View source for Dietary fiber

{{short description|Portion of plant-derived food that cannot be completely digested}}
{{use dmy dates|date=September 2022}}
{{use American English|date=December 2020}}
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[[File:Fruit, Vegetables and Grain NCI Visuals Online.jpg|thumb|Foods rich in fibers: fruits, vegetables and grains]]
[[File:WheatBran.jpg|thumb|[[Wheat]] [[bran]] has a high content of dietary fiber.]]

'''Dietary fiber,''' '''fibre,''' or '''roughage''' is the portion of plant-derived [[food]] that cannot be completely broken down by human [[digestive enzyme]]s.<ref name="lpi">{{cite web|url=https://lpi.oregonstate.edu/mic/other-nutrients/fiber|title=Fiber|publisher=Micronutrient Information Center, Linus Pauling Institute, Oregon State University|date=March 2019|access-date=20 March 2025}}</ref><ref name=bnf>{{cite web |title=Dietary fibre |url=https://www.nutrition.org.uk/nutritionscience/nutrients-food-and-ingredients/dietary-fibre.html?limitstart=0 |publisher=British Nutrition Foundation |access-date=26 July 2018 |date=2018 |archive-url=https://web.archive.org/web/20180726203523/https://www.nutrition.org.uk/nutritionscience/nutrients-food-and-ingredients/dietary-fibre.html?limitstart=0 |archive-date=26 July 2018 }}</ref> Dietary fibers are diverse in chemical composition and can be grouped generally by their [[solubility]], [[viscosity]] and [[Fermentation#Biological role|fermentability]] which affect how fibers are processed in the body.<ref name="lpi" /> Dietary fiber has two main subtypes: soluble fiber and insoluble fiber which are components of plant-based foods such as [[legume]]s, [[whole grain]]s, [[cereal]]s, [[vegetable]]s, [[fruit]]s, and [[nut (fruit)|nuts]] or [[seed]]s.<ref name=lpi/><ref name=USDA-IOM>{{cite book| title = Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids (2005), Chapter 7: Dietary, Functional and Total Fiber| publisher = US Department of Agriculture, National Agricultural Library and National Academy of Sciences, Institute of Medicine, Food and Nutrition Board| url = https://archive.org/details/isbn_9780309085250| doi = 10.17226/10490| year = 2005| isbn = 978-0-309-08525-0| url-access = registration}}</ref> A diet high in regular fiber consumption is generally associated with supporting health and lowering the risk of several diseases.<ref name=lpi/><ref name="veronese">{{cite journal | vauthors = Veronese N, Solmi M, Caruso MG, Giannelli G, Osella AR, Evangelou E, Maggi S, Fontana L, Stubbs B, Tzoulaki I  | title = Dietary fiber and health outcomes: an umbrella review of systematic reviews and meta-analyses | journal = The American Journal of Clinical Nutrition | volume = 107 | issue = 3 | pages = 436–444 | date = March 2018 | pmid = 29566200 | doi = 10.1093/ajcn/nqx082 | doi-access = free }}</ref> Dietary fiber consists of non-[[starch]] [[polysaccharide]]s and other plant components such as [[cellulose]], [[resistant starch]], resistant [[dextrin]]s, [[inulin]]s, [[lignin]]s, [[chitin]]s, [[pectin]]s, [[beta-glucan]]s, and [[oligosaccharide]]s.<ref name=bnf/><ref name=lpi/><ref name=USDA-IOM/>

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.<ref name="bnf" /><ref>{{cite book|url=http://www.nap.edu/catalog/10161/dietary-reference-intakes-proposed-definition-of-dietary-fiber|title=Dietary Reference Intakes, Proposed Definition of Dietary Fiber|date=2001|publisher=Institute of Medicine Press|isbn=978-0-309-07564-0|location=Washington, D.C.|page=25|doi=10.17226/10161 |pmid=25057569 }}</ref> Advantages of consuming fiber depend upon which type is consumed.<ref>{{cite book|title=Present Knowledge in Nutrition|vauthors=Gallaher DD|date=2006|publisher=ILSI Press|isbn=978-1-57881-199-1|edition=9th|location=Washington, D.C.|pages=102–110|chapter=8}}</ref> Bulking fibers &ndash; such as cellulose and [[hemicellulose]] (including [[psyllium]]) &ndash; absorb and hold water, promoting bowel movement regularity.<ref name="IOM">{{cite book|last1=Institute of Medicine|url=https://www.nap.edu/catalog/10161/dietary-reference-intakes-proposed-definition-of-dietary-fiber|title=Dietary Reference Intakes: Proposed Definition of Dietary Fiber|date=2001|publisher=National Academy Press|isbn=978-0-309-07564-0|location=Washington, D.C.|page=19}}</ref> Viscous fibers &ndash; such as beta-glucan and psyllium &ndash; thicken the fecal mass.<ref name="IOM" /> Fermentable fibers &ndash; such as resistant starch, [[xanthan gum]], and inulin &ndash; feed the bacteria and [[Gut microbiota|microbiota]] of the [[large intestine]] and are [[metabolized]] to yield short-chain fatty acids, which have diverse roles in gastrointestinal health.<ref>{{cite journal|vauthors=Bedford A, Gong J|date=June 2018|title=Implications of butyrate and its derivatives for gut health and animal production|journal=Animal Nutrition|volume=4|issue=2|pages=151–9|doi=10.1016/j.aninu.2017.08.010|pmc=6104520|pmid=30140754}}</ref><ref name="Cummings">{{cite book|url=https://www.crcpress.com/CRC-Handbook-of-Dietary-Fiber-in-Human-Nutrition-Third-Edition/Spiller/p/book/9780849323874|title=The Effect of Dietary Fiber on Fecal Weight and Composition|vauthors=Cummings JH|date=2001|publisher=CRC Press|isbn=978-0-8493-2387-4|edition=3rd|page=184}}</ref><ref>{{cite journal |last1=Ostrowski |first1=Matthew P. |last2=La Rosa |first2=Sabina Leanti |last3=Kunath |first3=Benoit J. |last4=Robertson |first4=Andrew |last5=Pereira |first5=Gabriel |last6=Hagen |first6=Live H. |last7=Varghese |first7=Neha J. |last8=Qiu |first8=Ling |last9=Yao |first9=Tianming |last10=Flint |first10=Gabrielle |last11=Li |first11=James |last12=McDonald |first12=Sean P. |last13=Buttner |first13=Duna |last14=Pudlo |first14=Nicholas A. |last15=Schnizlein |first15=Matthew K. |last16=Young |first16=Vincent B. |last17=Brumer |first17=Harry |last18=Schmidt |first18=Thomas M. |last19=Terrapon |first19=Nicolas |last20=Lombard |first20=Vincent |last21=Henrissat |first21=Bernard |last22=Hamaker |first22=Bruce |last23=Eloe-Fadrosh |first23=Emiley A. |last24=Tripathi |first24=Ashootosh |last25=Pope |first25=Phillip B. |last26=Martens |first26=Eric C. |title=Mechanistic insights into consumption of the food additive xanthan gum by the human gut microbiota |journal=Nature Microbiology |date=April 2022 |volume=7 |issue=4 |pages=556–569 |doi=10.1038/s41564-022-01093-0|pmid=35365790 |pmc=11537241 |s2cid=247866305 |hdl=11250/3003739 |hdl-access=free }}</ref>

Soluble fiber (''fermentable fiber'' or ''prebiotic fiber'') &ndash; which dissolves in water &ndash; is generally fermented in the [[colon (anatomy)|colon]] into gases and physiologically active [[by-product]]s such as [[short-chain fatty acid]]s produced in the colon by gut bacteria. Examples are beta-glucans (in oats, barley, and mushrooms) and raw [[guar gum]]. Psyllium {{ndash}} soluble, viscous, and non-fermented fiber {{ndash}} is a bulking fiber that retains water as it moves through the [[digestive system]], easing [[defecation]]. Soluble fiber is generally viscous and delays [[Stomach#Function|gastric emptying]] which in humans can result in an extended feeling of fullness.<ref name=lpi/> Inulin (in [[chicory]] root), [[wheat dextrin]], oligosaccharides, and resistant starches<ref name=keenan>{{cite journal | vauthors = Keenan MJ, Zhou J, Hegsted M, Pelkman C, Durham HA, Coulon DB, Martin RJ | title = Role of resistant starch in improving gut health, adiposity, and insulin resistance | journal = Advances in Nutrition | volume = 6 | issue = 2 | pages = 198–205 | date = March 2015 | pmid = 25770258 | pmc = 4352178 | doi = 10.3945/an.114.007419 }}</ref> (in legumes and bananas) are soluble non-viscous fibers.<ref name=lpi/> Regular intake of soluble fibers such as beta-glucans from oats or [[barley]] has been established to lower blood levels of [[low-density lipoprotein|LDL]] [[cholesterol]].<ref name=lpi/><ref name=veronese/><ref name=FDAc/> Soluble fiber supplements also significantly lower LDL cholesterol.<ref>{{cite journal |last1=Jovanovski |first1=Elena |last2=Yashpal |first2=Shahen |last3=Komishon |first3=Allison |last4=Zurbau |first4=Andreea |last5=Blanco Mejia |first5=Sonia |last6=Ho |first6=Hoang Vi Thanh |last7=Li |first7=Dandan |last8=Sievenpiper |first8=John |last9=Duvnjak |first9=Lea |last10=Vuksan |first10=Vladimir |title=Effect of psyllium (Plantago ovata) fiber on LDL cholesterol and alternative lipid targets, non-HDL cholesterol and apolipoprotein B: a systematic review and meta-analysis of randomized controlled trials |journal=The American Journal of Clinical Nutrition |date=1 November 2018 |volume=108 |issue=5 |pages=922–932 |doi=10.1093/ajcn/nqy115 |pmid=30239559 |issn=1938-3207|doi-access=free }}</ref><ref>{{cite journal |last1=Ho |first1=Hoang Vi Thanh |last2=Jovanovski |first2=Elena |last3=Zurbau |first3=Andreea |last4=Blanco Mejia |first4=Sonia |last5=Sievenpiper |first5=John L. |last6=Au-Yeung |first6=Fei |last7=Jenkins |first7=Alexandra L. |last8=Duvnjak |first8=Lea |last9=Leiter |first9=Lawrence |last10=Vuksan |first10=Vladimir |title=A systematic review and meta-analysis of randomized controlled trials of the effect of konjac glucomannan, a viscous soluble fiber, on LDL cholesterol and the new lipid targets non-HDL cholesterol and apolipoprotein B |journal=The American Journal of Clinical Nutrition |date=May 2017 |volume=105 |issue=5 |pages=1239–47 |doi=10.3945/ajcn.116.142158 |pmid=28356275 |issn=1938-3207|doi-access=free }}</ref><ref>{{cite journal |last1=Ghavami |first1=Abed |last2=Ziaei |first2=Rahele |last3=Talebi |first3=Sepide |last4=Barghchi |first4=Hanieh |last5=Nattagh-Eshtivani |first5=Elyas |last6=Moradi |first6=Sajjad |last7=Rahbarinejad |first7=Pegah |last8=Mohammadi |first8=Hamed |last9=Ghasemi-Tehrani |first9=Hatav |last10=Marx |first10=Wolfgang |last11=Askari |first11=Gholamreza |title=Soluble Fiber Supplementation and Serum Lipid Profile: A Systematic Review and Dose-Response Meta-Analysis of Randomized Controlled Trials |journal=Advances in Nutrition |date=1 May 2023 |volume=14 |issue=3 |pages=465–474 |doi=10.1016/j.advnut.2023.01.005 |pmid=36796439 |pmc=10201678 |issn=2161-8313|doi-access=free }}</ref>

