Dietary fiber

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Dietary fiber can be defined as the edible part of plants that resists digestion and absorption in the human small intestine and undergoes partial or total fermentation in the large intestine. This plant part is formed by a set of chemical compounds of a heterogeneous nature (polysaccharides, oligosaccharides, lignin and similar substances). From a nutritional point of view, and in a strict sense, dietary fiber is not a nutrient, since it does not directly participate in the body's basic metabolic processes. However, dietary fiber performs extremely important physiological functions such as stimulating intestinal peristalsis. The reason why the human body cannot process it is because the digestive system does not have the enzymes that can hydrolyze it. This does not mean that dietary fiber passes intact through the digestive system: although the intestine does not have enzymes to digest it, the enzymes of the intestinal microbiota partially ferment the fiber and break it down into various chemical compounds: gases (hydrogen, carbon dioxide and methane) and short chain fatty acids (acetate, propionate and butyrate). The latter can play an important role in the organism of living beings. Dietary fiber is found only in foods of plant origin that are not technologically processed, such as cereals, fruits, vegetables, and legumes.

Features and Functions

Dietary fiber fulfills the function of being the structural part of plants and, therefore, it is found in all foods derived from plant products such as vegetables, fruits, cereals and legumes. Most fibers are chemically considered to be polysaccharides, but not all polysaccharides are fibers (starch, for example, is not a vegetable fiber). The fibers are described as non-starch polysaccharides (non-starch polysaccharides). Some constituents of the fibers are cellulose, hemicelluloses, pectins, gums and mucilages. The fibers may also include some non-polysaccharide compounds such as lignin (polymers of several dozen molecules of phenol, an organic alcohol with strong internal bonds that make them impermeable to digestive enzymes), cutin, and tannins. As fiber has been investigated, other chemical components have been added to the list.

The terms that are sometimes mentioned crude fiber, neutral-detergent fiber, dietary fiber refer to fiber in general and they merely reflect different methodologies used to estimate fiber content in foods, since different types of fiber cannot be identified with these methods. For example, the chemical structure of cellulose and other polysaccharide fibers are similar.

Components of dietary fiber

Figure 2. Unrefined cereals are a primary source of fiber.

Vegetable fiber is sometimes referred to as a heterogeneous set of complex molecules, the benefits are various and for this reason it is convenient to ingest it from various sources rather than from a single one. Fibers often contain compounds such as:

Jealousy: Insoluble part of the dietary fiber, abundant in whole grain flour (Figure 2), bran and vegetables as alcachokes, spinach and green beans. Cellulose is part of the plant cell walls.
Hemicelulose: glucose mixture, galactous, xylous, arabinous, handy, and uronic acids, forming part of the insoluble fiber found in bran and whole grains of different cereals.
Pesticides: are polymers of methyl acid D-galacturonic. They are mostly found in the skin of certain fruits such as apple or pulp of other vegetables such as citrus, strawberry, quince and carrot. Since they retain water easily, forming very viscous gels, they are used to confer certain texture characteristics. In addition, the intestinal microorganisms ferment them and thus increase the fecal volume. Its main food use is that of thickening in the manufacture of jams and confectionery products. To do this, it is sufficient that they are at 1% concentrations in the product.
Sturdy starch: in tubers such as potato and seeds, also in fruits, rhizomes and marrow of many plants. This starch, which is not hydrolysed throughout the digestion process, constitutes 20% of the starch ingested in the diet. This ratio is reduced when food is subjected to thermal treatment.
Inulina: it is a reserve carbohydrate found in the achicoria, onion, garlic, cardo and alcachofa. It is soluble in water and is not digestible by digestive enzymes, but by those of the intestine's populated microorganisms.
Non-carbohydrated compounds: like the lignine that has a lot of acids and phenylpropylic alcohols forming the insoluble fiber with great ability to join and drag other substances by the digestive tract, forms the structure of the hardest part or woody of the vegetables, such as acelga, lettuce, the clot of the cereals, among others.
Gomas: formed by uronic acid, xylose, arabinous or handy, such as guar, arabic, karaya and tragacanto. They're soluble fiber.
Mucílagos: are highly branched polysaccharides of pentase (rabinous and xylose) that secrete the plants against the lesions. The composition depends on the degree of ripening of the plant. The higher their maturation, the higher the amount of cellulose and lignin and the less the amount of mucilages and rubbers. They are part of the seed of the plant Ovata plantagoSome algae and acacia and tomato seeds. They are part of soluble fibers and some have laxative function.
Other substances: cutin, tannins, suberine, phytic acid, proteins, ions such as calcium, potassium and magnesium.

