inflammatory markers [eg-C-reactive protein, interleukin-6] and with tissue plasminogen activator. Alteration for these markers attenuated the increased risk of DM(8). So dietary fiber is related with reduced DM risk in older men, which may be partly explained by inflammatory markers and hepatic fat deposition. This study was done in a predominantly white European male population, and further studies are required in women and other ethnic groups(8). The biological mechanisms of which dietary fiber may be beneficial for diabetes are still unclear(8). Several studies have shown inverse relationship between dietary fiber and markers of inflammation, insulin sensitivity, and hepatic function. A high fiber diet [at least 20 g fiber/day] in older men may reduce the risk of DM, and this appears to be partly explained(8).
2.4 Dietary Fiber and Constipation
The main component of faeces is water(6). It is responsible for 70-80% of stool weight. The dry matter is unfermentated fiber and bacteria(6).
On a low-fiber diet the dry matter consists of about 36% protein about 18% of each of the following:
fats and sterols, minerals, short-chain fatty acids and carbohydrates(6).
On a high-fiber diet solid factions may contain about 30% carbohydrate, at least 70% of the total sugars being in the form of cellulose, xylose and arabinose(6).
The dietary fibers are helping to increase the faecal volumes, mainly in the large intestine which stimulate peristaltic contractions and allowing the excretion of the faeces more easily(5). The amount of stool bulking by fiber depends on the source of the fiber. Dietary fiber may be taken as a hydrated sponge passing along the gastrointestinal tract(6). Vegetable and fruit fiber with a water-holding capacity of about 50 g water per grame, are more accessible for fermentation and so by themselves slightly change the bulk of stool(6).
People change in their definition of constipation, but there are several features constituting satisfactory or unsatisfactory defecation(6). Most people regard once or twice daily as preferable. Defecation should not require straining(6).
2.5 The Dietary Fibers and Heart Diseases.
The role of dietary factors including fish and n-3 fatty acids, flavonoids and polyphenols, in the prevention of coronary heart diseases(3). There is an inverse relationship between dietary fiber intake and the risk of myocardial infarction and cardiovascular diseases(1,3). The exact mechanism of cardioprotective effect of dietary fiber and cereal is still border border uncertain(3). Increased fecal excretion of cholesterol, increased insulin sensitivity, reduced activity of pro-coagulant factors, and decreased risk of incident diabetes with consumption of fiber and all grain cereals have been suggested(3). Inadequate dietary fiber intake is common in modern diets, especially in children(3). The physiological effects of fiber relate to the physical properties of volume, viscosity, and water-holding capacity that the fiber imparts to food leading to important influences over the energy density of food(1). Beyond these physical properties, fiber directly impacts a complex array of microbiological, biochemical, and neurohormonal effects directly through the modification of the kinetics of digestion and through its metabolism into constituents such as short chain fatty acids, which are both energy substrates and important enteroendocrine ligands. The dietary fiber plays in glucoregulation, appetite, and satiety(1). Increasing the consumption of dietary fiber to reduce the caloric density of food and reduce the glycemic impact of the food is generally considered to play an important, unless essential, part of long-term weight management(1). Many studies have showed that certain fibers decrease the glycemic response to food, advance satiety, reduce serum cholesterol bowel regularity,. The dietary fibers have been related with moderation in blood glucose and cholesterol concentrations, prolonged gastric emptying, and slower transit time through the small intestine. Rapidly fermented fiber sources supply substrates for short chain fatty acid production by microflora in the large bowel, but slowly or incompletely fermented fiber sources increase bowel health by increasing laxation, decreasing colonic transit time, and increasing stool weight(1).
Consumption of dietary fiber is related to lower risk of cardiovascular disease(CVD) for some time. But the hypothesis that dietary fiber intake can directly protect against CVD is introduce recently. The dietary fiber reduces the plasma cholesterol level(7). The soluble fiber binds bile acid and decrease their reabsorption. So increase the fecal excretion of bile acids. Consequencely decrease the hepatic bile acid and tends increase catabolism of cholesterol to maintain the bile acid pool. Soluble fiber decrease cholesterol absorption as well(4). So it is reduced the risk of atherosclerosis. Many studies have not shown an independent effect of dietary fiber on coronary heart disease(CHD). The American Heart Association says a total dietary fiber intake should be 25 g/day to 30 g/day to minimize the cholesterol. The American Dietetic Association and the Institute of Medicine say an intake of 14 g of dietary fiber per 1000 kcal, or 25 g/day for adult women and 38 g/day for adult men, to protect against CVD(7).
