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Dietary Fiber Information

It acts by changing the nature of the contents of the gastrointestinal tract and by changing how other nutrients and chemicals are absorbed.[2] Soluble fiber absorbs water to become a gelatinous, viscous substance and is fermented by bacteria in the digestive tract. Insoluble fiber has bulking action and is not fermented.[3] Lignin, a major dietary insoluble fiber source, may alter the fate and metabolism of soluble fibers.[1]

Chemically, dietary fiber consists of non-starch polysaccharides such as arabinoxylans, cellulose, and many other plant components such as resistant dextrins, inulin, lignin, waxes, chitins, pectins, beta-glucans, and oligosaccharides.[1] A novel position has been adopted by the US Department of Agriculture to include functional fibers as isolated fiber sources that may be included in the diet.[1] The term "fiber" is something of a misnomer, since many types of so-called dietary fiber are not actually fibrous.

Food sources of dietary fiber are often divided according to whether they provide (predominantly) soluble or insoluble fiber. Plant foods contain both types of fiber in varying degrees, according to the plant's characteristics.

Advantages of consuming fiber are the production of healthful compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive hygroscopic properties) to increase bulk, soften stool, and shorten transit time through the intestinal tract.

Disadvantages of a diet high in fiber is the potential for significant intestinal gas production and bloating. Constipation can occur if insufficient fluid is consumed with a high-fiber diet.

Contents

History of definition

Originally, fiber was defined to be the components of plants that resist human digestive enzymes, a definition that includes lignin and polysaccharides. The definition was later changed to also include resistant starches, along with inulin and other oligosaccharides.[3]

Sources of fibre

Dietary fibre is found in plants. While all plants contain some fibre, plants with high fibre concentrations are generally the most practical source.

Fibre-rich plants can be eaten directly. Or, alternatively, they can be used to make supplements and fibre-rich processed foods.

The American Dietetic Association (ADA) recommends consuming a variety of fiber-rich foods.

Plant sources of fibre

Legumes contain healthful dietary fibres.

Some plants contain significant amounts of soluble and insoluble fibre. For example plums (or prunes) have a thick skin covering a juicy pulp. The plum's skin is an example of an insoluble fibre source, whereas soluble fibre sources are inside the pulp.[4]

Soluble fibre is found in varying quantities in all plant foods, including:

Sources of insoluble fibre include:

Fiber supplements

These are a few example forms of fibre that have been sold as supplements or food additives. These may be marketed to consumers for nutritional purposes, treatment of various gastrointestinal disorders, and for such possible health benefits as lowering cholesterol levels, reducing risk of colon cancer, and losing weight.

Soluble fibre supplements may be beneficial for alleviating symptoms of irritable bowel syndrome, such as diarrhea and/or constipation and abdominal discomfort.[6] Prebiotic soluble fiber products, like those containing inulin or oligosaccharides, may contribute to relief from inflammatory bowel disease,[7] as in Crohn's disease,[8] ulcerative colitis,[9][10] and Clostridium difficile,[11] due in part to the short-chain fatty acids produced with subsequent anti-inflammatory actions upon the bowel.[12][13] fiber supplements may be effective in an overall dietary plan for managing irritable bowel syndrome by modification of food choices.[14]

Inulins

Main article: Inulin

Chemically defined as oligosaccharides occurring naturally in most plants, inulins have nutritional value as carbohydrates, or more specifically as fructans, a polymer of the natural plant sugar, fructose. Inulin is typically extracted by manufacturers from enriched plant sources such as chicory roots or Jerusalem artichokes for use in prepared foods.[15] Subtly sweet, it can be used to replace sugar, fat, and flour, is often used to improve the flow and mixing qualities of powdered nutritional supplements, and has significant potential health value as a prebiotic fermentable fiber.[16]

Inulin is advantageous because it contains 25–30% the food energy of sugar or other carbohydrates and 10–15% the food energy of fat. As a prebiotic fermentable fiber, its metabolism by gut flora yields short-chain fatty acids (discussed above) which increase absorption of calcium,[17] magnesium,[18] and iron,[19] resulting from upregulation of mineral-transporting genes and their membrane transport proteins within the colon wall. Among other potential beneficial effects noted above, inulin promotes an increase in the mass and health of intestinal Lactobacillus and Bifidobacterium populations.