Types of dietary fiber

Dietary fiber, traditionally considered as a complex carbohydrate, has been divided into two main groups according to its chemical characteristics and its effects on the human organism. This classification is arbitrary and is only based on chemical separation maintaining certain conditions controlled pH and enzymes that try to simulate physiological conditions. Two fractions are thus obtained: insoluble fiber and soluble fiber.

La insoluble fiber: consists of substances (cellulose, hemicelulous, lignin and resistant starch) that retain little water and swell little. This type of fiber predominates in foods such as wheat bran, whole grains, some vegetables and in general in all cereals. The components of this type of fiber are not fermentable and resist the action of the microorganisms of the intestine. Its main effect in the body is to clean, like a natural brush, the walls of the intestine by removing the debris attached to it; in addition to increasing the volume of the feces and diminishing its consistency and transit time through the digestive tract. As a result, this type of fiber, when ingested daily, facilitates depositions and prevents constipation.
La soluble fiber: consists of components (inulin, pectins, rubbers and fructooligosaccharides) that capture a lot of water and are able to form viscous gels. It is very fermentable by intestinal microorganisms, so it produces a lot of gas in the intestine. Being very fermentable favors the creation of bacterial flora that makes 1/3 of the fecal volume, so this type of fiber also increases the volume of the feces and decreases its consistency. This type of fiber predominates in legumes, cereals (avena and barley) and some fruits. The soluble fiber, in addition to capturing water, is able to decrease and slow down the absorption of fats and sugars from food (glycemic index), which contributes to regular levels of cholesterol and glucose in blood.

In general, the fiber consumed should have a ratio of 3:1 between insoluble and soluble. It should always be advised that the sources of fiber are varied, that its ingestion is throughout the day and that an adequate water intake is carried out. Recommendations from various food agencies mention that adults should consume approximately 30-35 grams of dietary fiber per day. However, the inhabitants of some Western countries have an average consumption that is less than 12-18 g/day (considering a reference diet of 2000 kcal).^,^,. The ingestion of several types of foods rich in fiber is recommended, instead of just one.

Nutritional characteristics

Food fiber is resistant to digestion: inatacable by human ferments and digestive enzymes, so they cannot degrade it, contrary to the digestive system of ruminants and rodents, which has cellulose produced by eating bacteria.
Fiber has high absorption and water retention capacity, being an osmotically active substance. All fibers do so to a greater or lesser extent. Influencing many variables such as particle size, pH, media electrolytes. In the case of particle size it has been proven that the greater the particle size, the more water absorption capacity it has, very important to take into account the refinement of some foods such as flour.
Fixing organic and inorganic substances: the substances that kidnap the fiber can be simply caught between the networks that naturally generate fibers or bonds by means of links of very different types, which makes the possibility of escape of these substances minimal. Among them we find:

Proteins, glucids and fats that delay their absorption in the presence of fibers.
Bile salts: Fiber increases its removal by stools, with carcinogenic protective effect (although not scientifically proven yet), lowers biliary cholesterol and lithogenicity of bile and also decreases the absorption of fats as these bile conveyors and emulsifiers of ingested fats. It is also known that the fiber of bread and whole grains, such as rye, accelerates the intestinal transit and drags the carcinogenic substances that may be in the digestive tract by preventing stomach and colon cancer. By preventing cancer from being absorbed, it protects against cancer of the pancreas and breast to a greater extent.
Minerals such as calcium (Ca), zinc (Zn), magnesium (Mg), phosphorus (P), iron (Fe) and vitamins. By joining the dietary fiber, its absorption can also be reduced, although large amounts of fiber or patients that already had some type of deficit would be required to have clinical impact.

Fermentation in the large intestine by the colon bacteria. Fiber reaches the unaltered colon and there is attacked by bacterial enzymes. This reaction produces short-chain fatty acids that drop pH levels from 7 to 6 and raises temperature to 0.7 °C. Fermentation depends on the speed of intestinal transit and whether it is complete food or insulated fiber among other things. From the point of view of fermentation in large intestines fibers can be:

Little fermentable: fibers rich in cellulose and lignin that are quite resistant to bacterial degradation of the colon and are expelled by intact feces such as wheat bran. They are the ones we have previously called insoluble fibers.
Very fermentable: fibers rich in hemicelulous, arabinoxylan, glucuronic acid and pectins that are fermented and degraded by the colon's flora. It corresponds to those we have previously called soluble fibers.

