Fish Nutrition & Feed Technology

Unit 5 - Carbohydrates

5.2.Classification

The carbohydrates can be divided into two major groups according to their chemical structure; the sugars and non-sugars. The simplest sugars are called monosaccharides, and these in turn can be divided into five sub-groups depending on the number of carbon atoms present in the molecule: Trioses (C₃H₆O₃), Tetroses (C₄H₈O₄), Pentoses (C₅H₁₀O₅), and Hexoses (C₆H₁₂O₆). These monosaccharides may also inturn be linked together (with the elimination of water) to form di, tri or polysaccharides containing two, three or more monosaccharide units or residues respectively. Here the term ‘sugar’ is restricted to those carbohydrates containing less than 10 monosaccharide units. Non-sugars are therefore carbohydrates which contain more than 10 monosaccharide units and which do not possess a sweet taste. The non-sugars can be divided into two sub-groups, homopolysaccharides and heteropolysaccharides; the former consisting of identical monosaccharide units and the latter of mixtures of different monosaccharide units.

The carbohydrates have limited role to play in fish nutrition as the ability of liver and tissues to store glycogen is limited. By contrast, juvenile shrimp P. japonicus have been found to contain glucose, acetyl glucosamine and trehalose as the major carbohydrates of body tissue. Fish do not mobilize liver glycogen rapidly when they are starved. In fact it has been shown within starving fish that the oxidation of non-carbohydrate substrates takes precedence over the mobilization and hydrolysis of glycogen. This suggests that the capacity of fish to oxidize glucose aerobically is somewhat limited.

Thus, although carbohydrates may be regarded as non-essential dietary nutrients for fish and shrimp, their inclusion in practical diets is warranted because:

• They represent an inexpensive source of valuable dietary energy for non-carnivorous fish and shrimp species.
• Their careful use in practical diets can spare more valuable protein for growth by way of ‘protein sparing’ action.
• They serve as essential dietary constituents for the manufacture of water stable diets when used as binders,
• Certain carbohydrate sources serve as dietary components which can increase feed palatability and reduce the dust content of finished feeds (cane or beet molasses).