Anabolism is the part of metabolism that builds larger molecules. One way of categorizing metabolic processes, whether at the cellular, organ or organism level is as anabolic or catabolic.

Anabolic processes tend toward "building up" organs and tissues. These processes produce growth and differentiation of cells and increase in body size, a process that involves synthesis of complex molecules. Examples of anabolic processes include growth and mineralization of bone and increase of muscle mass.

Catabolism is the part of metabolism that breaks down molecules into smaller units. It is made up of degradative chemical reactions in the living cell. Large polymeric molecules (polysaccharides, nucleic acids and proteins) are processed into their constituent monomeric units (i.e. monosaccharides, nucleotides and amino acids, respectively).

Cells use monomers to construct new polymeric molecules and disassemble them to simple cellular metabolites (lactic acid, acetic acid, carbon dioxide, ammonia, urea, etc.). The creation of cellular metabolites is an oxidation process involving a release of chemical free energy, not all of which is lost as heat, but some of which is partially conserved through the coupled synthesis of adenosine triphosphate. The hydrolysis of this compound is subsequently used to drive almost every energy-requiring reaction in the cell. Catabolism provides the chemical energy necessary for the maintenance of the living cell. Examples of catabolic processes include breakdown of muscle protein in order to use amino acids as substrates for gluconeogenesis and breakdown of fat in adipose to fatty acids.

Because it is counterproductive to have anabolic and catabolic processes occurring in cells simultaneously, there are many signals that switch on anabolic processes while switching off catabolic processes and vice versa. Most of the known signals are hormones and the molecules involved in metabolism itself. Endocrinologists have traditionally classified many of the hormones as anabolic or catabolic.

• Classic anabolic hormones include
  • Growth hormone
  • IGF1 and other insulin-like growth factors
  • Insulin
  • Testosterone
  • Estrogen
• Classic catabolic hormones include
  • Cortisol
  • Glucagon
  • Adrenalin and other catecholamines
  • Cytokines
• Newer hormones associated with the balance of the catabolic and anabolic states include
  • Orexin and Hypocretin (a hormone pair)
  • Melatonin