The parts of the body that break down food make up the digestive system. Digestion begins in the mouth, which breaks down food physically. Swallowed food then passes to the stomach and intestines, where it is broken down chemically. The intestines also absorb the products of digestion and expel the undigested remains.
Food is a mixture of many different organic compounds, including carbohydrates, fats, and proteins. These compounds are made of long, chainlike molecules that are too big to pass into the blood and enter body tissues. The process of digestion breaks down these large molecules into smaller units that are easy to absorb.
Digestion turns carbohydrates into sugars, proteins into amino acids, and fats into fatty acids.
Digestive organs produce chemicals called enzymes, which break the bonds in food molecules to turn the long chain molecules into smaller units. There are many different enzymes, each one specialized to break down a particular type of food molecule. The enzyme sucrase, for example, breaks down sucrose (table sugar) in the intestines.
The main nutrients in food are proteins, which help the body build and repair tissues; carbohydrates, which provide energy; and fats, which are used for storing energy. A healthy diet should also include plant fiber, which is indigestible but helps the intestines work, as well as essential chemicals called vitamins and minerals, which are needed in small amounts.
A varied mixture of different types of food helps ensure a balanced diet.
Scientists measure the energy in food in calories. The amount of energy you need depends on how active and how old you are. If you eat more calories than you need, the extra energy is stored in your body as fat. Regularly eating too much can make you overweight, which can lead to health problems, especially in later life.