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.

Carbohydrates in fish nutrition: effects on growth, glucose metabolism and hepatic enzymes

Teleost fishes represent a highly diverse group consisting of more than 20,000 species living across all aquatic environments. This group has significant economical, societal and environmental impacts, yet research efforts have concentrated primarily on salmonid and cyprinid species. This review examines carbohydrate/glucose metabolism and its regulation in these model species including the role of hormones and diet. Over the past decade, molecular tools have been used to address some of the downstream components of these processes and these are incorporated to better understand the roles played by carbohydrates and their regulatory paths. Glucose metabolism remains a contentious area as many fish species are traditionally considered glucose intolerant and, therefore, one might expect that the use and storage of glucose would be considered of minor importance. However, the actual picture is not so clear since the apparent intolerance of fish to carbohydrates is not evident in herbivorous and omnivorous species and even in carnivorous species, glucose is important for specific tissues and/or for specific activities. Thus, our aim is to up-date carbohydrate metabolism in fish, placing it to the context of these new experimental tools and its relationship to dietary intake. Finally, we suggest that new research directions ultimately will lead to a better understanding of these processes.

Glucose metabolism in fish: a review

This review summarizes information regarding digestion and absorption of carbohydrates in cultivated fish Relevant results of studies of digestive enzymes, eg 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

Carbohydrates in fish nutrition: digestion and absorption in postlarval stages

Farming of Atlantic salmon (Salmo salar) began in Norway in the late 1960s. During the 1980s and 1990s the production spread, mainly to other northwestern European countries and to Chile. In 1980 annual world production was less than 10,000 t and this increased to 850,000 t whole fish produced in 2000. It is projected to approach 2 million t in 2010. The growth rate of the fish in the farms is continuously improving, due to genetic selection, improved diets and feeding and improved management. The improvement in growth ascribed to genetic selection can be as much as 15–20% per generation if the salmon are from a combined family–phenotype selection programme (Gjedrem, 1983). Recent Canadian results have shown that transgenic salmon, which express growth hormone receptor activity in muscle tissues, have a growth rate 2.6–2.9 times that of the control population, mainly because of increased feed intake (Cook et al., 2000a). Selection or transgenic techniques may also influence the composition of the growth, result in improved feed conversion (Thodesen et al., 1999; Cook et al., 2000a) and change the rate of metabolism (Thodesen et al., 1999; Cook et al., 2000b,c). Economical feed conversions in Norwegian salmon farming have changed from more than 2 kg feed dry matter (DM) kg−1 gain in 1980 to approximately 1.15 kg kg−1 at the end of the 1990s. In salmon the biological feed conversion can vary from less than 0.6 to more than 1.3 kg intake kg−1 gain, and the proportion of protein and energy in diet partitioned into gain can vary from 25 to 60%, depending on the formulation of the diet and the size of the fish. Thus, requirement figures must be based on the requirements of the animal, related to growth, nutrient partitioning and the specific physiological processes in which the nutrient is required. Requirements differ throughout the life cycle. This is especially important in salmon, which undergo physiological changes during smoltification and transfer from a life in fresh water to salt water. In addition to developing the ability to maintain the ion balance in the hyperosmotic salt water, the salmon also

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Atlantic Salmon, Salmo Salar

The use of immunostimulants in fish larval aquaculture.

Five groups of Altantic salmon, Salmo salar L., (80 g postsmolt) were fed moist diets containing increasing levels of starch from 0% to 31% and concomitant decreasing levels of protein. The results showed that feeding a diet containing 22% lipid with no starch or a starch inclusion higher than 22% exerts negative effects on growth and feed utilization. A starch inclusion above 9% resulted in decreased starch digestibility, while protein digestibility was not influenced by the dietary starch content. Undigested starch is suggested to affect lipid digestibility in the same manner as dietary fibre.



The reduced digestibility by high levels of dietary starch led to increased loss of particulate matter to the environment. Taking into account feed utilization and environmental aspects, the present experiment suggests that a diet containing approximately 9% starch is optimal.

Carbohydrate nutrition in Atlantic salmon, Salmo salar L.: growth and feed utilization

This publication reports on analytical data from a large-scale experiment, using 3360 Atlantic salmon, Salmo salar L., distributed into 12 sea cages. Salmon were grown from an average 600 g to an average 3.5 kg. Samples for analyses were taken when fish were in the range of 1.8 and 3.5 kg. Dietary changes between groups were increased starch from 24 to 230 g kg−1 and balanced with protein. The diets were isolipic.