Vegetable gums

Vegetable gum fiber supplements are relatively new to the market. Often sold as a powder, vegetable gum fibers dissolve easily with no aftertaste. In preliminary clinical trials, they have proven effective for the treatment of irritable bowel syndrome.[20] Examples of vegetable gum fibers are guar gum and acacia Senegal gum.

Mechanism

The main action of dietary fiber is to change the nature of the contents of the gastrointestinal tract, and to change how other nutrients and chemicals are absorbed.[1][2]

Physico-chemical properties

Food has distinct physico chemical properties, meat is fibrous, bread is a solid foam , hard cheese is a particulate composit and fresh fruit and vegetables are cellular materials . Most semi solid foods, fibre and fat are a combination of gel matrices which are hydrated or collapsed with microstructural elements, globules, solutions or encapsulating walls. Many foods are cross-linked , either through chemical covalent bonds or cross-links through molecular entanglement or hydrogen or ionic bond cross-linking. Particulate size and interfacial interactions with adjacent matrices affect the mechanical properties of food composites. The variables include chemical structure, polymer concentration, molecular weight, degree of chain branching, the extent of ionisation (for electrolytes), solution pH, ionic strength and temperature. Most concentrated solutions of polymers and gels possess viscoelastic properties, i.e. their inner structure displays a spectrum of characteristic relaxation or retardation times. Many food systems consist of one phase dispersed or suspended in another phase eg foams, emulsions, doughs (which contain air bubbles suspended in a liquid), emulsion oil droplets in water and dough with starch granules in gluten.Proteins, carbohydrates and some times lipids interact in an aqueous environment to create the native or processed structure of foods. A variety of foods, particularly bakery products,are foams or sponges with a porous and open macrostructure, due to dispersed gas bubbles in a matrix. The cells of cooked potatoes and legumes are gels filled with gelatinised starch granules. The cellular structures of fruits and vegetables are foams with a closed cell geometry filled with a gel, surrounded by cell walls which are composites with an amorphous matrix strengthened by complex carbohydrate fibres. [58],[59],[60],[61],[62],[63] Food ingredients can soluble in and/or plasticised by water. Water molecules in a solution are a single physical state with degrees of hindered mobility. Individual water molecules are transiently hydrogen bonded to individual polar sites in the solute. Adding a few water molecules to an anhydrous solute profoundly changes the visco-elastic properties of the solute through water plasticisation, which increases the free volume and decreases the local viscosity. This has a profound effect on the hindered diffusion of water molecules and produces viscous drag. These less mobile water molecules are pulled along with the solute during flow and are freely exchangeable with all of the water in the solution, i.e. the water is not bound. Water is the most important plasticiser, particularly in biological systems changing mechanical properties. The initial sorbed water fraction is most strongly plasticising and unfreezable or bound. The later sorbed water fraction is freezable, and is free, mobile or loosely bound. Plasticisation in amorphous regions of polymers leads to increased intermolecular space or free volume, decreases local viscosity and increases mobility, by increasing free volume and decreasing local viscosity. The traditional view of the "structuring" effect of solutes on water and of water activity lead to the concept of bound water. This idea is now replacedthe concept of the mobilising effect of water acting as a plasticiser. The term bound and free implied that there are two types of water, which can be distinguished chemically or physically (energetically). Plasticising and non plasticising conditions (temperature and time dependent) result from different amounts of water but not different types of water, two different operational conditions. Cooking and chewing food alters these physico-chemical properties and hence absorption and movement through the stomach and along the intestine.

Physical events along the gastrointestinal tract.

Ingestion

The rate of eating is the very important. A slowly eaten meat will enter the absorptive phase of the gastrointestinal tract more slowly than a gulped meal of similar composition. Many of the differences between low and high glycaemic foods would disappear if a meal was eaten slowly. The chemical and physico chemical nature ( lipid, protein , carbohydrate ) of the meal will also influence the gastric emptying of the food multiphase system .

Upper gastrointestinal tract.

