Dietary fiber
Dietary fiber (fibre in Commonwealth English) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes.[1] Dietary fibers are diverse in chemical composition and can be grouped generally by their solubility, viscosity and fermentability which affect how fibers are processed in the body.[2] Dietary fiber has two main components: soluble fiber and insoluble fiber which are components of plant-based foods such as legumes, whole grains, cereals, vegetables, fruits, and nuts or seeds.[2][3] A diet high in regular fiber consumption is generally associated with supporting health and lowering the risk of several diseases.[2][4] Dietary fiber consists of non-starch polysaccharides and other plant components such as cellulose, resistant starch, resistant dextrins, inulin, lignins, chitins, pectins, beta-glucans, and oligosaccharides.[1][2][3]
Food sources of dietary fiber have traditionally been divided according to whether they provide soluble or insoluble fiber. Plant foods contain both types of fiber in varying amounts according to the fiber characteristics of viscosity and fermentability.[1][5] Advantages of consuming fiber depend upon which type of fiber is consumed and which benefits may result in the gastrointestinal system.[6] Bulking fibers – such as cellulose and hemicellulose (including psyllium) – absorb and hold water, promoting bowel movement regularity.[7] Viscous fibers – such as beta-glucan and psyllium – thicken the fecal mass.[7] Fermentable fibers – such as resistant starch, xanthan gum, and inulin – feed the bacteria and microbiota of the large intestine and are metabolized to yield short-chain fatty acids, which have diverse roles in gastrointestinal health.[8][9][10]
Soluble fiber (fermentable fiber or prebiotic fiber) – which dissolves in water – is generally fermented in the colon into gases and physiologically active by-products such as short-chain fatty acids produced in the colon by gut bacteria. Examples are beta-glucans (in oats, barley, and mushrooms) and raw guar gum. Psyllium – a soluble, viscous, non- fermented fiber – is a bulking fiber that retains water as it moves through the digestive system, easing defecation. Soluble fiber is generally viscous and delays gastric emptying which in humans can result in an extended feeling of fullness.[2] Inulin (in chicory root), wheat dextrin, oligosaccharides, and resistant starches[11] (in legumes and bananas) are soluble non-viscous fibers.[2] Regular intake of soluble fibers such as beta-glucans from oats or barley has been established to lower blood levels of LDL cholesterol.[2][4][12] Soluble fiber supplements also significantly lower LDL cholesterol.[13][14][15]
Insoluble fiber – which does not dissolve in water – is inert to digestive enzymes in the upper gastrointestinal tract. Examples are wheat bran, cellulose, and lignin. Coarsely ground insoluble fiber triggers the secretion of mucus in the large intestine providing bulking. Finely ground insoluble fiber does not have this effect and can actually have a constipating effect.[2] Some forms of insoluble fiber, such as resistant starches, can be fermented in the colon.[16]
Definition[edit]
Dietary fiber is defined to be plant components that are not broken down by human digestive enzymes.[1] In the late 20th century, only lignin and some polysaccharides were known to satisfy this definition, but in the early 21st century, resistant starch and oligosaccharides were included as dietary fiber components.[1][17] The most accepted definition of dietary fiber is "all polysaccharides and lignin, which are not digested by the endogenous secretion of the human digestive tract".[18] Currently, most animal nutritionists are using either a physiological definition, "the dietary components resistant to degradation by mammalian enzymes", or a chemical definition, "the sum of non-starch polysaccharides (NSP) and lignin".[18]
Dietary recommendations[edit]
European Union[edit]
According to the European Food Safety Authority (EFSA) Panel on Nutrition, Novel Foods and Food Allergens (NDA), which deals with the establishment of Dietary Reference Values for carbohydrates and dietary fibre, "based on the available evidence on bowel function, the Panel considers dietary fibre intakes of 25 g per day to be adequate for normal laxation in adults".[88][89]
United States[edit]
Current recommendations from the United States National Academy of Medicine (NAM) (formerly Institute of Medicine) of the National Academy of Sciences state that for Adequate Intake, adult men ages 19–50 consume 38 grams of dietary fiber per day, men 51 and older 30 grams, women ages 19–50 to consume 25 grams per day, women 51 and older 21 grams. These are based on three studies observing that people in the highest quintile of fiber intake consumed a median of 14 grams of fiber per 1,000 Calories and had the lowest risk of coronary heart disease, especially for those who ate more cereal fiber.[2][90][3]
The United States Academy of Nutrition and Dietetics (AND, previously ADA) reiterates the recommendations of the NAM.[91] A 1995 research team'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).[92][93] The NAM's current recommendation for children is 19 g/day for age 1–3 years and 25 g/day for age 4–8 years.[2] 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.
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.[94] 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. The FDA classifies which ingredients qualify as being "fiber", and requires for product labeling that a physiological benefit is gained by adding the fiber ingredient.[95] As of 2008, the FDA approved health claims for qualified fiber products to display labeling that regular consumption may reduce blood cholesterol levels – which can lower the risk of coronary heart disease[96] – and also reduce the risk of some types of cancer.[97]
Viscous fiber sources gaining FDA approval are:[2]
The Cereals & Grains Association 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."[103]
In this definition, "edible parts of plants" indicates that some parts of a plant that are eaten—skin, pulp, seeds, stems, leaves, roots—contain fiber. Both insoluble and soluble sources are in those plant components. "Carbohydrates" refers to complex carbohydrates, such as long-chained sugars also called starch, oligo saccharides, or poly saccharides, which are sources of soluble fermentable fiber. "Resistant to digestion and absorption in the human small intestine" refers to compounds that are not digested by gastric acid and digestive enzymes in the stomach and small intestine, preventing the digesting animal from utilizing the compounds for energy. 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" describes the digestive processes of the large intestine, which comprises a segment called the colon within which additional nutrient absorption occurs through the process of fermentation. Fermentation occurs through the action of colonic bacteria on the food mass, producing gases and short-chain fatty acids. These short-chain fatty acids have been shown to have significant health properties.[104] They include butyric, acetic (ethanoic), propionic, and valeric acids.
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,[98]
fiber compounds with partial or low fermentability include:
fiber compounds with high fermentability include:
When fermentable fiber is fermented, short-chain fatty acids (SCFA) are produced.[18] SCFAs are involved in numerous physiological processes promoting health, including:[104]
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.[106][107]
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.
The United States FDA allows manufacturers of foods containing 1.7 g per serving of psyllium husk soluble fiber or 0.75 g of oat or barley soluble fiber as beta-glucans to claim that regular consumption may reduce the risk of heart disease.[12]
The FDA statement template for making this claim is:
Eligible sources of soluble fiber providing beta-glucan include:
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).[12]
In December 2016, FDA approved a qualified health claim that consuming resistant starch from high-amylose corn may reduce the risk of type 2 diabetes due to its effect of increasing insulin sensitivity. The allowed claim specified: "High-amylose maize resistant starch may reduce the risk of type 2 diabetes. FDA has concluded that there is limited scientific evidence for this claim."[109] In 2018, the FDA released further guidance on the labeling of isolated or synthetic dietary fiber to clarify how different types of dietary fiber should be classified.[110]