Insoluble fiber &ndash; which does not dissolve in water &ndash; 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. However, finely ground insoluble fiber does not have this effect and instead can cause a constipation.<ref name=lpi/> Some forms of insoluble fiber, such as resistant starches, can be fermented in the colon.<ref name=lockyer>{{cite journal|doi=10.1111/nbu.12244|title=Health effects of resistant starch|journal=Nutrition Bulletin|volume=42|pages=10–41|year=2017| vauthors = Lockyer S, Nugent AP |doi-access=free}}</ref>

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==Definition==
Dietary fiber is defined to be plant components that are not broken down by human digestive enzymes.<ref name=bnf/> In the late 20th century, only [[lignin]] and some [[polysaccharide]]s were known to satisfy this definition, but in the early 21st century, [[resistant starch]] and [[oligosaccharide]]s were included as dietary fiber components.<ref name=bnf/><ref name=Anderson>{{cite journal | vauthors = Anderson JW, Baird P, Davis RH, Ferreri S, Knudtson M, Koraym A, Waters V, Williams CL  | title = Health benefits of dietary fiber | journal = Nutrition Reviews | volume = 67 | issue = 4 | pages = 188–205 | date = April 2009 | pmid = 19335713 | doi = 10.1111/j.1753-4887.2009.00189.x | s2cid = 11762029 | url = https://cloudfront.escholarship.org/dist/prd/content/qt8d0680bq/qt8d0680bq.pdf }}</ref> 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".<ref name=":0"/> 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".<ref name=":0">{{cite journal | vauthors = Jha R, Mishra P | title = Dietary fiber in poultry nutrition and their effects on nutrient utilization, performance, gut health, and on the environment: a review | journal = Journal of Animal Science and Biotechnology | volume = 12 | issue = 1 | page = 51 | date = April 2021 | pmid = 33866972 | pmc = 8054369 | doi = 10.1186/s40104-021-00576-0 | doi-access = free }}</ref>

==Types and sources==
{{More citations needed section|date=February 2021}}

{| class="wikitable"
|+ water-insoluble dietary fibers
|- style="background:#B3B7FF;"
! scope="col" | Nutrient 
! scope="col" | [[Food additive]] 
! scope="col" | Source/Comments
|- style="background:#E0E0E0;"
|align="left" colspan=3 | [[Beta-glucan|β-glucans]] (a few of which are water-soluble)
|-
|align="left" |&nbsp;&nbsp;&nbsp;[[Cellulose]] || E&nbsp;460 || align="left" | cereals, fruit, vegetables (in all plants in general)
|-
|align="left" |&nbsp;&nbsp;&nbsp;[[Chitin]] || — || align="left" | in [[Fungus|fungi]], exoskeleton of [[Entomophagy|insects]] and [[crustacean]]s
|-
|- style="background:#E0E0E0;"
|align="left" |[[Hemicellulose]] ||  || align="left" | cereals, [[bran]], [[timber]], legumes
|-
|align="left" |&nbsp;&nbsp;&nbsp;[[Hexose]]s || — || align="left" | [[wheat]], [[barley]]
|-
|align="left" |&nbsp;&nbsp;&nbsp;[[Pentose]] || — || align="left" | [[rye]], [[oat]]
|- style="background:#E0E0E0;"
|align="left" |[[Lignin]] || — || align="left" | [[Drupe|stones]] of fruits, vegetables (filaments of the [[garden bean]]), cereals
|- style="background:#E0E0E0;"
|align="left" |[[Xanthan gum]] || E&nbsp;415 || align="left" | production with [[Xanthomonas]]-bacteria from sugar substrates
|-
|- style="background:#E0E0E0;"
|align="left" |[[Resistant starch]] ||  || align="left" | Can be starch protected by seed or shell (type RS1), granular starch (type RS2) or retrograded starch (type RS3)<ref name=lockyer/>
|-
|align="left" |&nbsp;&nbsp;&nbsp;[[Resistant starch]] || — || align="left" | high amylose corn, [[barley]], high amylose wheat, legumes, raw bananas, cooked and cooled pasta and potatoes<ref name=lockyer/>
|}

{| class="wikitable"
|+ water-soluble dietary fibers
! scope="col" | Nutrient
! scope="col" | [[Food additive]]
! scope="col" | Source/Comments
|- style="background:#E0E0E0;"
|align="left" |[[Arabinoxylan]] (a [[hemicellulose]])|| — || align="left" | [[psyllium]]<ref>{{cite journal | vauthors = Fischer MH, Yu N, Gray GR, Ralph J, Anderson L, Marlett JA | title = The gel-forming polysaccharide of psyllium husk (Plantago ovata Forsk) | journal = Carbohydrate Research | volume = 339 | issue = 11 | pages = 2009–17 | date = August 2004 | pmid = 15261594 | doi = 10.1016/j.carres.2004.05.023 }}</ref>
|- style="background:#E0E0E0;"
|align="left" |[[Fructan]]s ||  || align="left" |  replace or complement in some [[plant]] taxa the [[starch]] as storage carbohydrate
|-
|align="left" |&nbsp;&nbsp;&nbsp;[[Inulin]] || — || align="left" | in diverse plants, e.g. [[topinambour]], [[chicory]], etc.
|-
|- style="background:#E0E0E0;"
|align="left" colspan=3 |[[Polyuronide]]
|-
|align="left" |&nbsp;&nbsp;&nbsp;[[Pectin]] || E&nbsp;440 || align="left" | in the fruit skin (mainly [[apple]]s, [[quince]]s), vegetables
|-
|align="left" |&nbsp;&nbsp;&nbsp;[[Alginic acid]]s (Alginates) || E&nbsp;400–E&nbsp;407 || align="left" | in [[Algae]]
|-
|align="left" |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[[Sodium alginate]] || E&nbsp;401
|-
|align="left" |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[[Potassium alginate]] || E&nbsp;402
|-
|align="left" |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[[Ammonium alginate]]|| E&nbsp;403
|-
|align="left" |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[[Calcium alginate]] || E&nbsp;404
|-
|align="left" |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[[Propylene glycol alginate]] (PGA) || E&nbsp;405
|-
|align="left" |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[[agar]] || E&nbsp;406
|-
|align="left" |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[[carrageenan]] || E&nbsp;407 || align="left" | [[red algae]]
|-
|- style="background:#E0E0E0;"
|align="left" |[[Raffinose]] || — || align="left" | [[legume]]s
|- style="background:#E0E0E0;"
|align="left" |[[Polydextrose]] || E&nbsp;1200 || align="left" | synthetic polymer, c. 1 kcal/g
|-
|}

===Contents in food===
[[File:Kids ‘n Fiber (6121371164).jpg|thumb|Children eating fiber-rich food]]

Dietary fiber is found in fruits, vegetables and [[whole grains]]. The amounts of fiber contained in common foods are listed in the following table:<ref name="usda28">{{cite web |date=2015|title=Search, USDA Food Composition Databases|url=https://fdc.nal.usda.gov/index.html|url-status=live|archive-url=https://web.archive.org/web/20190422152836/https://fdc.nal.usda.gov/index.html |archive-date=22 April 2019 |access-date=18 November 2017|website=|publisher=Nutrient Data Laboratory. USDA National Nutrient Database, US Department of Agriculture, Standard Release 28}}</ref>

{| class="wikitable sortable"
|+ {{Screen reader-only|Fiber content of some common food}}
|-
! scope="col" | Food group || scope="col" | Serving mean || scope="col" | Fibermass per serving
|-
| Fruit || 120&nbsp;[[:millilitre|mL]] (0.5 cup)<ref>[http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&rgn=div8&view=text&node=21:2.0.1.1.2.1.1.6&idno=21 U.S. Government Printing Office—Electronic Code of Federal Regulations] {{webarchive|url=https://web.archive.org/web/20090813113845/http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=77734a162c4f7ddd997233b4d623c029&rgn=div8&view=text&node=21%3A2.0.1.1.2.1.1.6&idno=21 |date=13 August 2009 }}</ref><ref>[https://web.archive.org/web/20140801062026/http://www.fda.gov/Food/GuidanceRegulation/GuidanceDocumentsRegulatoryInformation/ucm063102.htm U.S. Food and Drug Administration—Guidelines for Determining Metric Equivalents of Household Measures]</ref> || 1.1 [[Gram|g]]
|-
| Dark green vegetables || 120&nbsp;mL (0.5 cup) || 6.4 g
|-
| Orange vegetables || 120&nbsp;mL (0.5 cup) || 2.1 g
|-
| Cooked dry beans (legumes) || 120&nbsp;mL (0.5 cup) || 8.0 g
|-
| Starchy vegetables || 120&nbsp;mL (0.5 cup) || 1.7 g
|-
| Other vegetables || 120&nbsp;mL (0.5 cup) || 1.1 g
|-
| Whole grains || 28 g (1&nbsp;oz) || 2.4 g
|-
| Meat || 28 g (1&nbsp;oz) || 0.1 g
|}