Fiber and short-chain fatty acids Short-chain fatty acids are used by the intestinal mucosa or absorbed through the colonic wall towards the portal circulation (avoiding the entirehepatic circulation) and from there they are transported to the general circulation. Particularly butyric fatty acid (short chain) has extensive physiological actions, with favorable health effects, among which we have:

Stabilizes blood glucose (sugar) levels, acting on the pancreatic release of insulin and liver control of glucogenolisis.
It suppresses the synthesis of liver cholesterol and reduces levels of cholesterol and triglycerides, responsible for cardiovascular diseases (such as atherosclerosis)
Reduces the colonic pH, which prevents the formation of colonic polyps and increases the absorption of minerals.
It increases the proliferation of bacterial colonic flora (bifidobacteria and lactobacillus), which stimulates intestinal health.

Benefits of its intake

Although currently highly questioned by various researchers, the inclusion of foods rich in dietary fiber in the diet can prevent or alleviate different diseases such as:

Constipation: the best known effect of fiber is its ability to facilitate defecation. Fiber increases the volume of feces by creating solid residue and absorbing water which produces more voluminous and less consistent feces. In addition, intestinal transit time decreases, i.e., accelerates the evacuation process, increasing its frequency. Therefore, an adequate fiber content in food is essential to prevent and relieve constipation.^,
Diverticulosis or diverticular disease: disease characterized by the appearance of small bags on the walls of the colon in the form of glove finger called diverticulum. Divericulosis increases with age, as in older people the intestinal wall is weaker and the pressure exerted within the colon facilitates the creation of diverticules. The excessive pressure that has to exert the muscle layer of the colon wall by trying to remove the feces with little volume increases the pressure inside the colon and can contribute to the development of diverticular disease. Today it is accepted that diverticulosis is due to a greater deposit of elastin on the walls of the colon and a loss of the vagal inervation. Although diverticulosis has been predicted to be associated with poor food fiber diets, there is no scientific evidence to support its prevention through the consumption of food-rich foods.
Obesity: obesity is a disease that is associated with arterial hypertension, ischemic heart disease, diabetes mellitus and many types of cancer. Therefore, maintaining a proper body weight is a very healthy measure.^,Fiber-rich diets can help control obesity for several reasons: first, fiber-rich diets have less calories in the same volume of food; second, this type of diets facilitate the ingestion of less amount of food because they prolong the mastication time and by its volume, help to produce more quickly the sensation of savor, and finally, the diets rich in fiber 'secure'.
Colon and rectum cancer: Although in isolation a fiber-rich diet does not protect from colorectal cancer, the first observational epidemiological studies noted that populations who consumed fiber-rich diets had a lower incidence of colon cancer. However, these epidemiological studies on the protective effect of fiber against this type of cancer are contradictory, probably due to the diversity of components that form part of the food fiber. It has even been pointed out that the reverse relationship between fiber consumption and the development of colorectal adenomas, one of the precursors of colon cancer, is not significant. It is now accepted that the beneficial effect is in the diet in general: vegetable consumption (preferably fresh vegetables and fruits), reduced intake of fats and red meats, adequate contribution of micronutrients, etc.^,^,
Diabetes mellitus: an increase in in intake of food fiber, particularly insoluble, could improve the control of glucose, decreasing hyperinsulinemia and lipid plasma concentrations in type 2 diabetics.^,which would give an ideal profile for cardiovascular protection. However, while the inclusion of fiber-rich foods in the diabetic diet is recommended, scientific evidence supporting the prevention of type 2 diabetes is very weak through fiber-rich foods.
Hypercholesterolemia: fiber intake provides lower cholesterol absorption, leading to the prevention and treatment of conditions characterized by high blood cholesterol levels.

Adverse Effects of Fiber

Fiber fermentation by anaerobic bacteria in the colon can cause flatulence, bloating (bloating) and abdominal pain. It is recommended that fiber intake be done gradually so that the gastrointestinal tract adapts. Some cases of intestinal obstruction and the formation of phytobezoars have been described with the ingestion of high doses of non-fermentable fiber, especially when there is little water supply.

Uses in the diet

The inclusion of fresh fruits, vegetables and cereal derivatives (as long as they do not have refined flour - in what is called white bread), together with legumes, provides a good dose of dietary fiber in the diet. You can also resort to supplements or supplements and foods with a high fiber content that can be found on the market. The quality that the mixture of nutrients and the potentiation of different elements present in food supposes is much more beneficial than the pure fiber of the supplements, but in any case it is better to supplement the diet with some fiber than not to take it at all (as occurs with diets rich in protein from meat). Its main unwanted effect is flatulence and bloating, which can be uncomfortable in some people who suffer from it, but it usually subsides with continued intake of fiber. The ideal is to gradually accustom the intestinal tract to the appearance of fiber food and go ingesting amounts of liquid so that transit is possible.

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