Digestion is the physiological process by which food is broken down, mechanically and chemically, into particles small enough to pass through the walls of the intestinal tract and into the blood. Once in the bloodstream, these tiny particles can then be distributed throughout the body and used for nourishment. The breaking-down process takes place in almost all parts of the digestive tract, beginning at the mouth and ending, on average, more than fifteen feet later, at the anus.

The digestive process starts as soon as food, taken into the mouth, begins to be chewed into smaller pieces. While being chewed, the food is mixed with saliva that contains the enzyme ptyalin, the first of many enzymes that will help convert complex and indigestible food molecules into smaller and easier-to-absorb ones. When in the mouth, the activity of ptyalin is already at work, converting some of the complex starches...