Editing Chewing

{{Redirect|Chew}}
{{short description|Mechanical procedure for crushing the food and its first enzymatic splitting}}
[[File:Macaca arctoides.png|thumb|right|250px|A [[stump-tailed macaque]] using mastication to process tough plant matter]]
'''Chewing''' or '''mastication''' is the process by which [[food]] is [[comminution|crushed and ground]] by the [[teeth]]. It is the first step in the process of [[digestion]], allowing a greater surface area for [[digestive enzyme]]s to break down the foods.

During the mastication process, the food is positioned by the [[cheek]] and [[tongue]] between the teeth for grinding. The [[muscles of mastication]] move the [[jaw]]s to bring the teeth into intermittent contact, repeatedly [[occlusion (dentistry)|occluding]] and opening. As chewing continues, the food is made softer and warmer, and the enzymes in saliva begin to break down [[carbohydrates]] in the food. After chewing, the food (now called a [[Bolus (digestion)|bolus]]) is swallowed. It enters the [[esophagus]] and via [[peristalsis]] continues on to the stomach, where the next step of digestion occurs.<ref>{{cite web|title=Prehension, Mastication and Swallowing|url=http://biology.about.com/library/organs/blpathodigest4.htm|access-date=2012-05-24|archive-date=2015-07-15|archive-url=https://web.archive.org/web/20150715020626/http://biology.about.com/library/organs/blpathodigest4.htm|url-status=dead}}</ref> Increasing the number of chews per bite stimulates the production of digestive enzymes and peptides and has been shown to increase diet-induced thermogenesis (DIT) by activating the [[sympathetic nervous system]].<ref name=":0" /> Studies suggest that thorough chewing may facilitate digestion and nutrient absorption, improve insulin secretion and glucose homeostasis, and decrease food intake and levels of self-reported hunger.<ref name=":0">{{Cite journal|last1=Miquel-Kergoat|first1=Sophie|last2=Azais-Braesco|first2=Veronique|last3=Burton-Freeman|first3=Britt|last4=Hetherington|first4=Marion M.|date=2015-11-01|title=Effects of chewing on appetite, food intake and gut hormones: A systematic review and meta-analysis|journal=Physiology & Behavior|volume=151|pages=88–96|doi=10.1016/j.physbeh.2015.07.017|issn=1873-507X|pmid=26188140|doi-access=free}}</ref> [[Chewing gum]] has been around for many centuries; there is evidence that northern Europeans chewed [[Birch tar|birch bark tar]] 9,000 years ago. 

Mastication, as it requires specialized teeth, is mostly a mammalian adaptation that appeared in early [[Synapsid]]s, although some later herbivorous dinosaurs, now extinct, also developed chewing, too. Today only modern mammals chew in the strictest sense of the word, but some fish species exhibit a somewhat similar behavior.  By contrast, mastication is not found in any living birds, amphibians, or reptiles.

[[Premastication]] is sometimes performed by human parents for infants who are unable to do so for themselves. The food is masticated in the mouth of the parent into a bolus and then transferred to the infant for consumption<ref name=HolmesEA>{{citation|first=Wendy |last=Holmes |title=Influences on maternal and child nutrition in the highlands of the northern Lao PDR |journal=Asia Pac J Clin Nutr |year=2007 |volume=16 |number=3 |pages=537–545|pmid=17704036}}</ref> (some other animals also premasticate).

[[Cattle]] and some other animals, called [[ruminant]]s, chew food more than once to extract more nutrients. After the first round of chewing, this food is called [[cud]].

[[File:Trodden on chewing gum2.jpg|thumb|A piece of chewing gum after being trodden on.]]

==Chewing motor program==
[[File:Water buffalo chewing cud - 01.ogv|thumbnail|A [[water buffalo]] chewing cud]]
Chewing is primarily an unconscious ([[autonomic nervous system|semi-autonomic]]) act, but can be mediated by higher conscious input. The motor program for mastication is a hypothesized central nervous system function by which the complex patterns governing mastication are created and controlled.

It is thought that feedback from [[proprioceptive]] nerves in teeth and the [[Temporomandibular joint|temporomandibular]] joints govern the creation of neural pathways, which in turn determine duration and force of individual muscle activation (and in some cases muscle fiber groups as in the masseter and temporalis).

