The process in which food is broken down into usable energy is called digestion. During digestion, the body mechanically and chemically breaks food into smaller substances that can be absorbed and used by cells.
Digestion begins in the mouth, where chewing breaks food apart and saliva starts digesting starches. The food then travels through the esophagus to the stomach, where acids and enzymes help break down proteins. In the small intestine, digestive enzymes from the pancreas and bile from the liver help process fats, carbohydrates, and proteins.
The small intestine absorbs most nutrients, including glucose, amino acids, fatty acids, vitamins, and minerals. These nutrients enter the bloodstream or lymphatic system and are transported around the body. Cells use molecules such as glucose in cellular respiration to release energy in the form of ATP.
Digestion is often confused with absorption or cellular respiration. Absorption is the movement of digested nutrients into the body, while cellular respiration is the later process in which cells extract usable energy from those nutrients. Fermentation is a separate process involving microorganisms or oxygen-limited cells.