The movement of chyme along the gastrointestinal tract can be regarded as flow in a disperse system, with a multiplicity of particles and complex lipid / micellar / aqueous / hydrocolloid hydrophobic and hydrophilic phases, as well as solid, liquid, colloidal and gas bubble phases. As chyme moves along the gastrointestinal tract, polymer flow and diffusion becomes important. Suspended rigid particles will affect viscosity and flow through the lumen. Micelles are colloid-sized clusters of molecules which form in conditions above the critical micelle concentration and above the kraft temperature,as an organised solvent cage. In the upper gastrointestinal tract these consist of bile acids, triacyl glycerols, cholesterol and di- and mono- acyl glycerols . There is agitation of fluid flow inside the tube, with resultant effects on boundary or surface layers during the flow. The rate of movement in the centre of a tube is different to that at the boundary layer. The further from the wall the less the fluid velocity.[63],[64],[65],[66] Two mechanisms bring nutrients into contact with the epithelium.

  1. Intestinal contractions create turbulence
  2. convection currents direct luminal contents from the centre of the lumen to the epithelial surface.

The multiple physical phases in the intestinal tract slow the rate of absorption compared to that of the suspension solvent alone. Nutrients diffuse through the thin, relatively unstirred layer of fluid adjacent to the epithelium. Increased luminal content viscosity alters both convection and diffusion of the nutrients across the unstirred layer. The immobolising of aqueous carbohydrate solution within complex polysacchride molecules affects their release and subsequent absorption from the small intestine , a phenomenonon important to the glycaemic idex. Molecules begin to interact as their concentration increases. During enteric absorption water must be absorbed at a rate commensurate with the absorption of solutes. The transport of actively and passively absorbed nutrients across epithelium is affected by the unstirred water layer covering the microvillus membrane . The thickness of the microvillus membrane unstirred water varies along the villus with health and disease, intestinal movements and changes in the passive permeability properties of the membrane. The presence of mucus or fibre eg pectin or guar in the unstirred layer may alter the viscosity and alter the solute diffusion coefficient. Adding viscous polysaccharides to carbohydrate meals can reduce post prandial blood glucose concentrations. Wheat and maize but not oats modify glucose absorption rate dependent upon the particle size. This effect was originally explained as a delay in gastric emptying time. The reduction in absorption rate with guar gum may be due to the increased resistance by viscous solutions to the convective flows created by intestinal contractions Dietary fibre interacts with pancreatic and enteric enzymes and their substrates. Human pancreatic enzyme activity is reduced when incubated with most fibre sources. Fibre may effect amylase activity and hence the rate of hydrolysis of starch The more viscous polysaccharides extend the mouth to caecum transit time; guar, tragacanth and pectin being slower than wheat bran[67].

Sterol metabolism

Dietary fibre has been shown to have an effect on sterol metabolism. This effect is not simple and the effect maybe indirect or direct. [68] Indirectly dietary fibre may displace fat from the diet, or increased polyunsaturated fats are frequently eaten in conjunction with the fibre. The direct effect of fibre on sterol metabolism may be through one of several mechanisms: There are a number of ways

  1. replacement of energy foods from the diet through bulking effect
  2. slowing of gastric emptying time.
  3. glycaemic index type of action on absorption
  4. slowing of bile acid absorption in the ileum and carriage through to the caecum
  5. altered or increased bile acid metabolism in the caecum
  6. indirectly by absorbed short-chain fatty acids, especially propionic acid, resulting from fibre fermentation.
  7. sequestration of bile acids to fibre or bacteria in the caecum and enhanced faecal loss from the entero-hepatic circulation.

