Chewing

Chewing or mastication is the process by which food is crushed and ground by the teeth. It is the first step in the process of digestion, allowing a greater surface area for digestive enzymes and bile 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 jaws to bring the teeth into intermittent contact, repeatedly occluding and opening. As chewing continues, the food is made softer and warmer, and the enzymes in saliva (especially amylase and lingual lipase) begin to break down carbohydrates and other nutrients in the food. After chewing, the food (now called a bolus) is swallowed. It enters the esophagus and via peristalsis continues on to the stomach, where the next step of digestion occurs.^[1] 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.^[2][3][4] Studies suggest that thorough chewing may facilitate digestion and nutrient absorption, improve cephalic insulin release and glucose excursions, and decrease food intake and levels of self-reported hunger.^[2][4] More thorough chewing of foods that are high in protein or difficult to digest such as nuts, seeds, and meat, may help to release more of the nutrients contained in them,^[5][6] whereas taking fewer chews of starchy foods such as bread, rice, and pasta may actually help slow the rate of rise in postprandial glycemia by delaying gastric emptying and intestinal glucose absorption.^[7][8] However, slower rates of eating facilitated by more thorough chewing may benefit postprandial glucose excursions by enhancing insulin production and help to curb overeating by promoting satiety and GLP-1 secretion.^[9] Chewing gum has been around for many centuries; there is evidence that northern Europeans chewed birch bark tar 9,000 years ago.

Mastication, as it requires specialized teeth, is mostly a mammalian adaptation that appeared in early Synapsids, 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^[10] (some other animals also premasticate).

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