2.6 The Dietary Fibers and Obesity.
The relation between fiber intake and obesity is confirmed by several studies(1). Supplementation of the diet with highly functional fibers may prove to play an important role in long-term management of obesity(1).
Dietary fiber can change body weight by various mechanisms. Fiber rich food normally have lower energy content, which contribute to a decrease into the energy density of the diet. Foods rich in fiber should chew longer. It is leading to an increase in satiety and a decrease in energy consumption.
Epidemiological studies show that an inverse relationship between dietary fiber intake and body weight(9).
2.7 Dietary Fiber and bile acid absorption
Dietary fiber reduces the concentrations of the bile acids in the large intestine(6). Bile acids may be adsorbed onto the fiber, or dissolved in the interstitial water in the hydrated fiber. Bile acid conjugates are minimally adsorbed to the fiber in the jejunum(6). Deconjugation and dehydroxylation to secondary acids in the terminal ileum, and calcium adsorption to fiber. Adsorption disturbs to the entero-hepatic circulation of bile salts and increase bile acid and cholesterol excretion(6). Adsorption protects the colonic mucosa from the secondary bile acids as well, either directly or by reducing the efficiency of conversion of the primary to the secondary bile acids(6). Secondary factors may include binding of bile salts to specific fiber components and inhibition of diffusion across the unstirred layer(2).
Pectins are the main component of dietary fiber that bind bile acids(6). Of the fibers tested, adsorption to carrot fiber is high, with 25-29% deoxycholic acid being adsorbed. Citrus pectin also has a high bile acid binding capacity(6).
All the non-absorbed carotenoids and dietary fiber along with entrapped lipids and bile salts pass to the large intestine, where the polysaccharides are hydrolyzed by bacterial enzymes and the carotenoids may exert their antioxidant activity in the large intestine(2).
2.8 Dietary Fiber and Absorption of Minerals and Vitamins.
In vitro studies show that dietary fiber binds Calcium in propotion to the rhamnogalacuronic acid content of the fiber. Peptic substances are usually rich in these polysaccharides. Bound Calcium may be liberated and absorbed in the colon when the fiber is degraded by colonic bacteria(6).
Feeding entire grain has been shown to disrupt the Calcium balance, attributed to a high phytic acid content of the grain. But entire grain also contain dietary fiber. The ability of dietary fiber to interfere with Calcium bioavailability may depend on the type of dietary fiber and colonic interactions(6).
Changes in Zinc nutritional status caused by high fiber diets have been shown. Hemicellulose and pectin increase faecal zinc excretion more than cellulose. But the clear evidence for zinc binding by fibrous material does not help the ending that fiber is only responsible(6).
High fiber diet reduce iron absorption(9). In the Iranian bread studies serum iron fell in subjects on high fiber bread and faecal iron increased still further with cellulose supplemented bread(9). On the other hand, some studies show that wide variety of natural dietary fiber have no effect on iron requirements. It is not necessarily the dietary fiber component of complex carbohydrate food which is primarily responsible for the mineral binding effect(6).
Magnesium balance and retention are lower whether dietary fiber is fed in the form of high-fiber breads, fruits and vegetables or hemicellulose as well(6). In some studies pectin has begun retention, while wheat bran begun faecal excretion(6). There is also information that magnesium balance may positive later on period of adaptation(6).
It is shown that not all fibers behave similar in affecting mineral bio-availability. Wheat bran is the most active. Fiber seems to have little or no nutritional effects on persons on enough mineral intakes. But the results of high fiber products have yet to be evaluated(6). In persons on a marginal intake of mineral, especially micronutrients, increase of taking dietary fiber may increase results(6).
2.9 Dietary Fiber and Bacterial Flora
Dietary fibers positively influence colonic microflora, provide nutritional substrates for colonic mucosal cells, increase mucosal barrier function(1). Dietary fiber pronounced influence on the bacterial flora of the gastrointestinal tract [GIT], mainly in the colon(6). The dietary fibers are helping the fermentation of bacteria(5).
