Anabolism, also known as biosynthesis, is the process by which organisms make complex molecules and substances from less complex components. Anabolism leads to growth in organisms as molecules are produced from smaller components and added to bones or muscles. This process of growth requires raw components and energy which are produced through a process known as catabolism, in which large and complex molecules are broken down into simpler substances. Together, anabolic and catabolic processes make up the set of chemical processes known as metabolism, which is the set of life-sustaining reactions that occur in organisms for the maintenance of life.
Anabolic processes are responsible for producing nearly all of the chemicals that organisms are composed of. Processes of anabolism produce proteins, lipids, nucleic acids, and many other substances that are essential to the development, growth, maintenance, and repair of the body. Anabolism is a divergent process, which means that it produces a massive variety of different complex molecules from a relatively low number of simple substances.
Catabolism makes up the other half of metabolism as a whole, and is essential to the proper functioning of anabolism. Catabolic processes, such as digestion, are responsible for breaking large, complex molecules down into simpler forms so that they can be used for raw materials and energy in anabolic processes. While anabolism is responsible for building new molecules, catabolism is responsible for breaking down other molecules. If more anabolic activity than catabolic activity takes place, then net growth occurs, but net loss occurs when the rate of catabolism is higher than the rate of anabolism.
There are many hormones that are related to metabolism; these hormones are typically labeled anabolic or catabolic hormones based on whether they stimulate anabolic or catabolic pathways. Many of these hormones are natural and are completely necessary for proper functioning of the metabolic systems. Others, however, are administered artificially to stimulate growth or to increase strength and energy, legally or illegally. Anabolic steroids, for example, are used therapeutically to treat a variety of different conditions, such as delayed puberty. They increase muscle growth and are closely related to testosterone, so they stimulate many other masculine features such as voice depth and hair growth.
Anabolic steroids have had a controversial history because of their performance-enhancing use by athletes. The use of steroids in sports is generally forbidden around the world. In some nations, anabolic steroids are closely controlled substances.