Carbohydrates in fish nutrition:: digestion and absorption in postlarval stages

This review summarizes information regarding digestion and absorption of carbohydrates in cultivated fish. Relevant results of studies of digestive enzymes, e.g. amylase, chitinase, cellulase and brush border disaccharidases are presented. Fish amylases appear to be molecularly closely related and to have characteristics comparable to mammalian amylases. Whether chitinases and cellulases are endogenous enzymes of some fish species is still a matter of speculation, although recent molecular evidence, at least for chitinase seems to settle the issue in favour of endogenous sources. Feed and intestinal microbes may be the source of polysaccharidases in fish feeding on nutrients-containing non-starch polysaccharides. Knowledge regarding monosaccharide transport in fish intestine as interpreted from studies of brush border membrane vesicles, everted sleeves of fish intestinal sections and molecular biology is discussed. Glucose transporters of the intestinal brush border show characteristics similar to those found in mammals. A tabulatory presentation of experimental details and results reported in the literature regarding starch digestibility is included as a basis for discussion. Although numerous investigations on digestion of starch and other carbohydrates in fish have been published, the existing information is highly fragmentary. As yet, it is impossible to derive a cohesive picture on the integrated process of carbohydrate hydrolysis and absorption and interaction with diet composition for any of the fish species under cultivation. The physiological mechanisms behind the species differences are not known.