Carbohydrates in fish nutrition: effects on growth, glucose metabolism and hepatic enzymes
TL;DR: The focus then shifts to selected aspects of hormonal regulation of carbohydrate metabolism and dietary carbohydrates and their variable effects on glycogen and glucose turnover, and the correlation of dietary carbohydrates with fish health.
Abstract: 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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TL;DR: The 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 and it is suggested that new research directions ultimately will lead to a better understanding of these processes.
Abstract: 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.
632 citations
Cites background from "Carbohydrates in fish nutrition: ef..."
...This limit is due to an observed decrease in growth associated with a persistent postprandial hyperglycaemia (Hemre et al. 2002; Moon 2001; Panserat 2009)....
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...…carp, Cyprinus carpio and the Nile tilapia, Oreochromis niloticus are able to use high levels of carbohydrates (50 % in the diet) in contrast to fish at a higher trophic level (the carnivorous fish) which represent the majority of cultured species (Enes et al. 2011; Hemre et al. 2002; Wilson 1994)....
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...After feeding digestible carbohydrates, many fish species (mainly carnivorous) exhibit a persistent postprandial hyperglycaemia, which is associated with a concomitant decrease in growth and sometimes a ‘‘fatty liver’’ (Enes et al. 2011; Hemre et al. 2002; Panserat 2009)....
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...Feeding fish with enriched carbohydrate diets have been primarily studied in cultured species (Hemre et al. 2002; Panserat 2009; Wilson 1994) even though it is generally accepted that fish do not have a specific requirement for dietary glucose (NRC 2011)....
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...Although the only endogenous source of amylase in fish is the cells of the exocrine pancreas (Hemre et al. 2002), amylase can also be produced by the fish gut microbiota (Sugita et al. 1997). a-Amylase has been characterized biochemically in numerous species and cloned in some (Krogdahl et al.…...
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TL;DR: This review summarizes information regarding digestion and absorption of carbohydrates in cultivated fish and reports results of studies of digestive enzymes, e.g. amylase, chitinase, cellulase and brush border disaccharidases, which appear to be molecularly closely related and to have characteristics comparable to mammalian amylases.
Abstract: 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
542 citations
TL;DR: Overall, data strongly suggest that the liver of most fish species is apparently capable of regulating glucose storage, and the persistent high level of endogenous glucose production independent of carbohydrate intake level may lead to a putative competition between exogenous glucose and endogenous glucose as the source of energy.
Abstract: Glucose plays a key role as energy source in the majority of mammals, but its importance in fish appears limited. Until now, the physiological basis for such apparent glucose intolerance in fish has not been fully understood. A distinct regulation of hepatic glucose utilization (glycolysis) and production (gluconeogenesis) may be advanced to explain the relative inability of fish to efficiently utilize dietary glucose. We summarize here information regarding the nutritional regulation of key enzymes involved in glycolysis (hexokinases, 6-phosphofructo-1-kinase and pyruvate kinase) and gluconeogenesis (phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase and glucose-6-phosphatase) pathways as well as that of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. The effect of dietary carbohydrate level and source on the activities and gene expression of the mentioned key enzymes is also discussed. Overall, data strongly suggest that the liver of most fish species is apparently capable of regulating glucose storage. The persistent high level of endogenous glucose production independent of carbohydrate intake level may lead to a putative competition between exogenous (dietary) glucose and endogenous glucose as the source of energy, which may explain the poor dietary carbohydrate utilization in fish.
400 citations
Cites background from "Carbohydrates in fish nutrition: ef..."
...Glucose plays a key role as energy source in the majority of mammals, but its importance in fish appears limited (Wilson 1994; Hemre et al. 2002; Stone 2003)....
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TL;DR: This review intends to put together pieces of the puzzle of dietary carbohydrate utilisation in fish based on new insights gained with respect to the various biological, nutritional and environmental factors influencing carbohydrate use, and promising future approaches to augment carbohydrate use in fish.
337 citations
TL;DR: In fish, despite low muscle protein synthesis rates, the efficiency of protein deposition appears to be high, and research on specific signalling pathways involved in protein synthesis and degradation have been initiated to elucidate the reasons for high dietary protein/amino acid supply required and their utilization.
Abstract: Optimising the amino acid supply in tune with the requirements and improving protein utilization for body protein growth with limited impacts on the environment in terms of nutrient loads is a generic imperative in all animal production systems. With the continued high annual growth rate reported for global aquaculture, our commitments should be to make sure that this growth is indeed reflected in provision of protein of high biological value for humans. The limited availability of fish meal has led to some concerted efforts in fish meal replacement, analysing all possible physiological or metabolic consequences. The rising costs of plant feedstuffs make it necessary to strengthen our basic knowledge on amino acid availability and utilization. Regulation of muscle protein accretion has great significance with strong practical implications. In fish, despite low muscle protein synthesis rates, the efficiency of protein deposition appears to be high. Exploratory studies on amino acid flux, inter-organ distribution and particularly of muscle protein synthesis, growth and degradation and the underlying mechanisms as affected by dietary factors are warranted. Research on specific signalling pathways involved in protein synthesis and degradation have been initiated in order to elucidate the reasons for high dietary protein/amino acid supply required and their utilization.
320 citations
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TL;DR: This review attempts to delineate common themes on the physiological and metabolic roles of cortisol in teleost fishes and to suggest new approaches that might overcome some of the inconsistencies on the role of this multifaceted hormone.
