A Comparative Study On Dietary Carbohydrate Utilization By Gibel Carp And Chinese Longsnout Catfish

The present study was to investigate the nutritional function of dietary carbohydrate for fish with different food habits and to compare the difference in mechanism of carbohydrate utilization and metabolism between omnivorous fish and carnivorous fish. Omnivorous gibel carp (Carassius auratus gibelio (Bloch)) and carnivorous Chinese longsnout catfish (Leiocassis longirostris Günther), which are two important aquaculture species in China, were tested. Three 8-week growth trials were carried out under similar experimental conditions, and growth performance, feed efficiency, blood biochemistry, liver characteristic, body composition as well as gluconeogenetic and lipogenic enzymes in fish livers were determined. First, five carbohydrate sources, including glucose, sucrose, dextrin, soluble starch and α-cellulose, were tested to find suitable carbohydrate source for the two fish species. The inclusion level of each carbohydrate source was 6% of dry matter for Chinese longsnout catfish and 20% for gibel carp. Then, the optimal dietary carbohydrate to lipid (CHO:L) ratio for each species was determined at the same dietary gross energy level using corn starch and fish oil as energy sources. The CHO:L ratios tested were 0.75, 1.48, 1.98, 2.99 and 5.07 for Chinese longsnout catfish and 0.79, 1.33, 2.32, 3.70 and 7.51 for gibel carp, respectively. In the last experiment, four starch sources-corn meal, brown rice meal, sticky rice meal and resistant starch product were used to provide equal content of total starch and to determine the effect of starch fractions on fish growth and metabolism. The total starch content was 25.0% in gibel carp diets and 18.0% in Chinese longsnout catfish. The results are shown as follows: 1) The higher specific growth rate (SGR) and feed efficiency (FE) of gibel carp were observed in those fed diets containing soluble starch and cellulose (P<0.05), while dextrin and sucrose for Chinese longsnout catfish (P<0.05). 2) Apparent digestibility coefficient of dry matter (ADCd) and apparent digestibility coefficient of energy (ADCe) in gibel carp decreased significantly as dietary carbohydrate complexity increased (P<0.05), except that ADCd in soluble starch group was higher than other groups (P<0.05). ADCd and ADCe significantly (P<0.05) decreased as dietary carbohydrate complexity increased in Chinese longsnout catfish except that glucose diet had a medium ADCd and ADCe. In both species, no significant difference of apparent digestibility coefficient of protein (ADCp) was observed between different carbohydrate sources. 3) Liver phophoenolpyruvate carboxykinase (PEPCK), pyruvate kinase (PK), glucose 6-phosphate dehydrogenase (G6PD) and malic enzyme (ME) activities also varied according to dietary carbohydrate complexity (P<0.05). Fish with different food habits showed different ability to synthesize liver glycogen, and the liver glycogen content in gibel carp was significantly higher than that in Chinese longsnout catfish. The influence of carbohydrate source on gluconeogenesis and lipogenesis was also different in the two fish species. Chinese longsnout catfish showed high gluconeogenesis and lower lipogenesis to regulate glucose metabolism when given glucose, sucrose, dextrin or soluble starch while gibel carp showed lower gluconeogenesis and high ability ofcarbohydrate utilization by increasing ME activity (except sucrose-based diet). 4) Dietary carbohydrate sources significantly affected fish body composition. Gibel carp fed glucose showed the highest body levels of dry matter, lipid, ash and energy while the lowest crude protein level (P<0.05), and fish fed sucrose showed the lowest body contents of dry matter, protein, lipid, and energy while the highest ash levels (P<0.05). In Chinese longsnout catfish, body contents of dry matter, lipid and gross energy in fish fed cellulose were significantly lower than other groups (P<0.05). Body protein of the fish fed glucose was significantly higher than fish given sucrose diet (P<0.05). 5) Chinese longsnout catfish could not tolerate higher dietary carbohydrate and skeletal malformations was observed at dietary 5.07 CHO:L ratio, while gibel carp could tolerate dietary CHO:L ratio up to 7.51. 6) Gibel carp fed diets of 1.33 CHO:L ratio showed higher SGR and FE (P<0.05), while Chinese longsnout catfish fed diets of 1.98 CHO:L ratio showed higher SGR and FE (P<0.05). 7) Fish body dry matter, lipid, and energy contents significantly increased as CHO:L ratio decreased in the two fish species (P<0.05). Fish with different food habits showed no significant trend in variation of liver characteristic, blood biochemistry and liver histology with increased dietary CHO:L ratio. But pathological changes were observed in liver of Chinese longsnout catfish fed diet of 5.07 CHO:L ratio. 8) In both fish species, ADCd decreased significantly as dietary CHO:L ratio decreased (P<0.05), but ADCp showed no significant difference as dietary CHO:L ratio varied (P>0.05). ACDe significantly decreased in gibel carp as dietary CHO:L ratio decreased (P<0.05), while in Chinese longsnout catfish, ACDe in 1.98 CHO:L ratio group was significantly higher than other groups (P<0.05).9) The optimal carbohydrate to lipid ratio was found to be from 0.79 to 1.33 for gibel carp and 1.98 for Chinese longsnout catfish, respectively. 10) Starch source showed no significant effect on SGR and FE of gibel carp (P>0.05), while fish fed with resistant starch showed significantly higher feeding rate (FR), lower protein retention efficiency (PRE) and energy retention efficiency (ERE) than other groups (P<0.05). In Chinese longsnout catfish, fish fed with sticky rice meal showed significantly higher SGR than those fed with corn meal (P<0.05), while fish fed resistant starch showed significantly higher FR, and lower FE, PRE and ERE than other groups (P<0.05). 11) Gibel carp showed the highest ADCd value in sticky rice meal and brown rice meal groups, followed by corn meal group, and the lowest in resistant starch group (P<0.05), while significantly lower ADCp and ADCe were only observed in resistant starch group (P<0.05). Chinese longsnout catfish only showed significantly lower ADCp in resistant starch group (P<0.05), no variation in ADCd and ADCe values was observed (P>0.05). 12) Only gibel carp fed with resistant starch showed significantly lower body dry matter, lipid and gross energy contents, as well as significantly higher ash contents (P<0.05). Whereas, lower body dry matter and lipid contents in Chinese longsnout catfish fed with resistant starch were significantly only in comparison with sticky rice meal (P<0.05). No significant difference between other groups was observed (P>0.05). 13) Higher resistant starch fraction in diets significantly improved the G6PD activity of gibel carp and the ME activity of Chinese longsnout catfish (P<0.05). In the two fish species fed with resistant starch , some epithelial cells desquamated from the top of the intestine folds,and loosened structure and dropsy were also observed in part of submucosa of intestine. 14) Corn meal, brown rice meal, and sticky rice meal, which showed similardigestible and resistant starch fractions, are suitable for dietary carbohydrate source in gibel carp, but not in Chinese longsnout catfish. In conclusion, dietary carbohydrate source, starch fractions and carbohydrate to lipid ratios had significant effect on fish growth and carbohydrate metabolism. Fish with different food habits showed different response in growth and metabolism to diet treatments.