Under normal conditions there are no bacteria in the stomach, due to the low pH(6). As the mucosa ages and atrophies the rate of acid secretion decreases. So, gastric pH is increased.
In the large intestine the mean transit time of contents may be 20-80 h, while in small intestine just 2h. So that there is enough time for a rich bacterial flora to develop. Dietary fiber increases the supply of nutrients such as lipids trapped in gel-forming polymers and both anions and cations bound to them(6). Because of the action of bacteria on dietary fiber forms short-chain fatty acids [eg-acetic, propionic, butyric] which are absorbed and utilized by the body(6). The amount of fiber degraded bacteria alters with the nature of a fiber. Pectins, pentosans and hemicelluloses are almost entirely degraded. Cellulose partially degraded and lignin actually undegraded(6).
2.10 Dietary Fiber and GI symptoms
The severity of symptoms in all groups was minimal(10). Patient with a greater feeling of fullness or bloating symptom severity or emotional upset due to the low fiber dose. Patients with faecal incontinence experience a variety of GI symptoms. Symptom severity and emotional upset seem to influence fiber tolerance(10). Knowledge of the course of adverse symptoms from dietary fiber supplementation is lacking. Dietary supplementation with gum arabic and psyllium fiber in previous clinical studies have been involved with few and minor adverse GI symptoms.Greater severity of several GI symptoms increased in people with faecal incontinence whether they intake a fiber supplement or not(10).
The dietary fiber increases the viscosity of intestinal content(2). Consequently in reduced absorption of antioxidants due to slowed enzymatic activity in the pancreas and increase difficulty in contacting intestinal enterocytes.
2.11 Dietary Fiber and Antioxidant
The main physiological effect of dietary fiber in the small intestine is to reduce the rate of release of nutrients or antioxidants(2). The dominant factors involved in the influence of dietary fiber on antioxidant digestion are; physical trapping of antioxidants with in structured assemblies such as fruit tissue, and enhanced viscosity of gastric fluids limiting the peristaltic mixing process that supports transport of enzymes to their substrates, bile salts to unmicellized fat, and soluble antioxidants to the intestinal wall(2).
Antioxidants may interact with other food components in the gut by binding to macromolecules like dietary fiber and forming chemical complexes and colloid structures(11). That structures reduce or increase their bio-availability(11). This important consequences in assessing the nutritional rate and real impact of many fruit and vegetables phytochemicals in the prevention and therapy of some chronic diseases(11).
The rate of release antioxidants from fibrous particles into the surrounding intestinal is inversely proportional to particle size and is directly proportional to particle size and is directly proportional to solute gradient(2). It is also affected by the following factors; the physical state of the solute, the physical structure of the particle, and the surface properties of the particle(2). The concentration of antioxidants within the continuous aqueous phase is constantly decreased by enteric absorption and refill, by the release of material from food particles(2). The process of these sequential release process is influenced by transit time as well(2). The main reasons why phenolic compounds and carotenoids are not bio-accessible due to the presence of dietary fiber because they are not well released from fruit and vegetable, dietary fiber entraps the phenolic compounds during digestion in the upper intestine, and some antioxidants may be bound to be absorbed, which is restricted by the action of dietary fiber matrices formed in the chyme(2).
3.0 Disadvantages of dietary fiber.
The dietary fiber can bind trace elements [eg-Zinc] and reduce the absorption of fat soluble vitamins. And dietary fiber is decreased the absorption of other carotenoids and probably that of alpha-tocopherol and polyphenols compounds(2).
So the diet with excess fiber is not recommended(4).
If the amount of dietary fiber is small,the diet is known as to lack bulk(12). When the amount of material in the colon is small, the colon is inactive and bowel movements are decreased. In addition, starvation and parenteral nutrition lead to atrophy of the mucosa of the colon and this is reversed when substances like pectin are placed in the colon. So called bulk laxatives work by providing a large volume of indigestible material to the colon(12).
With increase the level of dietary fiber, fiber acts with dietary minerals, and changing the micronutrient balance.
The ability of fiber to bind cations, more correctly retain cations, influences mineral absorption. Various studies have shown that the fiber content of the diet is involved with increased excretion of electrolytes in faeces. Other studies show that a diet rich in dietary fiber is also high in electrolytes. One advantage of the cation exchange capacity of dietary fiber is the decreasing in heavy metal toxicity due to the irreversible binding(12).