Abstract: Cortisol is the principal corticosteriod in teleost fishes and its plasma concentrations rise dramatically during stress. The relationship between this cortisol increase and its metabolic consequences are subject to extensive debate. Much of this debate arises from the different responses of the many species used, the diversity of approaches to manipulate cortisol levels, and the sampling techniques and duration. Given the extreme differences in experimental approach, it is not surprising that inconsistencies exist within the literature. This review attempts to delineate common themes on the physiological and metabolic roles of cortisol in teleost fishes and to suggest new approaches that might overcome some of the inconsistencies on the role of this multifaceted hormone. We detail the dynamics of cortisol, especially the exogenous and endogenous factors modulating production, clearance and tissue availability of the hormone. We focus on the mechanisms of action, the biochemical and physiological impact, and the interaction with other hormones so as to provide a conceptual framework for cortisol under resting and/or stressed states. Interpretation of interactions between cortisol and other glucoregulatory hormones is hampered by the absence of adequate hormone quantification, resulting in correlative rather than causal relationships.
2,139 citations
TL;DR: It is shown, by means of intraperitoneal implantation of cortisol, that chronic elevation of plasma cortisol levels in the brown trout results in a dose-dependent increase in mortality due to common bacterial and fungal diseases.
Abstract: Basal levels of plasma cortisol in unstressed salmonid fish are normally in the range 0–5 ng ml−1. An acute stress such as handling or 1 h confinement caused a temporary elevation of the plasma cortisol levels of both brown trout,Salmo trutta L., and rainbow trout,Salmo gairdneri Richardson, in the range 40–200 ng ml−1 with a return to basal levels within 24–48 h. The extent of the cortisol elevation in response to an acute stress was dependent upon both the species and strain of trout. Chronic stresses, such as prolonged confinement or crowding, resulted in an elevation of plasma cortisol levels to approximately 10 ng ml−1. Under these circumstances, blood cortisol levels remained elevated for periods of up to 4 weeks before acclimation finally occurred. It is shown, by means of intraperitoneal implantation of cortisol, that chronic elevation of plasma cortisol levels in the brown trout results in a dose-dependent increase in mortality due to common bacterial and fungal diseases. This effect is apparent at plasma cortisol levels as low as 10 ng ml−1, levels below those often reported as being representative of ‘unstressed’ fish. These findings are discussed in relation to the known immunosuppressive effects of corticosteroids in teleost fish.
675 citations
TL;DR: It is suggested that the phenomena of insulin resistance and hyperglycemia might be more readily understood if viewed in the context of underlying abnormalities of lipid metabolism.
Abstract: Despite decades of intensive investigation, the basic pathophysiological mechanisms responsible for the metabolic derangements associated with diabetes mellitus have remained elusive. Explored here is the possibility that traditional concepts in this area might have carried the wrong emphasis. It is suggested that the phenomena of insulin resistance and hyperglycemia might be more readily understood if viewed in the context of underlying abnormalities of lipid metabolism.
649 citations
TL;DR: It is reported here that GLP-1 confers glucose sensitivity to glucose-resistant p-cells, a phenomenon the authors term glucose competence, which is similar to membrane depolarization, the generation of action potentials, and Ca2+ influx, events that are known to trigger insulin secretion.
Abstract: Non-insulin-dependent diabetes mellitus (NIDDM, type 2 diabetes) is a disorder of glucose homeostasis characterized by hyperglycaemia, peripheral insulin resistance, impaired hepatic glucose metabolism, and diminished glucose-dependent secretion of insulin from pancreatic beta-cells. Glucagon-like-peptide-1(7-37) (GLP-1) is an intestinally derived hormone that may be useful for the treatment of NIDDM because it acts in vivo to increase the level of circulating insulin, and thus lower the concentration of blood glucose. This therapeutic effect may result from the ability of GLP-1 to compensate for a defect in the glucose signalling pathway that regulates insulin secretion from beta-cells. In support of this concept we report here that GLP-1 confers glucose sensitivity to glucose-resistant beta-cells, a phenomenon we term glucose competence. Induction of glucose competence by GLP-1 results from its synergistic interaction with glucose to inhibit metabolically regulated potassium channels that are also targeted for inhibition by sulphonylurea drugs commonly used in the treatment of NIDDM. Glucose competence allows membrane depolarization, the generation of action potentials, and Ca2+ influx, events that are known to trigger insulin secretion.
585 citations
TL;DR: The central question of this review is “How can the G6PDH gene be constitutively expressed in some tissues while displaying adaptive regulation in others when there exists a single transcription unit for the gene?”
Abstract: The enzyme, glucose-6-phosphate dehydrogenase (G6PDH, EC1.1.1.49), has long been considered and studied as the archetypical X-linked "housekeeping" enzyme that is present in all cells, where it plays the key role in regulating carbon flow through the pentose phosphate pathway. Specifically, the enzyme catalyzes the first reaction in the pathway leading to the production of pentose phosphates and reducing power in the form of NADPH for reductive biosynthesis and maintenance of the redox state of the cell. It was in this latter function that the crucial importance of the enzyme was first appreciated with the description of the human deficiency syndrome. While the gene can be considered to be a constitutively expressed "housekeeping" gene in many tissues, there are several other tissues (liver, adipose, lung, and proliferating cells) wherein modulation of cellular G6PDH activity represents an important component of the integrated response to external stimuli (hormones, growth factors, nutrients, and oxidant stress). In this regard, adaptive regulation of G6PDH has been found to be exerted at transcriptional and posttranscriptional levels. However, the regulation observed is tissue-specific, which elicits the central question of this review, "How can the G6PDH gene be constitutively expressed in some tissues while displaying adaptive regulation in others when there exists a single transcription unit for the gene?" Future studies utilizing cloned genomic fragments of the human and other mammalian G6PDH genes should provide answers to this question.
520 citations