Dietary fiber is found in plants, typically eaten whole, raw or cooked, although fiber can be added to make [[dietary supplement]]s and fiber-rich [[processed foods]]. Grain bran products have the highest fiber contents, such as crude corn bran (79 g per 100 g) and crude wheat bran (43 g per 100 g), which are ingredients for manufactured foods.<ref name=usda28/> Medical authorities, such as the [[Mayo Clinic]], recommend adding fiber-rich products to the [[Standard American Diet]] because fiber intake is lower on such a diet that's rich in processed and artificially sweetened foods with minimal intake of vegetables and legumes.<ref>{{cite journal | title = Benefits and Harms of the Mediterranean Diet Compared to Other Diets | journal = VA Evidence-based Synthesis Program Reports | date = November 2015 | pmid = 27559560 | url = https://www.ncbi.nlm.nih.gov/books/NBK379574/pdf/Bookshelf_NBK379574.pdf | last1 = Bloomfield | first1 = H. E. | last2 = Kane | first2 = R. | last3 = Koeller | first3 = E. | last4 = Greer | first4 = N. | last5 = MacDonald | first5 = R. | last6 = Wilt | first6 = T. }}</ref><ref>{{cite web|title=Nutrition and healthy eating: Fiber|url=https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/fiber/art-20043983?pg=2|publisher=Mayo Clinic|access-date=18 November 2017|date=2017}}</ref>

===Plant sources===
Some plants contain significant amounts of soluble and insoluble fiber. For example, [[plum]]s and [[prune]]s have a thick skin covering a juicy pulp. The skin is a source of insoluble fiber, whereas soluble fiber is in the pulp. Grapes also contain a fair amount of fiber.<ref>{{cite journal | vauthors = Stacewicz-Sapuntzakis M, Bowen PE, Hussain EA, Damayanti-Wood BI, Farnsworth NR | title = Chemical composition and potential health effects of prunes: a functional food? | journal = Critical Reviews in Food Science and Nutrition | volume = 41 | issue = 4 | pages = 251–86 | date = May 2001 | pmid = 11401245 | doi = 10.1080/20014091091814 | s2cid = 31159565 }}</ref>

==== Soluble fiber ====
Found in varying quantities in all plant foods, including:
* [[legume]]s ([[pea]]s, [[soybean]]s, [[Lupin bean|lupins]] and other [[bean]]s)
* [[oat]]s, [[rye]], [[Salvia hispanica|chia]], and [[barley]]
* some [[fruit]]s (including [[ficus|figs]], [[avocado]]s, [[plum]]s, [[prune]]s, [[berry|berries]], ripe [[banana]]s, and the skin of [[apple]]s, [[quince]]s and [[pear]]s)
* certain [[vegetable]]s such as [[broccoli]], [[carrot]]s, and [[Jerusalem artichoke]]s
* [[root tuber]]s and [[root vegetable]]s such as [[sweet potato]]es and [[onion]]s (skins of these are sources of insoluble fiber also)
* [[psyllium seed husks]] (a [[mucilage]] soluble fiber) and [[Flax#Flax seeds|flax seeds]]
* [[nut (fruit)|nuts]], with [[almonds]] being the highest in dietary fiber

==== Insoluble fiber ====
Sources include:

* [[whole grain]] foods
* [[wheat]] and [[Maize|corn]] [[bran]]
* [[legume]]s such as beans and peas
* nuts and [[seed]]s
* [[potato]] skins
* [[lignan]]s
* vegetables such as [[green bean]]s, [[cauliflower]], [[zucchini]] (courgette), [[celery]], and [[nopal]]
* some fruits including [[avocado]], and unripe [[banana]]s
* the skins of some fruits, including [[kiwifruit]], [[grape]]s and [[tomato]]es<ref>{{cite journal | vauthors = Alvarado A, Pacheco-Delahaye E, Hevia P | title = Value of a tomato byproduct as a source of dietary fiber in rats | journal = Plant Foods for Human Nutrition | volume = 56 | issue = 4 | pages = 335–48 | year = 2001 | pmid = 11678439 | doi = 10.1023/A:1011855316778 | url = http://www.kluweronline.com/art.pdf?issn=0921-9668&volume=56&page=335 | s2cid = 21835355 }}</ref>

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

==== Soluble fiber ====
Soluble fiber supplements may be beneficial for alleviating symptoms of [[irritable bowel syndrome]], such as [[diarrhea]] or [[constipation]] and abdominal discomfort.<ref>{{cite journal | vauthors = Friedman G | title = Nutritional therapy of irritable bowel syndrome | journal = Gastroenterology Clinics of North America | volume = 18 | issue = 3 | pages = 513–24 | date = September 1989 | doi = 10.1016/S0889-8553(21)00639-7 | pmid = 2553606 }}</ref> [[Prebiotic (nutrition)|Prebiotic]] soluble fiber products, like those containing [[inulin]] or [[oligosaccharide]]s, may contribute to relief from [[inflammatory bowel disease]],<ref>{{cite journal | vauthors = Ewaschuk JB, Dieleman LA | title = Probiotics and prebiotics in chronic inflammatory bowel diseases | journal = World Journal of Gastroenterology | volume = 12 | issue = 37 | pages = 5941–50 | date = October 2006 | pmid = 17009391 | pmc = 4124400 | doi = 10.3748/wjg.v12.i37.5941 | doi-access = free }}</ref> as in [[Crohn's disease]],<ref>{{cite journal | vauthors = Guarner F | title = Inulin and oligofructose: impact on intestinal diseases and disorders | journal = The British Journal of Nutrition | volume = 93 | pages = S61-5 | date = April 2005 | issue = Suppl 1 | pmid = 15877897 | doi = 10.1079/BJN20041345 | doi-access = free }}</ref> [[ulcerative colitis]],<ref>{{cite journal | vauthors = Seidner DL, Lashner BA, Brzezinski A, Banks PL, Goldblum J, Fiocchi C, Katz J, Lichtenstein GR, Anton PA, Kam LY, Garleb KA, Demichele SJ  | title = An oral supplement enriched with fish oil, soluble fiber, and antioxidants for corticosteroid sparing in ulcerative colitis: a randomized, controlled trial | journal = Clinical Gastroenterology and Hepatology | volume = 3 | issue = 4 | pages = 358–69 | date = April 2005 | pmid = 15822041 | doi = 10.1016/S1542-3565(04)00672-X | doi-access = free }}</ref><ref>{{cite journal | vauthors = Rodríguez-Cabezas ME, Gálvez J, Camuesco D, Lorente MD, Concha A, Martinez-Augustin O, Redondo L, Zarzuelo A  | title = Intestinal anti-inflammatory activity of dietary fiber (Plantago ovata seeds) in HLA-B27 transgenic rats | journal = Clinical Nutrition | volume = 22 | issue = 5 | pages = 463–71 | date = October 2003 | pmid = 14512034 | doi = 10.1016/S0261-5614(03)00045-1 }}</ref> and ''[[Clostridioides difficile (bacteria)|Clostridioides difficile]]'',<ref>{{cite book |vauthors=Ward PB, Young GP |title=Mechanisms in the Pathogenesis of Enteric Diseases |chapter=Dynamics of Clostridium Difficile Infection: Control Using Diet |volume=412 |pages=63–75 |year=1997 |pmid=9191992 |doi=10.1007/978-1-4899-1828-4_8 |series=Advances in Experimental Medicine and Biology |isbn=978-1-4899-1830-7 }}</ref> due in part to the short-chain [[fatty acid]]s produced with subsequent [[anti-inflammatory]] actions upon the bowel.<ref>{{cite journal | vauthors = Säemann MD, Böhmig GA, Zlabinger GJ | title = Short-chain fatty acids: bacterial mediators of a balanced host-microbial relationship in the human gut | journal = Wiener Klinische Wochenschrift | volume = 114 | issue = 8–9 | pages = 289–300 | date = May 2002 | pmid = 12212362 }}</ref><ref>{{cite journal | vauthors = Cavaglieri CR, Nishiyama A, Fernandes LC, Curi R, Miles EA, Calder PC | title = Differential effects of short-chain fatty acids on proliferation and production of pro- and anti-inflammatory cytokines by cultured lymphocytes | journal = Life Sciences | volume = 73 | issue = 13 | pages = 1683–90 | date = August 2003 | pmid = 12875900 | doi = 10.1016/S0024-3205(03)00490-9 }}</ref> Fiber supplements may be effective in an overall dietary plan for managing irritable bowel syndrome by modification of food choices.<ref>{{cite journal | vauthors = MacDermott RP | title = Treatment of irritable bowel syndrome in outpatients with inflammatory bowel disease using a food and beverage intolerance, food and beverage avoidance diet | journal = Inflammatory Bowel Diseases | volume = 13 | issue = 1 | pages = 91–6 | date = January 2007 | pmid = 17206644 | doi = 10.1002/ibd.20048 | s2cid = 24307163 | doi-access = free }}</ref>

