Catabolism is the part of Metabolism where large complex molecules are broken down into smaller simpler ones. Its opposite is Anabolism where small simple molecules are assembled into large complex ones. Catabolism includes the processes used to release stored energy from carbohydrates, fats, and amino acids. Catabolic processes can go hand in hand with anabolic ones such as when muscle proteins are catabolized to yield their amino acids some of which are later reassembled to make other muscle proteins. Another example is when fat stores are catabolized to yield fatty acids to be used as a source of energy.