Digestion is the breakdown of food into particles small enough to cross the cellular barrier of the gastrointestinal (GI) system and be carried around the body in the circulation. This occurs by both mechanical and chemical processes that begin in the mouth and generally end in the small intestine, where 90% of absorption takes place. The other 10% takes place in the stomach and large intestine and often involves the help of the gut microbiota. A small amount of absorption is also thought to take place in the mouth.

Mechanical digestion begins in the mouth with chewing and continues with segmental muscle contractions in the stomach and intestines. Chemical digestion is primarily mediated by enzymes present in the secretions of the salivary glands, stomach, and pancreas, and on the epithelial lining (brush border) of the small intestine (Box 1).

Food comprises four major groups of nutrients: carbohydrates, proteins, lipids and vitamins, each of which has distinct digestion and absorption mechanisms. Water and minerals are also absorbed across the digestive tract and are often coupled with the absorption of other nutrients. Orally and rectally administered drugs are absorbed via similar pathways, depending on their chemical composition.