==== Insoluble fiber ====
One insoluble fiber, [[resistant starch]] from high-amylose corn, has been used as a supplement and may contribute to improving insulin sensitivity and glycemic management<ref name="Insulin-sensitizing effects on musc">{{cite journal | vauthors = Robertson MD, Wright JW, Loizon E, Debard C, Vidal H, Shojaee-Moradie F, Russell-Jones D, Umpleby AM  | title = Insulin-sensitizing effects on muscle and adipose tissue after dietary fiber intake in men and women with metabolic syndrome | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 97 | issue = 9 | pages = 3326–32 | date = September 2012 | pmid = 22745235 | doi = 10.1210/jc.2012-1513 | doi-access = free }}</ref><ref name="Resistant starch from high-amylose">{{cite journal | vauthors = Maki KC, Pelkman CL, Finocchiaro ET, Kelley KM, Lawless AL, Schild AL, Rains TM | title = Resistant starch from high-amylose maize increases insulin sensitivity in overweight and obese men | journal = The Journal of Nutrition | volume = 142 | issue = 4 | pages = 717–23 | date = April 2012 | pmid = 22357745 | pmc = 3301990 | doi = 10.3945/jn.111.152975 }}</ref><ref name="Resistant starch improves insulin s">{{cite journal | vauthors = Johnston KL, Thomas EL, Bell JD, Frost GS, Robertson MD | title = Resistant starch improves insulin sensitivity in metabolic syndrome | journal = Diabetic Medicine | volume = 27 | issue = 4 | pages = 391–7 | date = April 2010 | pmid = 20536509 | doi = 10.1111/j.1464-5491.2010.02923.x | s2cid = 27570039 }}</ref> as well as promoting regularity<ref>{{cite journal | vauthors = Phillips J, Muir JG, Birkett A, Lu ZX, Jones GP, O'Dea K, Young GP | title = Effect of resistant starch on fecal bulk and fermentation-dependent events in humans | journal = The American Journal of Clinical Nutrition | volume = 62 | issue = 1 | pages = 121–30 | date = July 1995 | pmid = 7598054 | doi = 10.1093/ajcn/62.1.121 | doi-access = free }}</ref> and possibly relief of diarrhea.<ref>{{cite journal | vauthors = Ramakrishna BS, Venkataraman S, Srinivasan P, Dash P, Young GP, Binder HJ | title = Amylase-resistant starch plus oral rehydration solution for cholera | journal = The New England Journal of Medicine | volume = 342 | issue = 5 | pages = 308–13 | date = February 2000 | pmid = 10655529 | doi = 10.1056/NEJM200002033420502 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Raghupathy P, Ramakrishna BS, Oommen SP, Ahmed MS, Priyaa G, Dziura J, Young GP, Binder HJ  | title = Amylase-resistant starch as adjunct to oral rehydration therapy in children with diarrhea | journal = Journal of Pediatric Gastroenterology and Nutrition | volume = 42 | issue = 4 | pages = 362–8 | date = April 2006 | pmid = 16641573 | doi = 10.1097/01.mpg.0000214163.83316.41 | s2cid = 4647366 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Ramakrishna BS, Subramanian V, Mohan V, Sebastian BK, Young GP, Farthing MJ, Binder HJ | title = A randomized controlled trial of glucose versus amylase resistant starch hypo-osmolar oral rehydration solution for adult acute dehydrating diarrhea | journal = PLOS ONE | volume = 3 | issue = 2 | article-number = e1587 | date = February 2008 | pmid = 18270575 | pmc = 2217593 | doi = 10.1371/journal.pone.0001587 | bibcode = 2008PLoSO...3.1587R | doi-access = free }} {{open access}}</ref> One preliminary finding indicates that resistant [[corn starch]] may reduce symptoms of ulcerative colitis.<ref>{{cite web|vauthors=James S|title=P208. Abnormal fibre utilisation and gut transit in ulcerative colitis in remission: A potential new target for dietary intervention|url=https://www.ecco-ibd.eu/index.php/publications/congress-abstract-s/abstracts-2012/item/p208-abnorm.html|work=Presentation at European Crohn's & Colitis Organization meeting, Feb 16–18, 2012 in Barcelona, Spain|publisher=European Crohn's & Colitis Organization|access-date=25 September 2016|archive-date=27 September 2016|archive-url=https://web.archive.org/web/20160927090534/https://www.ecco-ibd.eu/index.php/publications/congress-abstract-s/abstracts-2012/item/p208-abnorm.html}}</ref>

====Inulins====
{{Main|Inulin}}

Chemically defined as [[oligosaccharide]]s and occurring naturally in most plants, inulins have nutritional value as [[carbohydrates]], or more specifically as [[fructan]]s, a [[polymer]] of the natural plant sugar, [[fructose]]. Inulin is typically extracted by manufacturers from enriched plant sources such as [[chicory]] roots or [[Jerusalem artichoke]]s for use in prepared foods.<ref>{{cite journal | vauthors = Kaur N, Gupta AK | title = Applications of inulin and oligofructose in health and nutrition | journal = Journal of Biosciences | volume = 27 | issue = 7 | pages = 703–14 | date = December 2002 | pmid = 12571376 | doi = 10.1007/BF02708379 | url = http://www.ias.ac.in/jbiosci/dec2002/703.pdf | s2cid = 1327336 }}</ref> 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 potential health value as a [[prebiotic (nutrition)|prebiotic]] fermentable fiber.<ref>{{cite journal | vauthors = Roberfroid MB | title = Inulin-type fructans: functional food ingredients | journal = The Journal of Nutrition | volume = 137 | issue = 11 Suppl | pages = 2493S–2502S | date = November 2007 | pmid = 17951492 | doi = 10.1093/jn/137.11.2493S | doi-access = free }}</ref>

As a prebiotic fermentable fiber, inulin is [[metabolism|metabolized]] by [[gut flora]] to yield short-chain fatty acids ([[#Short-chain fatty acids|see below]]), which increase absorption of [[calcium]],<ref>{{cite journal | vauthors = Abrams SA, Griffin IJ, Hawthorne KM, Liang L, Gunn SK, Darlington G, Ellis KJ | title = A combination of prebiotic short- and long-chain inulin-type fructans enhances calcium absorption and bone mineralization in young adolescents | journal = The American Journal of Clinical Nutrition | volume = 82 | issue = 2 | pages = 471–6 | date = August 2005 | pmid = 16087995 | doi = 10.1093/ajcn.82.2.471 | doi-access = free }}</ref> [[magnesium]],<ref>{{cite journal | vauthors = Coudray C, Demigné C, Rayssiguier Y | title = Effects of dietary fibers on magnesium absorption in animals and humans | journal = The Journal of Nutrition | volume = 133 | issue = 1 | pages = 1–4 | date = January 2003 | pmid = 12514257 | doi = 10.1093/jn/133.1.1 | doi-access = free }}</ref> and [[iron]].<ref>{{cite journal | vauthors = Tako E, Glahn RP, Welch RM, Lei X, Yasuda K, Miller DD | title = Dietary inulin affects the expression of intestinal enterocyte iron transporters, receptors and storage protein and alters the microbiota in the pig intestine | journal = The British Journal of Nutrition | volume = 99 | issue = 3 | pages = 472–80 | date = March 2008 | pmid = 17868492 | doi = 10.1017/S0007114507825128 | doi-access = free }}</ref>

The primary disadvantage of inulin is its fermentation within the intestinal tract, possibly causing [[flatulence]] and digestive distress at doses higher than 15 grams/day in most people.<ref>{{cite journal | vauthors = Grabitske HA, Slavin JL | title = Gastrointestinal effects of low-digestible carbohydrates | journal = Critical Reviews in Food Science and Nutrition | volume = 49 | issue = 4 | pages = 327–60 | date = April 2009 | pmid = 19234944 | doi = 10.1080/10408390802067126 | s2cid = 205689161 }}</ref> Individuals with digestive diseases have benefited from removing fructose and inulin from their diet.<ref>{{cite journal | vauthors = Shepherd SJ, Gibson PR | title = Fructose malabsorption and symptoms of irritable bowel syndrome: guidelines for effective dietary management | journal = Journal of the American Dietetic Association | volume = 106 | issue = 10 | pages = 1631–9 | date = October 2006 | pmid = 17000196 | doi = 10.1016/j.jada.2006.07.010 }}</ref> While clinical studies have shown changes in the [[microbiota]] at lower levels of inulin intake, higher intake amounts may be needed to achieve effects on body weight.<ref>{{cite journal | vauthors = Liber A, Szajewska H | title = Effects of inulin-type fructans on appetite, energy intake, and body weight in children and adults: systematic review of randomized controlled trials | journal = Annals of Nutrition & Metabolism | volume = 63 | issue = 1–2 | pages = 42–54 | date = 2013 | pmid = 23887189 | doi = 10.1159/000350312 |url=https://www.karger.com/Article/FullText/350312| doi-access = free }}</ref>

====Vegetable gums====
[[Natural gum|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.<ref>{{cite journal | vauthors = Parisi GC, Zilli M, Miani MP, Carrara M, Bottona E, Verdianelli G, Battaglia G, Desideri S, Faedo A, Marzolino C, Tonon A, Ermani M, Leandro G  | title = High-fiber diet supplementation in patients with irritable bowel syndrome (IBS): a multicenter, randomized, open trial comparison between wheat bran diet and partially hydrolyzed guar gum (PHGG) | journal = Digestive Diseases and Sciences | volume = 47 | issue = 8 | pages = 1697–704 | date = August 2002 | pmid = 12184518 | doi = 10.1023/A:1016419906546 | s2cid = 27545330 }}</ref> Examples of vegetable gum fibers are [[guar gum]] and [[gum arabic]].

==Activity in the gut==
{{More citations needed section|date=February 2021}}
Many molecules that are considered to be "dietary fiber" are so because humans lack the necessary enzymes to split the [[glycosidic bond]] and they reach the large intestine. Many foods contain varying types of dietary fibers, all of which contribute to health in different ways.

Dietary fibers make three primary contributions: bulking, viscosity and fermentation.<ref name=Gallaher>{{cite book| vauthors = Gallaher DD |title=Dietary Fiber |year=2006 |publisher=ILSI Press |location=Washington, D.C. |isbn=978-1-57881-199-1 |pages=102–10}}</ref> Different fibers have different effects, suggesting that a variety of dietary fibers contribute to overall health. Some fibers contribute through one primary mechanism. For instance, cellulose and wheat bran provide excellent bulking effects, but are minimally fermented. Alternatively, many dietary fibers can contribute to health through more than one of these mechanisms. For instance, psyllium provides bulking as well as viscosity.

