The utilisation of dietary carbohydrates and their effects on fish metabolism are reviewed. Details on how dietary carbohydrates affect growth, feed utilisation and deposition of nutrients are discussed. Variations in plasma glucose concentrations emphasizing results from glucose tolerance tests, and the impact of adaptation diets are interpreted in the context of secondary carbohydrate metabolism. Our focus then shifts to selected aspects of hormonal regulation of carbohydrate metabolism and dietary carbohydrates and their variable effects on glycogen and glucose turnover. We analyse the interaction of carbohydrates with other nutrients, especially protein and protein sparing, and de novo synthesis of lipids, and finish by discussing the correlation of dietary carbohydrates with fish health.

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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.

During oxygen limitation in animals, glucose can be fermented via several metabolic pathways varying in energetic efficiency and leading to various end products (such as lactate, alanopine, octopine, succinate, or propionate). Because of opposite pH dependencies of proton production by fermentation and by hydrolysis of adenosine triphosphate formed in the fermentation, the total number of moles of protons generated is always two per mole of the fermentable substrate. However, two and three times more adenosine triphosphate can be turned over per mole of protons produced in succinate and propionate fermentations, respectively, than in lactate fermentation.

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