Dietary fibre or sometimes roughage is the indigestible portion of plant foods having two main components:
- soluble ( prebiotic, viscous) fibre that is readily fermented in the colon into gases and physiologically active byproducts, and
- insoluble fibre that is metabolically inert, absorbing water throughout the digestive system and easing defecation.
It acts by changing the nature of the contents of the gastrointestinal tract, and by changing how other nutrients and chemicals are absorbed. Soluble fibre 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, although a major dietary insoluble fibre source, lignin, may alter the fate and metabolism of soluble fibers.
Chemically, dietary fibre 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. 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. The term "fibre" is something of a misnomer, since many types of so-called dietary fibre are not fibers at all.
Food sources of dietary fiber are often divided according to whether they provide (predominantly) soluble or insoluble fiber. Plant foods contain both types of fibre in varying degrees, according to the plant's characteristics.
Advantages of consuming fiber are the production of salubrious compounds during the fermentation of soluble fiber, and insoluble fibre's ability (via its passive hygroscopic properties) to increase bulk, soften stool and shorten transit time through the intestinal tract.
History of definition
Originally, fibre 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.
The main action of dietary fibre is to change the nature of the contents of the gastrointestinal tract, and to change how other nutrients and chemicals are absorbed. Soluble fibre binds to bile acids in the small intestine, making them less likely to enter the body; this in turn lowers cholesterol levels in the blood. Soluble fibre also attenuates the absorption of sugar, reduces sugar response after eating, normalizes blood lipid levels and, once fermented in the colon, produces short-chain fatty acids as byproducts with wide-ranging physiological activities (discussion below). Although insoluble fibre is associated with reduced diabetes risk, the mechanism by which this occurs is unknown.
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 fibre intake.
Effects of fibre intake
Research has shown that fibre may benefit health in several different ways. Lignin and probably related materials that are resistant to enzymatic degradation, diminish the nutritional value of foods.
Colour coding of table entries:
- Applies to both soluble and insoluble fibre
- Applies to soluble fibre only
- Applies to insoluble fibre only
Dietary fibre functions and benefits
|Adds bulk to your diet, making you feel full faster||May reduce appetite|
|Attracts water and turns to gel during digestion, trapping carbohydrates and slowing absorption of glucose||Lowers variance in blood sugar levels|
|Lowers total and LDL cholesterol||Reduces risk of heart disease|
|Regulates blood sugar||May reduce onset risk or symptoms of metabolic syndrome and diabetes|
|Speeds the passage of foods through the digestive system||Facilitates regularity|
|Adds bulk to the stool||Alleviates constipation|
|Balances intestinal pH and stimulates intestinal fermentation production of short-chain fatty acids||May reduce risk of colorectal cancer|
Fiber does not bind to minerals and vitamins and therefore does not restrict their absorption, but rather evidence exists that fermentable fibre sources improve absorption of minerals, especially calcium. 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 fibre.
Guidelines on fibre 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 fibre is only 12–18 grams.
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 fibre 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 fibre intake of 18 g/day for healthy adults.
Fibre recommendations in North America
On average, North Americans consume less than 50% of the dietary fibre 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.
Recognizing the growing scientific evidence for physiological benefits of increased fibre 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 fibre.
In clinical trials to date, these fibre sources were shown to significantly reduce blood cholesterol levels, an important factor for general cardiovascular health, and to lower risk of onset for some types of cancer.
Soluble (fermentable) fibre sources gaining FDA approval are:
- Psyllium seed husk (7 grams per day)
- Beta-glucan from oat bran, whole oats, oatrim, or rolled oats (3 grams per day)
- Beta-glucan from whole grain or dry-milled barley (3 grams per day)
Other examples of fermentable fibre 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 fibre 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— 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 fibre can provide healthful benefits.
Insufficient fibre in the diet can complicate defecation. Low-fibre feces are dehydrated and hardened, making them difficult to evacuate—defining constipation and possibly leading to development of hemorrhoids or anal fissures.
Although many researchers believe that dietary fibre 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 fibre consumption and lower rates of colorectal cancer or adenomas.
