Physiology of Finfish and Shellfish (2+1)

4.3.2.5 Specific Dynamic Action (SDA)

Digested food, particularly proteins, is not fully available to a fish even after it has been absorbed into the blood stream. 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.

The proportion of amino acids which get deaminated varies with the food and the fish's circumstances. Fish, which are not growing because of low temperature or have their ration at maintenance level or below, would deaminate most or all of their amino acids. Fish kept at high rearing temperatures or at high activity levels and therefore having very high metabolic rates would do likewise. On the other hand, fish having rapid growth and high protein intake would deaminate a relatively small proportion of their digested protein, although the absolute quantity of amino acids deaminated could still be large enough to produce a relatively large SDA. The energy for deamination need not necessarily come from amino acids, but will be preferentially taken from carbohydrate or lipid, if available. Thus, salmonid aquaculturists long ago discovered this "protein-sparing" action of limited amounts of inexpensive carbohydrate in the diet as a way of reducing the cost of feed and still achieving a desired level of growth. The protein-sparing action of lipids appears to have been minimally investigated. One can thus minimize SDA costs,but not avoid them completely.