It is reasonable to see fibre acting in a well documented manner on each phase of ingestion, digestion, absorption and excretion. Delaying, slowing and then increasing excretion. It is repeatedly suggested that those foods and isolates which have high glycaemic index inhibit or slow absorption have a prime influence on sterol metabolism. An important action of some fibres is to reduce the reabsorption of bile acids in the ileum and hence the amount and type of bile acid and fats reaching the colon. Lignin in fibre adsorbs bile acids, but the unconjugated form of the bile acids are adsorbed more than the conjugated form. [69] In the ileum where bile acids are primarily absorbed there will be predominantly conjugated bile acids. Bile acids may be trapped within the lumen of the ileum either because of a high luminal viscosity or because they bind to the fibre. A reduction in the ileal reabsorption of bile acid has several direct effects. The enterohepatic circulation of bile acids may be changed and there is an increased flow of bile acids to the caecum, where they are deconjugated and 7-dehydroxylated. In this less water-soluble form, bile acids are adsorbed to dietary fibre The consequence of this is that the enterohepatic pool is diverted to the caecum and the faecal loss of sterols. This is dependent in part on the amount and type of fibre. A further factor is an increase in the bacterial mass and activity of the ileum further stimulating caecal bacterial activity. The enteric loss is renewed by increased synthesis of bile acids from cholesterol which in turn reduces body cholesterol. The fibres that are most effective in influencing sterol metabolism (e.g. pectin) are fermented in the colon, It is therefore unlikely that the reduction in body cholesterol is due to adsorption to this fermented fibre in the colon. This is in contrast to their important sequestrating effect of fibre in the ileum. There might be that there are alterations in the end-products of bile acid bacterial metabolism or the release of short chain fatty acids which are absorbed from the colon and return to the liver in the portal vein, modulating either the synthesis of cholesterol or its catabolism to bile acids. [70] The prime mechanism whereby fibre influences cholesterol metabolism is through bacteria binding bile acids in the colon after the initial deconjugation and dehydroxylation.[71] Fermentable fibres eg pectin will by virtue of their providing a medium for bacterial growth increase the bacterial mass in the colon. The sequestrated bile acids are then excreted in faeces. Other fibres, e.g. gum arabic, are associated with a significant decrease in serum cholesterol without increasing faecal bile acid excretion. The effect of fibre on sterol metabolism is through

  1. binding of bile acids to fibre in the ileum and hence increasing the amount of bile acids passing to the colon
  2. sequestration of deconjugated bile acids by bacteria in the colon and increased excretion in the faeces. The mass of bacteria will increase if the amount of fermentable bile acids in the caecum increases.

Caecal fermentation

The colon may be regarded as two organs, the right side a fermenter, the left side affecting continence. The right side of the colon is involved in nutrient salvage so that dietary fibre, resistant starch, fat and protein are utilised by bacteria and the end-products absorbed for use by the body. The presence of bacteria in the colon produces an ‘organ’ of intense, mainly reductive, metabolic activity.This is in contrast to the liver, which is oxidative. The range of metabolic transformations that the intestinal flora perform on ingested compounds is wide.The substrates utilised by the caecal have either passed along the entire intestine or are biliary excretion products . [70] The effects of dietary fibre in the colon may be summarised in terms of:

  1. susceptibility to bacterial fermentation
  2. ability to increase bacterial mass
  3. ability to increase bacterial saccharolytic enzyme activity
  4. water-holding capacity of the fibre residue after fermentation

Enlargement of the caecum is a common finding when some dietary fibres are fed and this is now believed to be part of a normal physiological adjustment. Such an increase may be due to a number of factors, prolonged caecal residence of the fibre, increased bacterial mass, or increased bacterial end-products. Some non-absorbed carbohydrates, e.g. pectin, gum arabic, oligosaccharides and resistant starch, are fermented to short-chain fatty acids (chiefly acetic, propionic and n-butyric), carbon dioxide, hydrogen and methane. Short-chain fatty acids are the predominant anions in the human faeces, derived by fermentation of complex carbohydrates, resistant starch and non-starch polysaccharides. The caecal fermentation of 40–50 g of complex polysaccharides will yield 400–500 mmol total short-chain fatty acids, 240–300 mmol acetate, and 80–100 mmol of both propionate and butyrate. Almost all of these short-chain fatty acids will be absorbed from the colon. This means that faecal short-chain fatty acid estimations do not reflect caecal and colonic fermentation, only the efficiency of absorption, the ability of the fibre residue to sequestrate short-chain fatty acids, and the continued fermentation of fibre around the colon, which presumably will continue until the substrate is exhausted. The production of short-chain fatty acids has several possible actions on the gut mucosa. All of the short-chain fatty acids are readily absorbed by the colonic mucosa, but only acetic acid reaches the systemic circulation in appreciable amounts. Butyric acid appears to be used as a fuel by the colonic mucosa and in vitro studies of isolated cells have indicated that the short-chain fatty acids and butyric acid in particular are the preferred energy sources of colonic cells.