Fibre recommendations in the UK
In June 2007, the British Nutrition Foundation issued a statement to define dietary fibre more concisely and list the potential health benefits established to date:
‘Dietary fiber’ 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 fibre’ by nature of its indigestibility results in many other indigestible components being isolated along with the carbohydrate components of dietary fibre. 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 fibre 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 fibre physiologically allows recognition of indigestible carbohydrates with structures and physiological properties similar to those of naturally occurring dietary fibers.
- helps prevent constipation
- reduces the risk of colon cancer
- improvements in gastrointestinal health
- improvements in glucose tolerance and the insulin response
- reduction of hyperlipidemia, hypertension, and other coronary heart disease risk factors
- reduction in the risk of developing some cancers
- increased satiety and hence some degree of weight management
Fibre and calories
Calories or kilojoules (as used on nutrition labels) are intended to be a measure of how much energy is available from the food source. This energy can be used immediately, for example allowing the body to move during exercise, or to make the heart beat. Energy that is not used immediately is stored as sugars in the short term and later converted to fats, which act as energy reserves.
Energy is extracted from food in a chemical reaction. Because of the principle of conservation of energy, energy can only be extracted when the chemical structure of food particles is changed. Since insoluble fibre particles do not change inside the body, the body should not absorb any energy (or Calories/kilojoules) from them.
Because soluble fiber is changed during fermentation, it could provide energy (Calories/kilojoules) to the body. As of 2009 nutritionists 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 fibre.
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/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 fibre may be (and usually is) treated as 0 Calories per gram and not mentioned on the label.
Soluble fibre fermentation
The American Association of Cereal Chemists has defined soluble fibre 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.” In this definition:
- edible parts of plants
- indicates that some parts of a plant we eat—skin, pulp, seeds, stems, leaves, roots—contain fibre. Both insoluble and soluble sources are in those plant components.
- complex carbohydrates, such as long-chained sugars also called starch, oligosaccharides, or polysaccharides, are sources of soluble fermentable fibre.
- 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 fibre).
- 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.
As an example of fermentation, shorter-chain carbohydrates (a type of fibre 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, fibers compounds with partial or low fermentability include:
- cellulose, a polysaccharide
- hemicellulose, a polysaccharide
- lignans, a group of phytoestrogens
- plant waxes
- resistant starches
Fibre compounds with high fermentability include:
- beta-glucans, a group of polysaccharides
- pectins, a group of heteropolysaccharides
- natural gums, a group of polysaccharides
- inulins, a group of polysaccharides
- oligosaccharides, a group of short-chained or simple sugars
- resistant dextrins
Short-chain fatty acids
When soluble fibre is fermented, short-chain fatty acids (SCFA) are produced. SCFA are involved in numerous physiological processes promoting health, including:
- stabilize blood glucose levels by acting on pancreatic insulin release and liver control of glycogen breakdown
- stimulate gene expression of glucose transporters in the intestinal mucosa, regulating glucose absorption
- provide nourishment of colonocytes, particularly by the SCFA butyrate
- suppress cholesterol synthesis by the liver and reduce blood levels of LDL cholesterol and triglycerides responsible for atherosclerosis
- lower colonic pH (i.e., raises the acidity level in the colon) which protects the lining from formation of colonic polyps and increases absorption of dietary minerals
- stimulate production of T helper cells, antibodies, leukocytes, cytokines, and lymph mechanisms having crucial roles in immune protection
- improve barrier properties of the colonic mucosal layer, inhibiting inflammatory and adhesion irritants, contributing to immune functions
SCFA 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, SCFA affect major regulatory systems, such as blood glucose and lipid levels, the colonic environment, and intestinal immune functions.
The major SCFA 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 fibre or 0.75g of oat or barley soluble fibre as beta-glucans to claim that reduced risk of heart disease can result from their regular consumption.
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 fibre source] necessary per day to have this effect.
Eligible sources of soluble fibre providing beta-glucan include:
- Oat bran
- Rolled oats
- Whole oat flour
- Whole grain barley and dry milled barley
- Soluble fibre 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 fibre 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 fibre from sources listed above associated with reduced risk of coronary heart disease are:
- 3g or more per day of beta-glucan soluble fibre from either whole oats or barley, or a combination of whole oats and barley
- 7g or more per day of soluble fibre from psyllium seed husk.
Soluble fibre 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).