Catabolism is the set of metabolic processes that break down complex molecules into simpler ones, releasing energy in the process. Digestion breaks food into absorbable units, and inside cells the catabolism of glucose, fatty acids, and amino acids feeds the citric acid cycle and electron transport chain to produce ATP, the cell’s energy currency. It is the destructive half of metabolism, balanced by anabolism, which builds molecules up. During fasting, illness, or intense exercise, the body raises catabolism of stored glycogen, fat, and eventually muscle protein. Hormones such as cortisol, glucagon, and adrenaline drive the response.