Faecal weight

Faeces are complex and consist of 75 % water; bacteria make a large contribution to the dry weight, the residue being unfermented fibre and excreted compounds. In a study in Edinburgh the individual variation was between 19 and 280 g over 24 hours. The amount of faeces excreted a day varies for any one individual over a period of time. Of the dietary constituents, only dietary fibre influenced faecal weight [72]. The most important mechanism whereby dietary fibre increases faecal weight is through the water-holding capacity of unfermented fibre. However, fibre may also influence faecal output through colonic microbial growth, stimulated by ingestion of fermentable fibres eg apple, guar or pectin, an uncertain route. There may also be an added osmotic effect of products of bacterial fermentation on faecal mass. Faeces are not a gel, however, but a plasticine-like material, heterogeneous without viscosity and made up of water, bacteria, lipids, sterols, mucus and fibre. The mechanism of this change, between the caecum and descending colon is not known, but the steps relate to the distribution of water.. In the colon water is distributed in three ways:

  1. Free water which can be absorbed from the colon.
  2. Water that is incorporated into bacterial mass.
  3. Water that is bound by fibre.

The amount of water in the caecal mass and the consequent faeces will be dependent upon

  1. The time available for water absorption through the colonic mucosa
  2. The incorporation of water into the residue of fibre after fermentation of the fibre
  3. The bacterial mass.

Wheat bran added to the diet increases faecal weight in a predictable linear manner and decreases intestinal transit time. The particle size of the fibre is all-important, coarse wheat bran being more effective than fine wheat bran. The greater the water-holding capacity of the bran, the greater the effect on faecal weight. Other faecal constituents, namely bile acids which in absolute amounts do not increase are diluted by faecal water and hence their concentration decreases. For most healthy individuals, an increase in wet faecal weight, depending on the particle size of the bran, is generally of the order of 3–5 g/g fibre. The fermentation of some fibres results in an increase in the bacterial content and possibly faecal weight. Other fibres, of which pectin is an important example, are fermented without any such effect. The degree to which free water is absorbed from the colon will be affected by a number of factors which are poorly understood. An increase in the short-chain fatty acid concentration of faeces appears to be related to an increased output of faecal water, which may suggest that under some circumstances short-chain fatty acid absorption is less efficient and in part determines faecal output. This is in sympathy with the view of Hellendoorn [70] who suggested an important role for fibre fermentation products on stool weight and transit time. The demonstration that short-chain fatty acids were absorbed rapidly in the colon suggested that they play no part in determining faecal output. However, it would appear that there is continued fermentation of some complex carbohydrates, e.g. ispaghula in the distal colon. Under these circumstances the faecal short-chain fatty acids may influence faecal water osmolality, absorption and stool weight. The effect of fibre on faecal weight may be calculated as:

where Wf, Wb and Wm are, respectively, the dry weights of fibre remaining after fermentation in the colon, bacteria present in the faeces, and osmotically-active metabolites and other substances in the colonic contents which could reduce the amount of free water absorbed, and Hf, H6 and Hm denote their respective ‘water-holding capacities’ (i.e. the weight of water resistant to absorption from the colon, per unit dry weight of each faecal constituent). A simple way toincrease stool weight A handful of bran and an apple and orange Or some other fruit to choice.

Not yet formally proposed as an essential macronutrient, dietary fiber is nevertheless regarded as important for the diet, with regulatory authorities in many developed countries recommending increases in fiber intake.[1][2][21][22]

Effects of fiber intake

Research has shown that fiber may benefit health in several different ways. Lignin and probably related materials that are resistant to enzymatic degradation, diminish the nutritional value of foods.[23]

Table legend

Color coding of table entries:

Dietary fiber functions and benefits

Functions Benefits[24][25]
Increases food volume without increasing caloric content, providing satiety May reduce appetite
Attracts water and forms a viscous gel during digestion, slowing the emptying of the stomach and intestinal transit, shielding carbohydrates from enzymes, and delaying absorption of glucose[26] Lowers variance in blood sugar levels
Lowers total and LDL cholesterol Reduces risk of cardiovascular disease
Regulates blood sugar May reduce glucose and insulin levels in diabetic patients and may lower risk of diabetes[27]
Speeds the passage of foods through the digestive system Facilitates regular defecation
Adds bulk to the stool Alleviates constipation
Balances intestinal pH[28] and stimulates intestinal fermentation production of short-chain fatty acids May reduce risk of colorectal cancer[29]

