<P> Underlying the process is muscle movement throughout the system through swallowing and peristalsis . Each step in digestion requires energy, and thus imposes an "overhead charge" on the energy made available from absorbed substances . Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures . Examples may be seen in humans, who differ considerably from other hominids (lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc .). </P> <P> The major part of digestion takes place in the small intestine . The large intestine primarily serves as a site for fermentation of indigestible matter by gut bacteria and for resorption of water from digests before excretion . </P> <P> In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach . Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches . The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes . Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with defecation . </P> <P> The human gastrointestinal tract is around 9 meters long . Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal, and the process of digestion normally takes between 24 and 72 hours . </P>

Where does the chemical breakdown of food begin