Bulking fibers can be soluble (e.g. psyllium) or insoluble (e.g. cellulose and hemicellulose). They absorb water and can significantly increase stool weight and regularity. Most bulking fibers are not fermented or are minimally fermented throughout the intestinal tract.<ref name=Gallaher/>

Viscous fibers thicken the contents of the intestinal tract and may attenuate the absorption of sugar, reduce sugar response after eating, and reduce lipid absorption (notably shown with cholesterol absorption). Their use in food formulations is often limited to low levels, due to their viscosity and thickening effects. Some viscous fibers may also be partially or fully fermented within the intestinal tract (guar gum, beta-glucan, glucomannan and pectins), but some viscous fibers are minimally or not fermented (modified cellulose such as methylcellulose and psyllium).<ref name=Gallaher/>

Fermentable fibers are consumed by the microbiota within the large intestines, mildly increasing fecal bulk and producing [[short-chain fatty acids]] as byproducts with wide-ranging physiological activities. [[Resistant starch]], inulin, [[fructooligosaccharide]] and [[galactooligosaccharide]] are dietary fibers which are fully fermented. These include insoluble as well as soluble fibers. This fermentation influences the expression of many genes within the large intestine,<ref>{{cite journal | vauthors = Keenan MJ, Martin RJ, Raggio AM, McCutcheon KL, Brown IL, Birkett A, Newman SS, Skaf J, Hegsted M, Tulley RT, Blair E, Zhou J  | title = High-amylose resistant starch increases hormones and improves structure and function of the gastrointestinal tract: a microarray study | journal = Journal of Nutrigenetics and Nutrigenomics | volume = 5 | issue = 1 | pages = 26–44 | date = 2012 | pmid = 22516953 | pmc = 4030412 | doi = 10.1159/000335319 }}</ref> which affect digestive function and lipid and glucose metabolism, as well as the immune system, inflammation and more.<ref>{{cite journal | vauthors = Simpson HL, Campbell BJ | title = Review article: dietary fibre-microbiota interactions | journal = Alimentary Pharmacology & Therapeutics | volume = 42 | issue = 2 | pages = 158–79 | date = July 2015 | pmid = 26011307 | pmc = 4949558 | doi = 10.1111/apt.13248 }}</ref>

Fiber fermentation produces gas (majorly carbon dioxide, hydrogen, and methane) and short-chain fatty acids. Isolated or purified fermentable fibers are more rapidly fermented in the fore-gut and may result in undesirable gastrointestinal symptoms ([[bloating]], [[indigestion]] and flatulence).<ref>{{cite journal | vauthors = Noack J, Timm D, Hospattankar A, Slavin J | title = Fermentation profiles of wheat dextrin, inulin and partially hydrolyzed guar gum using an in vitro digestion pretreatment and in vitro batch fermentation system model | journal = Nutrients | volume = 5 | issue = 5 | pages = 1500–10 | date = May 2013 | pmid = 23645025 | doi = 10.3390/nu5051500 | pmc = 3708332 | s2cid = 233676 | doi-access = free }}</ref>

Dietary fibers can change the nature of the contents of the [[gastrointestinal tract]] and can change how other nutrients and chemicals are absorbed through bulking and viscosity.<ref name=USDA-IOM/><ref name=Eastwood/> Some types of soluble fibers bind to [[bile acids]] in the small intestine, making them less likely to re-enter the body; this in turn lowers [[cholesterol]] levels in the blood from the actions of [[cytochrome P450]]-mediated oxidation of cholesterol.<ref name=Anderson/>

Insoluble fiber is associated with reduced risk of diabetes,<ref>{{cite web|title=Foods that spike a patient's blood glucose are not what you think|publisher=American Medical Association|url=https://www.ama-assn.org/delivering-care/diabetes/foods-spike-patient-s-blood-glucose-are-not-what-you-think|access-date=14 October 2020}}</ref> but the mechanism by which this is achieved is unknown.<ref>{{cite journal | vauthors = Weickert MO, Pfeiffer AF | title = Metabolic effects of dietary fiber consumption and prevention of diabetes | journal = The Journal of Nutrition | volume = 138 | issue = 3 | pages = 439–42 | date = March 2008 | pmid = 18287346 | doi = 10.1093/jn/138.3.439 | doi-access = free }}</ref> One type of insoluble dietary fiber, resistant starch, may increase insulin sensitivity in healthy people,<ref>{{cite journal | vauthors = Robertson MD, Currie JM, Morgan LM, Jewell DP, Frayn KN | title = Prior short-term consumption of resistant starch enhances postprandial insulin sensitivity in healthy subjects | journal = Diabetologia | volume = 46 | issue = 5 | pages = 659–65 | date = May 2003 | pmid = 12712245 | doi = 10.1007/s00125-003-1081-0 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Robertson MD, Bickerton AS, Dennis AL, Vidal H, Frayn KN | title = Insulin-sensitizing effects of dietary resistant starch and effects on skeletal muscle and adipose tissue metabolism | journal = The American Journal of Clinical Nutrition | volume = 82 | issue = 3 | pages = 559–67 | date = September 2005 | pmid = 16155268 | doi = 10.1093/ajcn.82.3.559 | doi-access = free }}</ref> in type 2 diabetics,<ref>{{cite journal | vauthors = Zhang WQ, Wang HW, Zhang YM, Yang YX | title = [Effects of resistant starch on insulin resistance of type 2 diabetes mellitus patients] | language = zh | journal = Zhonghua Yu Fang Yi Xue Za Zhi [Chinese Journal of Preventive Medicine] | volume = 41 | issue = 2 | pages = 101–4 | date = March 2007 | pmid = 17605234 }}</ref> and in individuals with insulin resistance, possibly contributing to reduced risk of type 2 diabetes.<ref name="Resistant starch improves insulin s"/><ref name="Resistant starch from high-amylose"/><ref name="Insulin-sensitizing effects on musc"/>

Not yet formally proposed as an essential [[macronutrient]], dietary fiber has importance in the diet, with regulatory authorities in many developed countries recommending increases in fiber intake.<ref name=USDA-IOM/><ref name=Eastwood>{{cite journal | vauthors = Eastwood M, Kritchevsky D | title = Dietary fiber: how did we get where we are? | journal = Annual Review of Nutrition | volume = 25 | pages = 1–8 | year = 2005 | pmid = 16011456 | doi = 10.1146/annurev.nutr.25.121304.131658 | author-link2 = David Kritchevsky }}</ref><ref>{{cite journal|title=Scientific Opinion on Dietary Reference Values for carbohydrates and dietary fiber |author=((EFSA Panel on Dietetic Products, Nutrition, and Allergies, European Food Safety Authority))|journal=EFSA Journal|volume=8|issue=3|page=1462|year=2010 |doi=10.2903/j.efsa.2010.1462|doi-access=free}}</ref><ref>{{cite journal | vauthors = Jones PJ, Varady KA | title = Are functional foods redefining nutritional requirements? | journal = Applied Physiology, Nutrition, and Metabolism | volume = 33 | issue = 1 | pages = 118–23 | date = February 2008 | pmid = 18347661 | doi = 10.1139/H07-134 | url = http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=1715-5312&volume=33&issue=1&startPage=118 | archive-url = https://archive.today/20120711055314/http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=1715-5312&volume=33&issue=1&startPage=118 | archive-date = 11 July 2012 | url-access = subscription }}</ref>

===Physicochemical properties===
Dietary fiber has distinct [[physical chemistry|physicochemical]] properties. Most semi-solid foods, fiber and fat are a combination of gel matrices which are hydrated or collapsed with microstructural elements, globules, solutions or encapsulating walls. Fresh fruit and vegetables are cellular materials.<ref>Hermansson AM. Gel structure of food biopolymers In:  Food Structure, its creation and evaluation.JMV Blanshard and JR Mitchell, eds. 1988 pp. 25–40 Butterworths, London.</ref><ref>Rockland LB, Stewart GF. Water Activity: Influences on Food Quality. Academic Press, New York. 1991</ref><ref>{{cite journal | vauthors = Eastwood MA, Morris ER | title = Physical properties of dietary fiber that influence physiological function: a model for polymers along the gastrointestinal tract | journal = The American Journal of Clinical Nutrition | volume = 55 | issue = 2 | pages = 436–42 | date = February 1992 | pmid = 1310375 | doi = 10.1093/ajcn/55.2.436 | doi-access = free }}</ref>
* The cells of cooked potatoes and legumes are gels filled with gelatinized 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 fibers.
* Particle size and interfacial interactions with adjacent matrices affect the mechanical properties of food composites.
* Food polymers may be soluble in and/or plasticized by water.
* The variables include chemical structure, polymer concentration, molecular weight, degree of chain branching, the extent of ionization (for electrolytes), solution pH, ionic strength and temperature.
* Cross-linking of different polymers, protein and polysaccharides, either through chemical covalent bonds or cross-links through molecular entanglement or hydrogen or ionic bond cross-linking.
* Cooking and chewing food alters these physicochemical properties and hence absorption and movement through the stomach and along the intestine<ref name=Eastwood92>{{cite journal |vauthors=Eastwood MA |title=The physiological effect of dietary fiber: an update |journal=Annu Rev Nutr |volume=12 |issue= |pages=19–35 |date=1992 |pmid=1323981 |doi=10.1146/annurev.nu.12.070192.000315 }}</ref>

===Upper gastrointestinal tract===

Following a meal, the stomach and upper gastrointestinal contents consist of
* food compounds
* complex lipids/[[micelles|micellar]]/[[aqueous]]/[[hydrocolloid]] and [[hydrophobic]] phases
* [[hydrophilic]] phases
* solid, liquid, colloidal and gas bubble phases.<ref name=Eastwood92/>
[[Micelle]]s are colloid-sized clusters of molecules which form in conditions as those above, similar to the critical micelle concentration of detergents.<ref name=Carey>{{cite journal |vauthors=Carey MC, Small DM, Bliss CM |title=Lipid digestion and absorption |journal=Annu Rev Physiol |volume=45 |issue= |pages=651–77 |date=1983 |pmid=6342528 |doi=10.1146/annurev.ph.45.030183.003251 }}</ref>
In the upper gastrointestinal tract, these compounds consist of bile acids and di- and monoacyl [[glycerol]]s which solubilize [[triacylglycerol]]s and cholesterol.<ref name=Carey/>

Two mechanisms bring nutrients into contact with the epithelium:
# intestinal contractions create turbulence; and
# convection currents direct contents from the [[lumen (anatomy)|lumen]] to the epithelial surface.<ref name=Edwards>{{cite journal | vauthors = Edwards CA, Johnson IT, Read NW | title = Do viscous polysaccharides slow absorption by inhibiting diffusion or convection? | journal = European Journal of Clinical Nutrition | volume = 42 | issue = 4 | pages = 307–12 | date = April 1988 | pmid = 2840277 }}</ref>
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.
# Immobilizing of nutrients and other chemicals within complex polysaccharide molecules affects their release and subsequent absorption from the small intestine, an effect influential on the [[glycemic index]].<ref name=Edwards/>
# Molecules begin to interact as their concentration increases. During 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.<ref name=Edwards/>
# The presence of mucus or fiber, e.g., pectin or guar, in the unstirred layer may alter the viscosity and solute diffusion coefficient.<ref name=Eastwood92/>

Adding viscous polysaccharides to carbohydrate meals can reduce [[post-prandial]] blood glucose concentrations. Wheat and maize but not oats modify glucose absorption, the rate being dependent upon the particle size. 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 fiber interacts with pancreatic and enteric enzymes and their substrates. Human pancreatic enzyme activity is reduced when incubated with most fiber sources. Fiber may affect [[amylase]] activity and hence the rate of hydrolysis of starch. The more viscous polysaccharides extend the mouth-to-[[cecum]] transit time; guar, [[tragacanth]] and pectin being slower than wheat bran.<ref>{{cite journal |vauthors=Schneeman BO, Gallaher D |title=Effects of dietary fiber on digestive enzyme activity and bile acids in the small intestine |journal=Proc Soc Exp Biol Med |volume=180 |issue=3 |pages=409–14 |date=December 1985 |pmid=2417249 |doi=10.3181/00379727-180-42197 }}</ref>

