Chewing

"Chew" redirects here. For other uses, seeChew (disambiguation).

Chewingormasticationis the process by whichfoodiscrushed and groundby theteeth.It is the first step in the process ofdigestion,allowing a greater surface area fordigestive enzymesto break down the foods.

Astump-tailed macaqueusing mastication to process tough plant matter

During the mastication process, the food is positioned by thecheekandtonguebetween the teeth for grinding. Themuscles of masticationmove thejawsto bring the teeth into intermittent contact, repeatedlyoccludingand opening. As chewing continues, the food is made softer and warmer, and the enzymes in saliva begin to break downcarbohydratesin the food. After chewing, the food (now called abolus) is swallowed. It enters theesophagusand viaperistalsiscontinues on to the stomach, where the next step of digestion occurs.Increasing the number of chews per bite increases relevant gut hormones.Studies suggest that chewing may decrease self-reported hunger and food intake.Chewing gumhas been around for many centuries; there is evidence that northern Europeans chewedbirch bark tar9,000 years ago.

Chewing, needing specialized teeth, is mostly a mammalian adaptation that appeared in earlySynapsids,though some later herbivorous dinosaurs, since extinct, had developed chewing too. Nowadays, only mammals chew in the strict sense of the word, though some fishes have a somewhat similar behavior. Neither birds, nor amphibians or any living reptiles chew.

Premasticationis 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(some other animals also premasticate).

Cattleand some other animals, calledruminants,chew food more than once to extract more nutrients. After the first round of chewing, this food is calledcud.

A piece of chewing gum after being trodden on.

Chewing motor program

Awater buffalochewing cud

Chewing is primarily an unconscious (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 fromproprioceptivenerves in teeth and thetemporomandibularjoints 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.This adaptation is a learned skill that may sometimes require relearning to adapt to loss of teeth or to dental appliances such asdentures.

It is thought that conscious mediation is important in the limitation ofparafunctional habitsas 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 toparafunctional habits.

Muscles

Main article:Muscles of mastication

Nutrition and health

Chewing stimulatessalivaproduction and increases sensory perception of the food being eaten, controlling when the food is swallowed.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.A 2015systemic reviewfound evidence that chewing can decrease self-reported hunger and therefore food intake.Eating food which does not require chewing, by choice or for medical reasons astooth loss,is known as asoft diet.Such a diet may lead to inadequate nutrition due to a reduction in fruit and vegetable intake.

Chewing also stimulates thehippocampusand is necessary to maintain its normal function.Chewing stimulates hippocampalneurogenesisin both humans and mice.

In other animals

Chewing is largely an adaptation formammalianherbivory.Carnivoresgenerally chew very little or swallow their food whole or in chunks.This act of gulping food (or medicine pills) without chewing has inspired the Englishidiom"wolfing it down".

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.

Ornithopods,a group ofdinosaursincluding theHadrosaurids( "duck-bills" ), developed teeth analogous to mammalianmolarsandincisorsduring theCretaceousperiod; 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 formidablesauropods,who depended on their massive gastrointestinal tracts to digest food without grinding it.

In machinery

Masticator on theZaca Fire

The process of chewing has, by analogy, been applied to machinery. TheU.S. Forest Serviceuses a machine called amasticator(also called aforestry mulching machine) to "chew" through brush and timber in order to clearfirelinesin advance of a wildfire.

See also

• Biting
• Gnathology
• Muscles of mastication
• Horace Fletcher
• Chewing gum

External links

• Masticatory Musclesat the U.S. National Library of MedicineMedical Subject Headings(MeSH)