Digestion involves mechanical breakdown, solubilization and absorption of nutrients. The nutrient profile of an ingredient may appear good, but, if these nutrients are not digested, absorbed, or utilized, they are of little value to the animal. Digestibility information is essential in evaluating the quality of an ingredient. In terms of dry matter, diets containing purified ingredients high in protein: casein, gelatin, soy protein and wheat gluten, were more digestible than the high carbohydrate diet containing corn starch. This suggests that proteins are more efficiently digested by shrimp than carbohydrates (Akiyama et al., 1992).

Shrimp require energy for growth, muscle activity and reproduction. The biological process of utilizing energy is defined as metabolism, while the rate at which energy is utilized is called the metabolic rate. Metabolic rate in shrimp is influenced by factors such as water temperature, species, age or body size, activity, physical condition and body functions. Other parameters, such as oxygen or carbon dioxide concentration, pH and salinity, also influence metabolic rate (Akiyama et al., 1992).

Shrimp are believed to have a lower dietary energy requirement than terrestrial animals. This lower requirement is due to a number of factors. First, shrimp do not have to maintain a constant body temperature. Second, they require relatively less energy to maintain their position and to move in water as compared to terrestrial animals. Lastly, because they excrete most of their nitrogenous waste as ammonia instead of urea or uric acid, less energy is lost in the protein catabolism and excretion of nitrogenous wastes (Akiyama et al., 1992).

Shrimp will utilize proteins, lipids and carbohydrates as energy sources. As protein is highly digestible by shrimp, excesses will be used for energy. The use of proteins for energy is not economically efficient. Therefore, adequate non-protein energy sources must be maintained in shrimp feeds (Akiyama et al., 1992).