Catabolism is the set of metabolic pathways that break down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules into smaller ones. Catabolism provides the chemical energy necessary for the maintenance and growth of cells.
Examples of catabolic processes include glycolysis, the citric acid cycle, the breakdown of muscle protein to use amino acids for the breakdown of fat in adipose tissue to fatty acids. There are many signals that control catabolism. Most of the major signals are hormones and molecules involved in metabolism itself.
Lean body mass represents the body compartment that contains protein. Because critical illness stimulates proteolysis and increased excretion of body nitrogen, this compartment is consistently reduced, not increased. The change in body composition is associated with a loss of body weight, an increase in total body water, and a decrease in body fat.