In order to obtain a fuller understanding of the effects of absorbed nutrients on maintenance and growth for the purpose of better evaluating their nutritional value, it is important to know the metabolic pathways of these nutrients in the fish body. These may proceed in two different directions: catabolism and anabolism. In catabolism the relatively large molecules of nutrients absorbed into the blood are broken down and transformed into smaller molecules of simpler compounds. This is done in two stages: (a) transformation of absorbed nutrients into a limited number of intermediate compounds of relatively smaller molecular weight, releasing about one third of the energy in the nutrient as free energy; (b) final oxidation of the intermediate compounds to carbon dioxide and water, releasing the remaining two thirds of the energy. In anabolism, complex constituents of the tissues and body fluids are formed from simple precursors, which are usually the intermediate products of the first stage of catabolism. The rather intricate web involved in the transformation of matter and energy is called ‘intermediate metabolism’. Processes of intermediate metabolism are similar in most animals, though variations may exist. The metabolic pathways of each of the major nutrients – carbohydrates, lipids and proteins – with specific regard to fish, will be discussed here separately, as was done in the case of digestibility (see Chapter 2). It should be borne in mind, however, that these pathways converge at crucial points and should, therefore, be considered as one comprehensive system.