Fiber does not bind to minerals and vitamins and therefore does not restrict their absorption, but rather evidence exists that fermentable fiber sources improve absorption of minerals, especially calcium.[30][31][32] Some plant foods can reduce the absorption of minerals and vitamins like calcium, zinc, vitamin C, and magnesium, but this is caused by the presence of phytate (which is also thought to have important health benefits), not by fiber.[33]

Guidelines on fiber intake

Current recommendations from the United States National Academy of Sciences, Institute of Medicine, suggest that adults should consume 20–35 grams of dietary fiber per day, but the average American's daily intake of dietary fiber is only 12–18 grams.[33][34]

The ADA recommends a minimum of 20–35 g/day for a healthy adult depending on calorie intake (e.g., a 2000 Cal/8400 kJ diet should include 25g of fiber per day). The ADA's recommendation for children is that intake should equal age in years plus 5 g/day (e.g., a 4 year old should consume 9 g/day). No guidelines have yet been established for the elderly or very ill. Patients with current constipation, vomiting, and abdominal pain should see a physician. Certain bulking agents are not commonly recommended with the prescription of opioids because the slow transit time mixed with larger stools may lead to severe constipation, pain, or obstruction.

The British Nutrition Foundation has recommended a minimum fiber intake of 18 g/day for healthy adults.[35]

Fiber recommendations in the USA

On average, North Americans consume less than 50% of the dietary fiber levels recommended for good health. In the preferred food choices of today's youth, this value may be as low as 20%, a factor considered by experts as contributing to the obesity levels seen in many developed countries.[36][37]

Recognizing the growing scientific evidence for physiological benefits of increased fiber intake, regulatory agencies such as the Food and Drug Administration (FDA) of the United States have given approvals to food products making health claims for fiber.

In clinical trials to date, these fiber sources were shown to significantly reduce blood cholesterol levels, an important factor for general cardiovascular health,[38] and to lower risk of onset for some types of cancer.[39]

Soluble (fermentable) fiber sources gaining FDA approval are:

Other examples of fermentable fiber sources (from plant foods or biotechnology) used in functional foods and supplements include inulin, resistant dextrins, fructans, xanthan gum, cellulose, guar gum, fructooligosaccharides (FOS), and oligo- or polysaccharides.

Consistent intake of fermentable fiber through foods like berries and other fresh fruit, vegetables, whole grains, seeds, and nuts is now known to reduce risk of some of the world’s most prevalent diseases[40][41][42][43]—obesity, diabetes, high blood cholesterol, cardiovascular disease, and numerous gastrointestinal disorders. In this last category are constipation, inflammatory bowel disease, ulcerative colitis, hemorrhoids, Crohn’s disease, diverticulitis, and colon cancer—all disorders of the intestinal tract where fermentable fiber can provide healthful benefits.[40]

Insufficient fiber in the diet can complicate defecation.[44] Low-fiber feces are dehydrated and hardened, making them difficult to evacuate—defining constipation[44] and possibly leading to development of hemorrhoids[44] or anal fissures.

Although many researchers believe that dietary fiber intake reduces risk of colon cancer, one study conducted by researchers at the Harvard School of Medicine of over 88,000 women did not show a statistically significant relationship between higher fiber consumption and lower rates of colorectal cancer or adenomas.[45]

Fiber recommendations in the UK

In June 2007, the British Nutrition Foundation issued a statement to define dietary fiber more concisely and list the potential health benefits established to date:[46][47]

‘Dietary fibre’ has been used as a collective term for a complex mixture of substances with different chemical and physical properties which exert different types of physiological effects. The use of certain analytical methods to quantify dietary fiber by nature of its indigestibility results in many other indigestible components being isolated along with the carbohydrate components of dietary fiber. These components include resistant starches and oligosaccharides along with other substances that exist within the plant cell structure and contribute to the material that passes through the digestive tract. Such components are likely to have physiological effects. Yet, some differentiation has to be made between these indigestible plant components and other partially digested material, such as protein, that appears in the large bowel. Thus, it is better to classify fiber as a group of compounds with different physiological characteristics, rather than to be constrained by defining it chemically. Diets naturally high in fiber can be considered to bring about several main physiological consequences:

Therefore, it is not appropriate to state that fiber has a single all encompassing physiological property as these effects are dependent on the type of fiber in the diet. The beneficial effects of high fiber diets are the summation of the effects of the different types of fiber present in the diet and also other components of such diets. Defining fiber physiologically allows recognition of indigestible carbohydrates with structures and physiological properties similar to those of naturally occurring dietary fibers.[47]