This motor program continuously adapts to changes in food type or occlusion.<ref>{{cite journal|last=Peyron|first=Marie-Agnès|author2=Olivier Blanc |author3=James P. Lund |author4=Alain Woda |date=2004-03-09|title=Influence of Age on Adaptability of Human Mastication |journal=[[Journal of Neurophysiology]] |volume=92|pages=773–779|url=http://jn.physiology.org/cgi/content/full/92/2/773|access-date=2008-07-02|doi=10.1152/jn.01122.2003|pmid=15277595|issue=2|url-access=subscription}}</ref> This adaptation is a learned skill that may sometimes require relearning to adapt to loss of teeth or to dental appliances such as [[dentures]].

It is thought that conscious mediation is important in the limitation of [[parafunctional habit]]s as most commonly, the motor program can be excessively engaged during periods of sleep and times of stress. It is also theorized that excessive input to the motor program from myofascial pain or occlusal imbalance can contribute to [[parafunctional habit]]s.

===Muscles===
{{Main|Muscles of mastication}}

== Nutrition and health ==

Chewing stimulates [[saliva]] production and increases sensory perception of the food being eaten, controlling when the food is swallowed.<ref>{{cite journal|last1=Peyron|first1=Marie-Agnès|last2=Gierczynski|first2=Isabelle|last3=Hartmann|first3=Christoph|last4=Loret|first4=Chrystel|last5=Dardevet|first5=Dominique|last6=Martin|first6=Nathalie|last7=Woda|first7=Alain|title=Role of Physical Bolus Properties as Sensory Inputs in the Trigger of Swallowing|journal=PLOS ONE|date=27 June 2011|volume=6|issue=6|pages=e21167|doi=10.1371/journal.pone.0021167|issn=1932-6203|pmc=3124480|pmid=21738616|bibcode=2011PLoSO...621167P|doi-access=free}}</ref> Evidence from one study suggests that chewing almonds 25-40 times kept people fuller while also allowing them to get more nutrients out of the almonds. The researchers also suggest that this is likely to be the case in other foods.<ref>{{Cite journal|last1=Cassady|first1=Bridget A|last2=Hollis|first2=James H|last3=Fulford|first3=Angie D|last4=Considine|first4=Robert V|last5=Mattes|first5=Richard D|date=2009-03-01|title=Mastication of almonds: effects of lipid bioaccessibility, appetite, and hormone response|journal=The American Journal of Clinical Nutrition|language=en|volume=89|issue=3|pages=794–800|doi=10.3945/ajcn.2008.26669|pmid=19144727|issn=0002-9165|doi-access=free}}</ref> A 2015 [[systemic review]] found evidence that chewing can decrease self-reported hunger and therefore food intake.<ref>{{cite journal|pmid=26188140 |date=2015 |last1=Miquel-Kergoat |first1=S. |last2=Azais-Braesco |first2=V. |last3=Burton-Freeman |first3=B. |last4=Hetherington |first4=M. M. |title=Effects of chewing on appetite, food intake and gut hormones: A systematic review and meta-analysis |journal=Physiology & Behavior |volume=151 |pages=88–96 |doi=10.1016/j.physbeh.2015.07.017 |s2cid=15350810 |doi-access=free }}</ref> Eating food which does not require chewing, by choice or for medical reasons as [[tooth loss]], is known as a [[soft diet]]. Such a diet may lead to inadequate nutrition due to a reduction in fruit and vegetable intake.<ref>{{cite journal|last1=N'Gom|first1=Papa Ibrahima|last2=Woda|first2=Alain|title=Influence of impaired mastication on nutrition|journal=The Journal of Prosthetic Dentistry|date=June 2002|volume=87|issue=6|pages=667–673|doi=10.1067/mpr.2002.123229|pmid=12131890}}</ref>