All salmon showed small stores of glycogen in all analysed organs, and only in heart, gills and kidney of large fish (3.5 kg) were the levels correlated with dietary starch. Minor differences between groups were found in liver NADPH production, but with substantially decreased NADPH production per g protein as the fish grew from 1.8 to 3.5 kg, indicating that increasing dietary starch did not lead to induction of liver hexokinase, and that the activity of this enzyme may decline as fish size increases.



An increase in plasma glucose concentrations was found as dietary starch increased, but all levels were moderate and ranged within reference values. Plasma total protein concentrations did not, however, vary according to decreased dietary protein, but increased substantially in all groups as the fish grew from 1.8 to 3.5 kg. Dietary treatments had no influence on haematological parameters, except for decreased haemoglobin concentrations as dietary starch increased in large fish (3.5 kg). No impared liver function was detected, evaluated by activities of ASAT, ALAT and LDH, and by histological analyses. Low serum lysozyme activities were recorded in all groups, and were not correlated with plasma glucose or liver glycogen concentrations.

Effect of gelatinized wheat and maize in diets for large Atlantic salmon (Salmo salar L.) on glycogen retention, plasma glucose and fish health

. The influence of dietary carbohydrate (CHO) on blood chemistry, immunity and disease resistance was studied in two experiments with Atlantic salmon, Salmo salar L. Moist diets with increasing amounts of digestible CHO ranging from 0 to 30% (dry weight) were used. In the first experiment with adult (0.5 kg) fish, blood haemoglobin concentration was negatively correlated with increasing dietary CHO level, while serum glucose and protein did not differ between the groups. Serum cortisol increased linearly in fish fed from 5 to 30% CHO. Serum haemolytic activity was negatively correlated with dietary levels of CHO. Humoral immune responses elicited after vaccination by intraperitoneal injection or by dip immersion with Vibrio salmonicida showed no differences according to diet 10 and 17 weeks post-vaccination. Mortality after challenge with live Aeromonas salmonicida by intraperitoneal injection was lowest in fish fed 10% CHO. In the second experiment with juvenile Atlantic salmon (3g), there were minor differences in body and organ weights. Plasma glucose, protein and cholesterol were elevated in fish fed the highest CHO levels. Fish exposed to immersion challenge with different water concentrations of Vibrio anguillarum showed no statistical differences in mortality. The studies indicate that varying dietary levels of CHO affected immunity and resistance to bacterial infections to a minor extent in Atlantic salmon at low water temperatures during freshwater and seawater stages.

Influence of dietary carbohydrate on blood chemistry, immunity and disease resistance in Atlantic salmon, Salmo salar L.

Individually tagged Atlantic salmon, Salmo salar L., were fed three different adaptation diets containing low, medium or high levels of extruded starch, before intraperitoneal injection of glucose. The injections were done at winter and at summer temperatures. This was to investigate whether the tolerance for glucose varied according to adaption diet and/or according to season. The results showed a temperature-dependent response. At winter temperatures (2°C) the Atlantic salmon had problems adapting to both medium and high levels of dietary starch. This was indicated by lower growth response in the adaptation period, as well as by delayed glucose regulation. At summer temperatures (12.5°C) the Atlantic salmon could tolerate and utilize both medium and high levels of dietary starch. In summer also all groups responded evenly to the tolerance tests with respect to blood glucose regulation. Incorporation of liver glycogen was higher at summer than at winter temperatures in the high- carbohydrate group. No effect was found on haematocrit values as a response to diet or season.

G.I. Hemre

Papers

Carbohydrates in fish nutrition: effects on growth, glucose metabolism and hepatic enzymes

Carbohydrate nutrition in Atlantic salmon, Salmo salar L.: growth and feed utilization

Effect of gelatinized wheat and maize in diets for large Atlantic salmon (Salmo salar L.) on glycogen retention, plasma glucose and fish health

Influence of dietary carbohydrate on blood chemistry, immunity and disease resistance in Atlantic salmon, Salmo salar L.

Glucose tolerance in Atlantic salmon, Salmo salar L., dependence on adaption to dietary starch and water temperature