===Colon===
The colon may be regarded as two organs,
# the right side (cecum and [[Large intestine#Ascending colon|ascending colon]]), a [[Fermentation|fermenter]].<ref>{{cite book |vauthors=Hellendoorn EW |date=1978 |chapter=6. Fermentation as the principal cause of the physiological activity of indigestible food residue |veditors=Spiller GA |title=Topics in dietary fiber research |publisher=Plenum Press |pages=127–168 |doi=10.1007/978-1-4684-2481-2_6 |isbn=978-1-4684-2481-2}}</ref> The right side of the colon is involved in nutrient salvage so that dietary fiber, resistant starch, fat and protein are utilized by bacteria and the end-products absorbed for use by the body
# the left side ([[Large intestine#Transverse colon|transverse]], [[Large intestine#Descending colon|descending]], and [[Large intestine#Sigmoid colon|sigmoid colon]]), affecting continence.
The presence of bacteria in the colon produces an 'organ' of intense, mainly reductive, metabolic activity, whereas the liver is oxidative.
The substrates utilized by the cecum have either passed along the entire intestine or are biliary excretion products.
The effects of dietary fiber in the colon are on
# bacterial fermentation of some dietary fibers
# thereby an increase in bacterial mass
# an increase in bacterial enzyme activity
# changes in the water-holding capacity of the fiber residue after fermentation
Enlargement of the cecum is a common finding when some dietary fibers are fed and this is now believed to be normal physiological adjustment. Such an increase may be due to a number of factors, prolonged cecal residence of the fiber, 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), and carbon dioxide, hydrogen and methane. Almost all of these short-chain fatty acids will be absorbed from the colon. This means that fecal short-chain fatty acid estimations do not reflect cecal and colonic fermentation, only the efficiency of absorption, the ability of the fiber residue to sequestrate short-chain fatty acids, and the continued fermentation of fiber 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 as the preferred energy source for colonic cells.

===Cholesterol metabolism===
Dietary fiber may act on each phase of ingestion, digestion, absorption and excretion to affect cholesterol metabolism,<ref>{{cite journal | vauthors = Brown L, Rosner B, Willett WW, Sacks FM | title = Cholesterol-lowering effects of dietary fiber: a meta-analysis | journal = The American Journal of Clinical Nutrition | volume = 69 | issue = 1 | pages = 30–42 | date = January 1999 | pmid = 9925120 | doi = 10.1093/ajcn/69.1.30 | doi-access = free }}</ref> such as the following:

# Caloric energy of foods through a bulking effect
# Slowing of gastric emptying time
# A glycemic index type of action on absorption
# A slowing of bile acid absorption in the [[ileum]] so bile acids escape through to the cecum
# Altered or increased bile acid metabolism in the cecum
# Indirectly by absorbed short-chain fatty acids, especially propionic acid, resulting from fiber fermentation affecting the cholesterol metabolism in the liver.
# Binding of bile acids to fiber or bacteria in the cecum with increased fecal loss from the entero-hepatic circulation.
One action of some fibers 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. A reduction in the reabsorption of bile acid from the ileum has several direct effects.
# Bile acids may be trapped within the lumen of the ileum either because of a high luminal viscosity or because of binding to a dietary fiber.<ref>{{cite journal | vauthors = Eastwood MA, Hamilton D | title = Studies on the adsorption of bile salts to non-absorbed components of diet | journal = Biochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism | volume = 152 | issue = 1 | pages = 165–73 | date = January 1968 | pmid = 5645448 | doi = 10.1016/0005-2760(68)90018-0 }}</ref>
# Lignin in fiber adsorbs bile acids, but the unconjugated form of the bile acids are adsorbed more than the conjugated form. In the ileum where bile acids are primarily absorbed the bile acids are predominantly conjugated.
# The enterohepatic circulation of bile acids may be altered and there is an increased flow of bile acids to the cecum, where they are deconjugated and 7alpha-dehydroxylated.
# These water-soluble form, bile acids e.g., deoxycholic and lithocholic are adsorbed to dietary fiber and an increased fecal loss of sterols, dependent in part on the amount and type of fiber.
# A further factor is an increase in the bacterial mass and activity of the ileum as some fibers e.g., pectin are digested by bacteria. The bacterial mass increases and cecal bacterial activity increases.
# The enteric loss of bile acids results in increased synthesis of bile acids from cholesterol which in turn reduces body cholesterol.
The fibers 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 fiber in the colon.
# There might be alterations in the end-products of bile acid bacterial metabolism or the release of short chain fatty acids which are absorbed from the colon, return to the liver in the portal vein and modulate either the synthesis of cholesterol or its catabolism to bile acids.
# The prime mechanism whereby fiber influences cholesterol metabolism is through bacteria binding bile acids in the colon after the initial deconjugation and dehydroxylation. The sequestered bile acids are then excreted in feces.<ref>{{cite journal | vauthors = Gelissen IC, Eastwood MA | title = Taurocholic acid adsorption during non-starch polysaccharide fermentation: an in vitro study | journal = The British Journal of Nutrition | volume = 74 | issue = 2 | pages = 221–8 | date = August 1995 | pmid = 7547839 | doi = 10.1079/BJN19950125 | doi-access = free }}</ref>
# Fermentable fibers e.g., pectin will increase the bacterial mass in the colon by virtue of their providing a medium for bacterial growth. 
# Other fibers, e.g., [[gum arabic]], act as [[Food additive#Categories|stabilizers]] and cause a significant decrease in serum cholesterol without increasing fecal bile acid excretion.

===Fecal weight===
Feces consist of a plasticine-like material, made up of water, bacteria, lipids, sterols, mucus and fiber.

# Feces are 75% water; bacteria make a large contribution to the dry weight, the residue being unfermented fiber and excreted compounds.
# Fecal output may vary over a range of between 20 and 280 g over 24 hours. The amount of feces egested a day varies for any one individual over a period of time.
# Of dietary constituents, only dietary fiber increases fecal weight.
Water is distributed in the colon in three ways:
# Free water which can be absorbed from the colon.
# Water that is incorporated into bacterial mass.
# Water that is bound by fiber.
Fecal weight is dictated by:
# the holding of water by the residual dietary fiber after fermentation.
# the bacterial mass.
# There may also be an added osmotic effect of products of bacterial fermentation on fecal mass.

==Effects of fiber intake==
Preliminary research indicates that fiber may affect health by different mechanisms.

Effects of fiber include:<ref name=bnf/><ref name=lpi/>
*Increases food volume without increasing caloric content to the same extent as digestible carbohydrates, providing satiety which may reduce appetite (both insoluble and soluble fiber)
*Attracts water and forms a [[viscosity|viscous]] gel during digestion, slowing the emptying of the stomach, shortening intestinal transit time, shielding carbohydrates from enzymes, and delaying absorption of glucose,<ref name=bnf/><ref>{{cite book | vauthors = Gropper SS, Smith JL, Groff JL | title = Advanced nutrition and human metabolism | url = https://books.google.com/books?id=rXSO9YLr72YC&q=soluble+fiber+viscosity+glucose&pg=PA114 | publisher = [[Cengage Learning]] | page = 114 | year = 2008 | edition = 5th | isbn = 978-0-495-11657-8 }}</ref> which lowers variance in blood sugar levels (soluble fiber)
*Lowers total and LDL cholesterol, which may reduce the risk of cardiovascular disease<ref name=bnf/> (soluble fiber)
*Reduces blood pressure, being negatively associated with all-cause and cardiovascular mortality,<ref>{{cite journal |last1= Reynolds |first1= Andrew |last2= Mann |first2= Jim |date= 10 January 2019 |title= Carbohydrate quality and human health: a series of systematic reviews and meta-analyses |url= https://linkinghub.elsevier.com/retrieve/pii/S0140-6736(18)31809-9 |journal= Lancet |volume= 393 |issue= 10170 |pages= 434–445 |doi= 10.1016/S0140-6736(18)31809-9 |pmid= 30638909 |access-date= 7 August 2024}}</ref><ref>{{cite journal |last1= Jama |first1= Hamdi |last2= Snelson |first2= Matthew |date= 12 July 2024 |title= Recommendations for the Use of Dietary Fiber to Improve Blood Pressure Control |url= https://www.ahajournals.org/doi/10.1161/CIRCRESAHA.124.324614 |journal= Circulation Research|volume= 135|issue= 4 |pages= 537–9 |doi= 10.1161/CIRCRESAHA.124.324614 |pmid= 39016011 |access-date= 7 August 2024|url-access= subscription }}</ref> and may prevent the development of cardiovascular disease in offspring<ref>{{cite journal |last1= Jama |first1= Hamdi |last2= Malathi |first2= Dona |date= 2 August 2024 |title= Maternal Diet and Gut Microbiota Influence Predisposition to Cardiovascular Disease in Offspring |url= https://www.ahajournals.org/doi/10.1161/HYPERTENSIONAHA.123.22575 |journal= Hypertension |volume= 81 |issue= 7 |pages= 1450–59 |doi= 10.1161/HYPERTENSIONAHA.123.22575 |pmid= 38586958 |access-date= 7 August 2024|url-access= subscription }}</ref> 
*Regulates blood sugar, which may reduce glucose and insulin levels in diabetic patients and may lower risk of diabetes<ref name=bnf/><ref>{{cite book|last = Food and Nutrition Board, [[Institute of Medicine]] of the National Academies | title = Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients) | publisher = National Academies Press | year = 2005 | pages = 380–82 | url = http://www.nap.edu/openbook/10490/png/380.png}}</ref> (insoluble fiber)
*Speeds the passage of foods through the digestive system, which facilitates regular defecation (insoluble fiber)
*Adds bulk to the stool, which alleviates constipation (insoluble fiber)
*Balances intestinal pH<ref>{{cite book | vauthors = Spiller G, Woods MN, Gorbach SL | title = Influence of fiber on the ecology of the intestinal flora; In: CRC Handbook of Dietary Fiber in Human Nutrition  | url = https://books.google.com/books?id=Jjdqw6n5zAQC&q=fiber+Balance+%22intestinal+pH%22&pg=PA257 | publisher = [[CRC Press]] | page = 257 | date = 27 June 2001 | access-date = 22 April 2009 | isbn = 978-0-8493-2387-4 }}</ref> and stimulates intestinal fermentation production of short-chain fatty acids.<ref name=bnf/>

