mastication

thumb|right|250px|A stump-tailed macaque using mastication to process tough plant matter 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 opening and closing. As chewing continues, and the digestive enzymes in saliva (especially amylase and lingual lipase) begin to break down carbohydrates and other nutrients, the food is made softer and warmer, forming a food bolus ready to be swallowed. It enters the esophagus and via peristalsis continues on to the stomach, where the next step of digestion occurs. 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. 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. 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, 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. 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. Chewing gum has been around for many centuries; there is evidence that northern Europeans chewed birch bark tar 9,000 years ago.