Carbohydrates in fish nutrition
An overview of what could decide, limit and improve the use of nutritive carbohydrates in fish by Biju Sam Kamalam and Stephane Panserat
arbohydrates are an excellent source of energy and carbon in feed formulations. They can be easily distinguished from the other energy yielding nutrients in terms of their abundance and low price. To illustrate, the collective global production of the major cereal grains i.e., maize, wheat and rice amounted to a colossal 2.5 billion tonnes in the year 2013 (FAO). The total carbohydrate content and the digestible fraction of starch and sugars in these grains can be roughly estimated to be about 2.1 and 1.75 billion tonnes, respectively (www.feedipedia.org). Besides, the unit cost of carbohydrate sources is almost three to five fold less than that of the protein and lipid sources of interest. Therefore, the inclusion level of carbohydrates in commercial fish feed assumes direct economic significance i.e., in terms of lower feed cost per unit weight gain. On the other hand, though not strictly essential in the biological sense, optimal inclusion of dietary carbohydrates is known to increase the retention of protein and lipid in farmed fishes and reduce nitrogen discharge in farm effluents. These are factors that are relevant to the sustainability of any aquaculture operation. Moreover, the presence of carbohydrates in the ingredient mixture during the process of cooking extrusion inevitably helps in pellet binding, stability and floatability. These are characteristics that minimize nutrient leaching and feed wastage. Taken as a whole, carbohydrate is an often underrated but vital cog in the fish feed manufacturing wheel. In the evolving context of fish feed production, it is important to note that increasing amounts of carbohydrates are inadvertently added when competitively priced plant origin ingredients are used to replace expensive and limited marine ingredients. Among the different forms of carbohydrates that are abundant in plant sources, only starch and sugars (energy reserves) have nutritive value in fish nutrition and therefore they will be the focus of this article. Whereas, structural non-starch polysaccharides (fibre) mostly have negative nutritional value and so will not be
discussed further. Farmed fishes have the entire biological machinery of digestive and metabolic enzymes, hormones, glucose transporters and glucose sensing components, which are essential to use glucose as a cellular energy currency. Nevertheless, certain divergence in regulatory mechanism makes them less able to use digestible forms of carbohydrates to meet energy requirements, when compared to other livestock. There are remarkable differences in carbohydrate utilisation between and even within fish species linked to their diverse feeding habits, anatomical features, physiology and rearing habitats. Particularly, farmed carnivorous fishes such as salmon and trout are considered to be less tolerant to carbohydrate rich meals mainly due to slow blood glucose clearance. Consequently, the dietary inclusion level and appropriate source of carbohydrate is decided based on protein sparing without any adverse effect on growth and physiology of the fish. The maximum recommended levels of dietary carbohydrate inclusion fall within 15-25 percent for salmonids and marine fish, while it can go up to 50 percent for herbivorous and omnivorous species (NRC, 2011).
What could decide carbohydrate utilisation in fish?
A complex array of biological, dietary and environmental factors determines the capacity of a fish to use a carbohydrate rich meal (Fig. 1). Among the biological factors, natural feeding habit and the resultant evolutionary adaptation is considered as the primary determinant. For instance, omnivorous and herbivorous fishes like carp, tilapia and catfish are known to have superior amylase activity, intestinal glucose uptake capacity and control of glycaemia as compared to carnivorous trout, salmon and seabass. At the same time, it is important to note that the optimum inclusion level of carbohydrates varies with the cultured size or age of the fish, irrespective of its feeding habit. The existence of genotypic differences within species also remains possible in fish, as shown in terms of glucose tolerance and metabolism in two experimental lines of rainbow trout. Likewise, transgenic salmon with growth hormone gene construct
20 | March | April 2016 - International Aquafeed