Digested food, particularly proteins, is not fully available to a fish even after it has been absorbed into the blood stream. Amino acids, if used for building new tissue, could be used as absorbed. If amino acids are to be oxidized for energy, however, deamination (removal of the amino group) must occur first - a reaction which requires input of energy. This process, known as specific dynamic action (SDA), can be measured externally in fish as an increase in oxygen consumption beginning soon after ingestion of food followed by an increase in ammonia excretion