Chewing also stimulates the [[hippocampus]] and is necessary to maintain its normal function.<ref>{{cite journal|last1=Chen|first1=Huayue|last2=Iinuma|first2=Mitsuo|last3=Onozuka|first3=Minoru|last4=Kubo|first4=Kin-Ya|title=Chewing Maintains Hippocampus-Dependent Cognitive Function|journal=International Journal of Medical Sciences|date=9 June 2015|volume=12|issue=6|pages=502–509|doi=10.7150/ijms.11911|pmc=4466515|issn=1449-1907|pmid=26078711}}</ref> Chewing stimulates hippocampal [[neurogenesis]] in both humans and mice.<ref>Smith, Natalie, Miquel-Kergoat, Sophie, and Thuret, Sandrine. 'The Impact of Mastication on Cognition: Evidence for Intervention and the Role of Adult Hippocampal Neurogenesis'. 1 Jan. 2015 : 115 – 123.</ref>

==In other animals==
Chewing is largely an adaptation for [[mammal]]ian [[herbivory]]. [[Carnivore]]s generally chew very little or swallow their food whole or in chunks.<ref>{{cite journal|last1=Hiiemae|first1=K.M.|last2=Crompton|first2=A.W.|title=Mastication, Food Transport, and Swallowing|journal=Functional Vertebrate Morphology|date=1985|doi=10.4159/harvard.9780674184404.c14|url=https://www.researchgate.net/publication/275344896}}</ref> This act of gulping food (or medicine pills) without chewing has inspired the English [[idiom]] "wolfing it down".<ref>{{cite web |title=Wiktionary |url=https://en.wiktionary.org/wiki/wolf_down}}</ref>

Other animals such as cows chew their food for long periods to allow for proper digestion in a process known as rumination. Rumination in cows has been shown by researchers to intensify during the night. They concluded that cows chewed more intently in the night time compared to the morning.<ref>{{Cite journal|last1=Zebeli|first1=Q.|last2=Tafaj|first2=M.|last3=Weber|first3=I.|last4=Dijkstra|first4=J.|last5=Steingass|first5=H.|last6=Drochner|first6=W.|date=April 2007|title=Effects of Varying Dietary Forage Particle Size in Two Concentrate Levels on Chewing Activity, Ruminal Mat Characteristics, and Passage in Dairy Cows|journal=Journal of Dairy Science|language=en|volume=90|issue=4|pages=1929–1942|doi=10.3168/jds.2006-354|pmid=17369233|doi-access=free}}</ref>

[[Ornithopoda|Ornithopods]], a group of [[dinosaur]]s including the [[Hadrosaurid]]s ("duck-bills"), developed teeth analogous to mammalian [[Molar (tooth)|molars]] and [[incisor]]s during the [[Cretaceous]] period; this advanced, cow-like dentition allowed the creatures to obtain more nutrients from the tough plant life. This may have given them the advantage needed to compete with the formidable [[Sauropoda|sauropods]], who depended on their massive gastrointestinal tracts to digest food without grinding it.<ref>{{cite journal|last1=Sanders|first1=P. Martin|last2=Clauss|first2=Marcus|title=Sauropod Gigantism|journal=Science|date=10 October 2008|volume=322|issue=5899|pages=200–201|doi=10.1126/science.1160904|pmid=18845734|s2cid=206514245}}</ref>

==In machinery==
[[File:Masticator-usfs image.jpg|thumb|right|Masticator on the [[Zaca Fire]]]]
The process of chewing has, by analogy, been applied to machinery. The [[U.S. Forest Service]] uses a machine called a ''masticator'' (also called a [[forestry mulching|forestry mulching machine]]) to "chew" through brush and timber in order to clear [[fireline]]s in advance of a wildfire.<ref>[http://inciweb.nwcg.gov/incident/pictures/large/770/0/ Masticator shown and described at interagency Inciweb.org]{{Dead link|date=November 2019 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

==See also==
*[[Biting]]
*[[Gnathology]]
*[[Muscles of mastication]]
*[[Horace Fletcher]]
*[[Chewing gum]]

==Notes==
{{reflist}}

==External links==
{{Commons category|Mastication}}
* {{MeSH name|Masticatory Muscles}}

{{Bone/cartilage physiology}}

{{Authority control}}

[[Category:Chewing| ]]
[[Category:Dentistry]]
[[Category:Digestive system]]
[[Category:Articles containing video clips]]