Fiber and calories

Fiber contributes less energy (measured in Calories or kilojoules) than sugars and starches because it cannot be fully absorbed by the body. Sugars and starches provide 4 Calories per gram, and the human body has specific enzymes to break them down into glucose, fructose, and galactose, which can then be absorbed by the body. The human body lacks enzymes to break down fiber. Insoluble fiber does not change inside the body, so the body cannot absorb it and nutritionists say that it contributes 0 Calories per gram. Soluble fiber is partially fermented, with the degree of fermentability varying with the type of fiber, and contributes some energy when broken down and absorbed by the body. Dietitians have not reached a consensus on how much energy is actually absorbed, but some approximate around 2 Calories (8.5 kilojoules) per gram of soluble fiber. Regardless of the type of fiber, the body absorbs fewer than 4 Calories (16.7 kilojoules) per gram of fiber, which can create inconsistencies for actual product nutrition labels. In some countries, fiber is not listed on nutrition labels, and is considered 0 Calories/gram when the food's total Calories are computed. In other countries all fiber must be listed, and is considered 4 Calories per gram when the food's total Calories are computed (because chemically fiber is a type of carbohydrate and other carbohydrates contribute 4 Calories per gram). In the US, soluble fiber must be counted as 4 Calories per gram, but insoluble fiber may be (and usually is) treated as 0 Calories per gram and not mentioned on the label.

Soluble fiber fermentation

The American Association of Cereal Chemists has defined soluble fiber this way: “the edible parts of plants or similar carbohydrates resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine.”[48] In this definition:

edible parts of plants
indicates that some parts of a plant we eat—skin, pulp, seeds, stems, leaves, roots—contain fiber. Both insoluble and soluble sources are in those plant components.
carbohydrates
complex carbohydrates, such as long-chained sugars also called starch, oligosaccharides, or polysaccharides, are sources of soluble fermentable fiber.
resistant to digestion and absorption in the human small intestine
foods providing nutrients are digested by gastric acid and digestive enzymes in the stomach and small intestine where the nutrients are released then absorbed through the intestinal wall for transport via the blood throughout the body. A food resistant to this process is undigested, as insoluble and soluble fibers are. They pass to the large intestine only affected by their absorption of water (insoluble fiber) or dissolution in water (soluble fiber).
complete or partial fermentation in the large intestine
the large intestine comprises a segment called the colon within which additional nutrient absorption occurs through the process of fermentation. Fermentation occurs by the action of colonic bacteria on the food mass, producing gases and short-chain fatty acids. It is these short-chain fatty acids—butyric, acetic (ethanoic), propionic, and valeric acids—that scientific evidence is revealing to have significant health properties.[49]

As an example of fermentation, shorter-chain carbohydrates (a type of fiber found in legumes) cannot be digested, but are changed via fermentation in the colon into short-chain fatty acids and gases (which are typically expelled as flatulence).

According to a 2002 journal article,[40] fibers compounds with partial or low fermentability include:

fiber compounds with high fermentability include:

Short-chain fatty acids

When soluble fiber is fermented, short-chain fatty acids (SCFA) are produced. SCFAs are involved in numerous physiological processes promoting health, including:[49]

SCFAs that are absorbed by the colonic mucosa pass through the colonic wall into the portal circulation (supplying the liver), and the liver transports them into the general circulatory system.

Overall, SCFAs affect major regulatory systems, such as blood glucose and lipid levels, the colonic environment, and intestinal immune functions.[52][53]

The major SCFAs in humans are butyrate, propionate, and acetate, where butyrate is the major energy source for colonocytes, propionate is destined for uptake by the liver, and acetate enters the peripheral circulation to be metabolized by peripheral tissues.

