Editing Human nutrition (section)

=== Fiber ===
{{Main|Dietary fiber}}
{{more medical citations needed|section|date=October 2024}}
Dietary fiber is a [[carbohydrate]], specifically a [[polysaccharide]], which is incompletely absorbed in humans and in some animals. Fiber slows down the absorption of sugar in the gut. The [[Microbiome of humans|microbiome]] converts fiber into signals that stimulate gut hormones, which in turn control how quickly the stomach empties, regulate blood sugar levels, and influence feelings of hunger. Like all carbohydrates, when fiber is digested, it can produce four [[calorie]]s (kilocalories) of energy per gram, but in most circumstances, it accounts for less than that because of its limited absorption and digestibility.

The two subcategories are ''insoluble'' and ''soluble'' fiber.

;Insoluble dietary fiber
:Includes [[cellulose]], a large carbohydrate polymer that is indigestible by humans, because humans do not have the required enzymes to break it down, and the human digestive system does not harbor enough of the types of microbes that can do so.
:Includes [[resistant starch]], an insoluble starch that resists digestion either because it is protected by a shell or food matrix (Type 1 resistant starch, RS1), maintains the natural starch granule (Type 2 resistant starch, RS2), is retrograded and partially crystallized (Type 3 resistant starch, RS3), has been chemically modified (Type 4 resistant starch, RS4) or has complexed with a lipid (Type 5 resistant starch, RS5).<ref name="Lockyer RS Review" /> Natural sources of resistant starch (RS1, RS2 and RS3) are fermented by the microbes in the human digestive system to produce short-chain fatty acids which are utilized as food for the colonic cells or absorbed.<ref name="Lockyer RS Review" />

;Soluble dietary fiber
:Comprises a variety of [[oligosaccharides]], [[waxes]], [[esters]], and other carbohydrates that dissolve or gelatinize in water. Many of these soluble fibers can be fermented or partially fermented by microbes in the human digestive system to produce short-chain fatty acids which are absorbed and therefore introduce some caloric content.<ref>{{Cite journal | last1=Hervik | first1=Astrid Kolderup | last2=Svihus | first2=Birger | date=2019 | title=The role of fiber in energy balance | journal=Journal of Nutrition and Metabolism | volume=2019 | pages=4983657 | doi=10.1155/2019/4983657 | issn=2090-0724 | pmc=6360548 | pmid=30805214 | doi-access=free}}</ref>

Whole grains, beans, and other [[legumes]], fruits (especially [[plum]]s, [[prune]]s, and [[fig]]s), and vegetables are good sources of dietary fiber. Fiber has three primary mechanisms, which in general determine their health impact: bulking, viscosity and fermentation.<ref name="Gallaher Fiber Review">{{cite book | last1=Gallaher | first1=Daniel D. | title=Chapter 8 – Dietary Fiber in Present Knowledge in Nutrition | date=2006 | publisher=ILSI Press | location=Washington DC | isbn=978-1-57881-199-1 | pages=102–110 | edition=Ninth Edition, Volume I}}</ref> Fiber provides bulk to the intestinal contents, and insoluble fiber facilitates [[peristalsis]]&nbsp;– the rhythmic muscular contractions of the intestines which move contents along the digestive tract. Some soluble and insoluble fibers produce a solution of high [[viscosity]]; this is essentially a gel, which slows the movement of food through the intestines. Fermentable fibers are used as food by the [[microbiome]], mildly increasing bulk, and producing [[short-chain fatty acid]]s and other metabolites, including vitamins, hormones, and glucose. One of these metabolites, [[butyrate]], is important as an energy source for colon cells, and may improve [[metabolic syndrome]].<ref>{{cite journal | last1=Bridgeman | first1=Stephanie C. | last2=Northrop | first2=Wendy | last3=Melton | first3=Phillip E. | last4=Ellison | first4=Gaewyn C. | last5=Newsholme | first5=Philip | last6=Mamotte | first6=Cyril D.S. | title=Butyrate generated by gut microbiota and its therapeutic role in metabolic syndrome | journal=Pharmacological Research | date=1 October 2020 | volume=160 | page=105174 | doi=10.1016/j.phrs.2020.105174 | pmid=32860943 | s2cid=221373075 | url=http://www.sciencedirect.com/science/article/pii/S1043661820314821}}</ref><ref>{{cite journal | last1=Wang | first1=Yong | last2=Chen | first2=Jing | last3=Song | first3=Ying-Han | last4=Zhao | first4=Rui | last5=Xia | first5=Lin | last6=Chen | first6=Yi | last7=Cui | first7=Ya-Ping | last8=Rao | first8=Zhi-Yong | last9=Zhou | first9=Yong |last10=Zhuang |first10=Wen | last11=Wu | first11=Xiao-Ting | title=Effects of the resistant starch on glucose, insulin, insulin resistance, and lipid parameters in overweight or obese adults: a systemic review and meta-analysis | journal=Nutrition & Diabetes | date=5 June 2019 | volume=9 | issue=1 | page=19 | doi=10.1038/s41387-019-0086-9 | pmid=31168050 | pmc=6551340 | url=https://doi.org/10.1038/s41387-019-0086-9 | access-date=22 February 2021 | archive-date=24 January 2023 | archive-url=https://web.archive.org/web/20230124005320/https://www.nature.com/articles/s41387-019-0086-9 | url-status=live}}</ref>

In 2016, the U.S. FDA approved a qualified [[health claim]] stating that resistant starch might reduce the risk of [[type 2 diabetes]], but with qualifying language for product labels that only limited scientific evidence exists to support this claim. The FDA requires specific labeling language, such as the guideline concerning resistant starch: "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."<ref name="FDA approved health claim RS2 resistant starch">{{cite web | last1=Balentine | first1=Douglas | title=Letter announcing decision for a health claim for high-amylose maize starch (containing type-2 resistant starch) and reduced risk of type 2 diabetes mellitus | url=https://www.regulations.gov/docket?D=FDA-2015-Q-2352 | website=www.regulations.gov | publisher=U.S. Food and Drug Administration | access-date=16 December 2016 | archive-date=20 December 2016 | archive-url=https://web.archive.org/web/20161220200555/https://www.regulations.gov/docket?D=FDA-2015-Q-2352 | url-status=live}}</ref>