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.<ref>{{cite journal | vauthors = Greger JL | title = Nondigestible carbohydrates and mineral bioavailability | journal = The Journal of Nutrition | volume = 129 | issue = 7 Suppl | pages = 1434S–5S | date = July 1999 | pmid = 10395614 | doi = 10.1093/jn/129.7.1434S | doi-access = free }}</ref><ref>{{cite journal | vauthors = Raschka L, Daniel H | title = Mechanisms underlying the effects of inulin-type fructans on calcium absorption in the large intestine of rats | journal = Bone | volume = 37 | issue = 5 | pages = 728–35 | date = November 2005 | pmid = 16126464 | doi = 10.1016/j.bone.2005.05.015 }}</ref><ref>{{cite journal | vauthors = Scholz-Ahrens KE, Schrezenmeir J | title = Inulin and oligofructose and mineral metabolism: the evidence from animal trials | journal = The Journal of Nutrition | volume = 137 | issue = 11 Suppl | pages = 2513S–2523S | date = November 2007 | pmid = 17951495 | doi = 10.1093/jn/137.11.2513S | doi-access = free }}</ref>

===Research===
As of 2019, preliminary [[clinical research]] on the potential health effects of a regular high-fiber diet included studies on the risk of several [[cancer]]s, [[cardiovascular disease]]s, and [[type II diabetes]].<ref name=lpi/><ref name=veronese/>

High-fiber intake is associated with a decreased risk of breast cancer, colon cancer and lower mortality.<ref>{{cite journal|vauthors=Arayici ME, Basbinar Y, Ellidokuz H |year=2023|title=High and low dietary fiber consumption and cancer risk: a comprehensive umbrella review with meta-meta-analysis involving meta-analyses of observational epidemiological studies|journal=Critical Reviews in Food Science and Nutrition|url=https://www.tandfonline.com/doi/abs/10.1080/10408398.2023.2298772|volume=28|issue=9|pages=1–14|doi=10.1080/10408398.2023.2298772|pmid=38153313|url-access=subscription}}</ref><ref>{{cite journal |vauthors=Yiallourou A, Pantavou K, Markozannes G, Pilavas A, Georgiou A, Hadjikou A, Economou M, Christodoulou N, Letsos K, Khattab E, Kossyva C, Constantinou M, Theodoridou M, Piovani D, Tsilidis KK, Bonovas S, Nikolopoulos GK |title=Non-genetic factors and breast cancer: an umbrella review of meta-analyses |journal=BMC Cancer |volume=24 |issue=1 |page=903 |date=July 2024 |pmid=39061008 |pmc=11282738 |doi=10.1186/s12885-024-12641-8 |doi-access=free}}</ref><ref>{{cite journal|vauthors=Oh H, Kim H, Lee DH, Lee A, Giovannucci EL, Kang SS, Keum N |year=2019|title=Different dietary fibre sources and risks of colorectal cancer and adenoma: a dose–response meta-analysis of prospective studies|journal=British Journal of Nutrition|url=https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/different-dietary-fibre-sources-and-risks-of-colorectal-cancer-and-adenoma-a-doseresponse-metaanalysis-of-prospective-studies/BDEA13DC175ACF0B446E5CC0021417FE|volume=122|issue=6|pages=605–615|doi=10.1017/S0007114519001454|pmid=31495339}}</ref><ref>{{cite journal|vauthors=Arayici ME, Mert-Ozupek N, Yalcin F, Basbinar Y, Ellidokuz H |year=2022|title=Soluble and Insoluble Dietary Fiber Consumption and Colorectal Cancer Risk: A Systematic Review and Meta-Analysis|journal=Nutrition and Cancer|url=|volume=74|issue=7|pages=2412–25|doi=10.1080/01635581.2021.2008990|pmid=34854791}}</ref>

==Dietary recommendations==
===European Union===
According to the [[European Food Safety Authority]] (EFSA) Panel on Nutrition, Novel Foods and Food Allergens, 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".<ref>{{cite journal|date=2010|title=Scientific Opinion on Dietary Reference Values for carbohydrates and dietary fibre|journal=EFSA Journal|language=en|volume=8|issue=3|page=1462|doi=10.2903/j.efsa.2010.1462|issn=1831-4732|doi-access=free}}</ref><ref name="jrc2017">{{cite web|url=https://ec.europa.eu/jrc/en/health-knowledge-gateway/promotion-prevention/nutrition/fibre|title=Dietary Fibre | vauthors = Maragkoudakis P |date=20 June 2017|website=EU Science Hub|publisher=[[Joint Research Centre]]|language=en|access-date=21 December 2019}}</ref>

===United States===
Current recommendations from the United States [[National Academy of Medicine]] (NAM) (formerly Institute of Medicine) of the [[National Academy of Sciences]] state that for [[Dietary Reference Intake|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.<ref name=lpi/><ref>{{cite book |last1=Institute of Medicine |title=Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids |date=2005 |publisher=The National Academies Press |location=Washington, DC |pages=387–8 |doi=10.17226/10490 |isbn=978-0-309-08525-0 |url=https://www.nap.edu/read/10490/chapter/9#387 |access-date=8 June 2021}}</ref><ref name=USDA-IOM/>

The U.S. [[Academy of Nutrition and Dietetics]] (AND) reiterates the recommendations of the NAM.<ref>{{cite web|url=https://www.eatright.org/food/vitamins-and-supplements/nutrient-rich-foods/fiber|title=Fiber|website=www.eatright.org|language=en|access-date=11 October 2019}}</ref> 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).<ref>{{cite journal |last1=Williams |first1=Christine L. |last2=Bollella |first2=Marguerite |last3=Wynder |first3=Ernst L. |title=A New Recommendation for Dietary Fiber in Childhood |journal=Pediatrics |date=November 1995 |volume=96 |issue=5 |pages=985–8 |doi=10.1542/peds.96.5.985 |pmid=7494677 |s2cid=39644070 |url=https://pediatrics.aappublications.org/content/96/5/985 |access-date=7 June 2021|url-access=subscription }}</ref><ref>{{cite journal |last1=Wilkinson Enns |first1=Cecilia |last2=Mickle |first2=Sharon J. |last3=Goldman |first3=Joseph D. |title=Trends in Food and Nutrient Intakes by Children in the United States |journal=Family Economics and Nutrition Review |date=2002 |volume=14 |issue=1 |page=64 |url=https://books.google.com/books?id=Y6cJlzOR4WQC |access-date=7 June 2021}}</ref> 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.<ref name=lpi/> 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 [[opioid]]s 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]].<ref>{{cite book | vauthors = Suter PM |title=Atherosclerosis: Diet and Drugs |chapter=Carbohydrates and dietary fiber |issue=170 |pages=231–61 |year=2005 |pmid=16596802 |doi=10.1007/3-540-27661-0_8 |volume=170 |series=Handbook of Experimental Pharmacology |isbn=978-3-540-22569-0|s2cid=37892002 }}</ref> Recognizing the growing scientific evidence for physiological benefits of increased fiber intake, regulatory agencies such as the U.S. [[Food and Drug Administration]] (FDA) 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.<ref>{{cite news|url=https://www.npr.org/sections/thesalt/2017/10/23/558761819/the-fda-will-decide-if-these-26-ingredients-count-as-fiber|title=The FDA Will Decide Whether 26 Ingredients Count As Fiber|publisher=National Public Radio| vauthors = Aubrey A |date=23 October 2017|access-date=19 November 2017|language=en}}</ref> As of 2008, the FDA approved [[health claim]]s for qualified fiber products to display labeling that regular consumption may reduce [[blood cholesterol]] levels &ndash; which can lower the risk of [[coronary heart disease]]<ref>[https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=101.77 Health claims: fruits, vegetables, and grain products that contain fiber, particularly soluble fiber, and risk of coronary heart disease.] Electronic Code of Federal Regulations: US Government Printing Office, current as of 20 October 2008</ref> &ndash; and also reduce the risk of some types of cancer.<ref>[https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=101.78 Health claims: fiber-containing grain products, fruits, and vegetables and cancer.] Electronic Code of Federal Regulations: US Government Printing Office, current as of 20 October 2008</ref>

Viscous fiber sources gaining FDA approval are:<ref name=lpi/>

* [[Psyllium]] seed husk (7&nbsp;grams per day)
* [[Beta-glucan]] from [[oat bran]], whole [[oat]]s, or [[rolled oats]]; or whole grain or dry-milled [[barley]] (3&nbsp;grams per day)

Other examples of bulking fiber sources used in [[functional food]]s and supplements include [[cellulose]], [[guar gum]] and [[xanthan gum]]. Other examples of fermentable fiber sources (from plant foods or biotechnology) used in functional foods and supplements include [[resistant starch]], [[inulin]], [[fructan]]s, fructooligo saccharides, oligo- or polysaccharides, and resistant [[dextrin]]s, which may be partially or fully fermented.

Consistent intake of fermentable fiber may reduce the risk of chronic diseases.<ref name=tungland/><ref>{{cite journal | vauthors = Lee YP, Puddey IB, Hodgson JM | title = Protein, fibre and blood pressure: potential benefit of legumes | journal = Clinical and Experimental Pharmacology & Physiology | volume = 35 | issue = 4 | pages = 473–6 | date = April 2008 | pmid = 18307744 | doi = 10.1111/j.1440-1681.2008.04899.x | s2cid = 25086200 | author-link1 = Yuan-Pern Lee }}</ref><ref>{{cite journal | vauthors = Theuwissen E, Mensink RP | title = Water-soluble dietary fibers and cardiovascular disease | journal = Physiology & Behavior | volume = 94 | issue = 2 | pages = 285–92 | date = May 2008 | pmid = 18302966 | doi = 10.1016/j.physbeh.2008.01.001 | s2cid = 30898446 }}</ref> Insufficient fiber in the diet can lead to [[constipation]].<ref name=WebMDConst>{{cite web|website=WebMD|title=What Is Constipation?|url=http://www.webmd.com/content/article/90/100651.htm|date=2017|access-date=19 November 2017}}</ref>

===United Kingdom===
In 2018, the [[British Nutrition Foundation]] issued a statement to define dietary fiber more concisely and list the potential health benefits established to date, while increasing its recommended daily minimum intake to 30 grams for healthy adults.<ref>{{cite journal|doi=10.1111/nbu.12141|title=30 g of fibre a day: An achievable recommendation?|journal=Nutrition Bulletin|volume=40|issue=2|pages=118–129|year=2015| vauthors = Hooper B, Spiro A, Stanner S |doi-access=free}}</ref><ref name=bnf/>

The use of certain analytical methods to quantify dietary fiber by nature of its indigestion ability 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.