FDA-approved health claims

The FDA allows producers of foods containing 1.7g per serving of psyllium husk soluble fiber or 0.75g of oat or barley soluble fiber as beta-glucans to claim that reduced risk of heart disease can result from their regular consumption.[54]

The FDA statement template for making this claim is: Soluble fiber from foods such as [name of soluble fiber source, and, if desired, name of food product], as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of [name of food product] supplies __ grams of the [necessary daily dietary intake for the benefit] soluble fiber from [name of soluble fiber source] necessary per day to have this effect.[54]

Eligible sources of soluble fiber providing beta-glucan include:

  1. Oat bran
  2. Rolled oats
  3. Whole oat flour
  4. Oatrim
  5. Whole grain barley and dry milled barley
  6. Soluble fiber from psyllium husk with purity of no less than 95%

The allowed label may state that diets low in saturated fat and cholesterol and that include soluble fiber from certain of the above foods “may” or “might” reduce the risk of heart disease.

As discussed in FDA regulation 21 CFR 101.81, the daily dietary intake levels of soluble fiber from sources listed above associated with reduced risk of coronary heart disease are:

Soluble fiber from consuming grains is included in other allowed health claims for lowering risk of some types of cancer and heart disease by consuming fruit and vegetables (21 CFR 101.76, 101.77, and 101.78).[54]

Potential longevity

A study of 388,000 adults ages 50 to 71 for nine years found that the highest consumers of fiber were 22% less likely to die over this period.[56] In addition to lowering the risk of death from heart disease, adequate consumption of fiber-containing foods, especially grains, also appeared to reduce the incidence of infectious and respiratory illnesses, and, particularly among males, lowered the risk of cancer-related death.

See also

Footnotes

  1. ^ a b c d e f "Dietary Reference Intakes for Energy, Carbohydrate, fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients) (2005), Chapter 7: Dietary, Functional and Total fiber.". US Department of Agriculture, National Agricultural Library and National Academy of Sciences, Institute of Medicine, Food and Nutrition Board. http://www.nal.usda.gov/fnic/DRI//DRI_Energy/339-421.pdf.
  2. ^ a b c Eastwood M, Kritchevsky D (2005). "Dietary fiber: how did we get where we are?". Annu Rev Nutr 25: 1–8. doi:10.1146/annurev.nutr.25.121304.131658. PMID 16011456.
  3. ^ a b Anderson JW, Baird P, Davis RH et al. (2009). "Health benefits of dietary fiber". Nutr Rev 67 (4): 188–205. doi:10.1111/j.1753-4887.2009.00189.x. PMID 19335713.
  4. ^ Stacewicz-Sapuntzakis M, Bowen PE, Hussain EA, Damayanti-Wood BI, Farnsworth NR (May 2001). "Chemical composition and potential health effects of prunes: a functional food?". Crit Rev Food Sci Nutr. 41 (4): 251–86. doi:10.1080/20014091091814. PMID 11401245.
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Dietary supplements
Types Amino acids • Bodybuilding supplement • Energy drink • Energy bar • Fatty acids • Herbal Supplements • Minerals • Prebiotics • Probiotics (Lactobacillus, Bifidobacterium) • Protein bar • Vitamins
Vitamins and "minerals" (chemical elements) Retinol (Vitamin A) • B vitamins: Thiamine (B1) • Riboflavin (B2) • Niacin (B3) • Pantothenic acid (B5) • Pyridoxine (B6) • Biotin (B7) • Folic acid (B9) • Cyanocobalamin (B12) • Ascorbic acid (Vitamin C) • Ergocalciferol and Cholecalciferol (Vitamin D) • Tocopherol (Vitamin E) • Naphthoquinone (Vitamin K) • Calcium • Choline • Chromium • Cobalt • Copper • Fluorine • Iodine • Iron • Magnesium • Manganese • Molybdenum • Phosphorus • Potassium • Selenium • Sodium • Sulfur • Zinc
Other common ingredients AAKG • Carnitine • Chondroitin sulfate • Cod liver oil • Copper gluconate • Creatine/Creatine supplements • Dietary fiber • Echinacea • Elemental calcium • Ephedra • Fish oil • Folic acid • Ginseng • Glucosamine • Glutamine • Grape seed extract • Guarana • Iron supplements • Japanese Honeysuckle • Krill oil • Lingzhi • Linseed oil • Lipoic acid • Milk thistle • Melatonin • Red yeast rice • Royal jelly • Saw palmetto • Spirulina • St John's wort • Taurine • Wheatgrass • Wolfberry • Yohimbine • Zinc gluconate
Related articles Codex Alimentarius • Enzyte • Hadacol • Nutraceutical • Multivitamin • Nutrition

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