|Rawles, Steven - Steve|
Submitted to: Aquaculture Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2007
Publication Date: 2/1/2008
Citation: Rawles, S.D., Smith, S.B., Gatlin, D.M. 2008. Hepatic glucose utilization and lipogenesis of hybrid striped bass (Morone chrysops x M. saxatilis) in response to dietary carbohydrate level and complexity. Aquaculture Nutrition. 14:40-50. Interpretive Summary: Carbohydrates, like milled grains, are an inexpensive feedstuff for livestock production. However, fish do not grow and perform well when fed diets that contain high amounts of carbohydrate. If we could increase the amount of carbohydrate in fish diets, we could significantly decrease the cost of fish feed and improve the competitiveness of the industry. Scientists at USDA/ARS H.K. Dupree Stuttgart National Aquaculture Research Center carried out experiments to define the limit of carbohydrate use in hybrid striped bass. Six diets containing the same amount of protein and energy but different kinds (glucose, maltose, or dextrin-starch) and levels (20% and 40%) of carbohydrates were fed to the fish for 15 weeks. At the end of the feeding, liver samples were taken from the fish and incubated with radioactive carbohydrate and fat to determine how the fish fed the different diets would use the radioactive nutrients. Hybrid striped bass fed diets with complex carbohydrate (maltose or starch) gained more weight than those fed simple carbohydrate (glucose). Feed efficiency did not differ among fish fed the different diets. Fish fed simple carbohydrate (glucose) had larger livers and gained more fat regardless of the level in the diet. Liver glycogen, a form of starch storage, was affected by both the level and complexity of carbohydrate in the diet. The radioactive nutrient experiments showed that hybrid striped bass fed high carbohydrate diets do not use the carbohydrate for energy but rather store it as different products. These data support the idea that the level of carbohydrate in the diet of hybrid striped bass should be limited to 20% or less for best efficiency.
Technical Abstract: The influence of dietary carbohydrate level and complexity on in vitro hepatic glucose utilization and lipogenesis and depots of dietary energy were determined in the most predominately cultured hybrid striped bass, Morone chrysops ' X M. saxatilis '. Six isocaloric, isonitrogenous diets were formulated to contain one of three kinds of carbohydrate (glucose, maltose, and dextrin) at two different levels (20% and 40%) in the diet. After 15 wk of feeding, growth and composition of growth were determined. Fish were killed and liver explants were obtained at near-maximum postprandial glycemic response and incubated with [1 14C]acetate or [U 14C]glucose and [9,10 3H]palmitate to determine glucose utilization and lipogenesis. Weight gains of fish fed the more complex carbohydrates (maltose or dextrin) were greater than those of fish fed GLC at 40% carbohydrate. Feed efficiency was not different among dietary treatments. Compositional indices were less favorable when fish were fed less complex carbohydrate, i.e. hepatosomatic index (HSI) and intraperitoneal fat (IPF) ratio values were greater in fish fed GLC at both 20% and 40% of the diet. Liver lipid was unresponsive to dietary treatments whereas liver glycogen was affected by both carbohydrate level and complexity. Glycogen synthesis in vitro from [14C]glucose was not different among dietary carbohydrate treatments and was less than 14CO2 formation. 14CO2 production increased as a function of carbohydrate level in the diet but was unrelated to carbohydrate complexity. There was no detectable conversion of [14C]glucose to lactate for any treatment. Rates of de novo lipogenesis from [1 14C]acetate were quite high in comparison to [U 14C]glucose or [9,10 3H]palmitate incorporation into liver lipids and differed in response to both carbohydrate level and complexity in the diet. Rates of [9,10 3H]palmitate esterification were an order of magnitude less than glycogen and CO2 production but four to ten times greater than rates of de novo synthesis from glucose. Palmitate incorporation, however, did not differ among treatments. Though lower than palmitate esterification, incorporation of [14C]glucose into liver lipids was higher in fish fed the higher level of carbohydrate. Of the portion of dietary carbohydrate actually sequestered intracellularly, the majority in the liver may be converted to CO2 via pentose cycle/triose phosphate pathway production of reducing equivalents (NADPH) in order to drive lipogenesis. These data support the idea that glucose is not a major oxidative substrate in hybrid striped bass and suggest that the level of soluble carbohydrate in the diet of hybrid striped bass should be limited to 20% or less.