Diets naturally high in fiber can be considered to bring about several main physiological consequences:<ref name=bnf/>
* increases [[fecal]] bulk and helps prevent [[constipation]] by decreasing fecal transit time in the [[large intestine]]
* improves [[human gastrointestinal tract|gastro-intestinal]] health
* improves [[glucose tolerance]] and the [[insulin]] response following a meal
* increases [[colonic]] [[fermentation]] and [[short-chain fatty acid]] production
* positively modulates colonic [[microflora]]
* reduces [[hyperlipidemia]], [[hypertension]], and other [[coronary heart disease]] risk factors
* increases [[satiety]] and hence may contribute to weight management

Fiber is defined by its physiological impact, with many heterogenous types of fibers. Some fibers may primarily impact one of these benefits (i.e., cellulose increases fecal bulking and prevents constipation), but many fibers impact more than one of these benefits (i.e., resistant starch increases bulking, increases colonic fermentation, positively modulates colonic microflora and increases satiety and insulin sensitivity).<ref name=lockyer/><ref name=keenan/> 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.<ref name=bnf/>

==Fermentation==
The [[American Association of Cereal Chemists|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."<ref>{{cite web |url=http://www.aaccnet.org/DietaryFiber/pdfs/dietfiber.pdf |title=The Definition of Dietary Fiber |access-date=12 May 2007 |archive-url=https://web.archive.org/web/20070928040750/http://www.aaccnet.org/DietaryFiber/pdfs/dietfiber.pdf |archive-date=28 September 2007|author=AACC International }}</ref>

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, oligosaccharides, or polysaccharides, 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 (anatomy)|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.<ref name=Wong>{{cite journal | vauthors = Wong JM, de Souza R, Kendall CW, Emam A, Jenkins DJ | title = Colonic health: fermentation and short chain fatty acids | journal = Journal of Clinical Gastroenterology | volume = 40 | issue = 3 | pages = 235–43 | date = March 2006 | pmid = 16633129 | doi = 10.1097/00004836-200603000-00015 | s2cid = 46228892 }}</ref> They include [[butyric acid|butyric]], [[acetic acid|acetic]] (ethanoic), [[propionic acid|propionic]], and [[valeric acid|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 acid]]s and gases (which are typically expelled as flatulence).

According to a 2002 journal article,<ref name=tungland>{{cite journal|vauthors=Tungland BC, Meyer D|title=Nondigestible oligo- and polysaccharides (dietary fiber): their physiology and role in human health and food|journal=Comprehensive Reviews in Food Science and Food Safety|volume=1|issue=3|pages=73–92|year=2002|doi=10.1111/j.1541-4337.2002.tb00009.x|pmid=33451232|doi-access=free}}</ref>
fiber compounds with partial or low fermentability include:
* [[cellulose]], a polysaccharide
* [[methyl cellulose]]
* [[hemicellulose]], a polysaccharide
* [[lignan]]s, a group of [[phytoestrogen]]s
* plant [[wax]]es

fiber compounds with high fermentability include:
* [[resistant starch]]es
* [[beta-glucans]], a group of polysaccharides
* [[pectin]]s, a group of [[heteropolysaccharide]]s
* [[natural gum]]s, a group of polysaccharides
* [[inulin]]s, a group of polysaccharides
* [[oligosaccharide]]s

==Short-chain fatty acids==
{{Main|Short-chain fatty acid}}
When fermentable fiber is fermented, short-chain fatty acids (SCFA) are produced.<ref name=":0" /> SCFAs are involved in numerous physiological processes promoting health, including:<ref name="Wong" />
* stabilize blood [[glucose]] levels by acting on pancreatic [[insulin]] release and liver control of [[glycogen]] breakdown
* stimulate [[gene expression]] of [[glucose transporter]]s in the [[intestinal mucosa]], regulating glucose absorption<ref>{{cite journal | vauthors = Drozdowski LA, Dixon WT, McBurney MI, Thomson AB | title = Short-chain fatty acids and total parenteral nutrition affect intestinal gene expression | journal = Journal of Parenteral and Enteral Nutrition | volume = 26 | issue = 3 | pages = 145–50 | year = 2002 | pmid = 12005453 | doi = 10.1177/0148607102026003145 }}</ref>
* provide nourishment of [[colonocytes]], particularly by the SCFA butyrate
* suppress [[cholesterol]] synthesis by the liver and reduce blood levels of [[Low-density lipoprotein|LDL cholesterol]] and [[triglyceride]]s responsible for [[atherosclerosis]]
* lower colonic [[pH]] (i.e., raises the acidity level in the [[Colon (anatomy)|colon]]) which protects the lining from formation of [[colonic polyp]]s and increases absorption of [[dietary mineral]]s
* stimulate production of [[T helper cell]]s, [[Antibody|antibodies]], [[leukocyte]]s, [[cytokine]]s, and [[lymph]] mechanisms having crucial roles in [[immune]] protection
* improve barrier properties of the colonic [[mucosal]] layer, inhibiting [[inflammation|inflammatory]] and [[adhesion]] irritants, contributing to immune functions

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.<ref>{{cite journal | vauthors = Roy CC, Kien CL, Bouthillier L, Levy E | title = Short-chain fatty acids: ready for prime time? | journal = Nutrition in Clinical Practice | volume = 21 | issue = 4 | pages = 351–66 | date = August 2006 | pmid = 16870803 | doi = 10.1177/0115426506021004351 }}</ref><ref>{{cite journal | vauthors = Scholz-Ahrens KE, Ade P, Marten B, Weber P, Timm W, Açil Y, Glüer CC, Schrezenmeir J  | title = Prebiotics, probiotics, and synbiotics affect mineral absorption, bone mineral content, and bone structure | journal = The Journal of Nutrition | volume = 137 | issue = 3 Suppl 2 | pages = 838S–46S | date = March 2007 | pmid = 17311984 | doi = 10.1093/jn/137.3.838S | doi-access = free }}</ref>

The major SCFAs in humans are [[butyric acid|butyrate]], [[propionic acid|propionate]], and [[acetic acid|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.{{Citation needed|date=March 2021}}

==FDA-approved health claims==
The 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 [[health claim|claim]] that regular consumption may reduce the risk of [[heart disease]].<ref name=FDAc>[http://www.cfsan.fda.gov/~dms/2lg-xc.html FDA/CFSAN A Food Labeling Guide: Appendix C Health Claims, April 2008] {{webarchive|url=https://web.archive.org/web/20080412090421/http://www.cfsan.fda.gov/~dms/2lg-xc.html |date=12 April 2008 }}</ref>

The FDA statement template for making this claim is:

{{Blockquote|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.<ref name=FDAc/>}}

Eligible sources of soluble fiber providing beta-glucan include:

* Oat bran
* Rolled oats
* Whole oat flour
* Oatrim
* Whole grain barley and dry milled barley
* 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:

* 3 g or more per day of beta-glucan soluble fiber from either whole oats or barley, or a combination of whole oats and barley
* 7 g or more per day of soluble fiber from psyllium seed husk.<ref>Soluble Fiber from Certain Foods and Risk of Coronary Heart Disease, U.S. Government Printing Office, Electronic Code of Federal Regulations, Title 21: Food and Drugs, part 101: Food Labeling, Subpart E, Specific Requirements for Health Claims, 101.81 [http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=77ed7da9463357d9a09892213e5c74db&rgn=div8&view=text&node=21:2.0.1.1.2.5.1.12&idno=21] {{webarchive|url=https://web.archive.org/web/20080601040856/http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=77ed7da9463357d9a09892213e5c74db&rgn=div8&view=text&node=21%3A2.0.1.1.2.5.1.12&idno=21|date=1 June 2008}}</ref>

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).<ref name=FDAc/>

In December 2016, the 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. The FDA has concluded that there is limited scientific evidence for this claim."<ref>{{cite web| vauthors = Balentine D |title=Petition for a Health Claim for High-Amylose Maize Starch (Containing Type-2 Resistant Starch) and Reduced Risk Type 2 Diabetes Mellitus (Docket Number FDA2015-Q-2352)|url=https://www.fda.gov/downloads/food/ingredientspackaginglabeling/labelingnutrition/ucm546963.pdf|archive-url=https://web.archive.org/web/20170406180416/https://www.fda.gov/downloads/Food/IngredientsPackagingLabeling/LabelingNutrition/UCM546963.pdf|archive-date=6 April 2017|publisher=Office of Nutrition and Food Labeling, Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration|date=12 December 2016|access-date=22 March 2018}}</ref> 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.<ref>{{cite web|url=https://www.foodnavigator-usa.com/Article/2018/06/15/FDA-unveils-dietary-fibers-guidance-Good-news-for-inulin-polydextrose-some-gray-areas-remaining|title=FDA unveils dietary fibers guidance: Good news for inulin, polydextrose, some gray areas remaining|publisher=FoodNavigatorUSA.com|first=Elaine |last=Watson|date=14 June 2018|language=en|access-date=24 June 2019}}</ref>

==See also==
* {{annotated link|Essential nutrient}}
* [[List of diets]]
* {{annotated link|List of macronutrients}}
* {{annotated link|List of micronutrients}}
* {{annotated link|List of phytochemicals in food}}
* {{annotated link|Low-fiber/low-residue diet}}

==References==
{{reflist}}

==Further reading==
{{refbegin}}
*{{cite journal |vauthors=Yusuf K, Saha S, Umar S |title=Health Benefits of Dietary Fiber for the Management of Inflammatory Bowel Disease |journal=Biomedicines |volume=10 |issue=6 |page=1242 |date=May 2022 |pmid=35740264 |pmc=9220141 |doi=10.3390/biomedicines10061242 |doi-access=free}}
{{refend}}

==External links==
{{commons category}}

{{Phytochemicals}}
{{carbohydrates}}
{{Dietary supplement}}
{{Diets}}
{{Plant-based diets}}
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{{DEFAULTSORT:Dietary fiber}}
[[Category:Dietary supplements]]
[[Category:Nutrition]]
[[Category:Dietary fiber| ]]