Abstract
Practical diets for commercial barramundi production rarely contain greater than 10% starch, used mainly as a binding agent during extrusion. Alternative ingredients such as digestible starch have shown some capacity to spare dietary protein catabolism to generate glucose. In the present study, a carnivorous fish species, the Asian seabass (Lates calcarifer) was subjected to two diets with the same digestible energy: Protein (P) – with high protein content (no digestible starch); and Starch (S) – with high digestible (pregelatinized) starch content. The effects of a high starch content diet on hepatic glycogen synthesis as well as the muscle and liver metabolome were studied using a complementary approach of 1H and 2H NMR. The hepatosomatic index was lower for fish fed high starch content diet while the concentration of hepatic glycogen was similar between groups. However, increased glycogen synthesis via the direct pathway was observed in the fish fed high starch content diet which is indicative of increased carbohydrate utilization. Multivariate analysis also showed differences between groups in the metabolome of both tissues. Univariate analysis revealed more variations in liver than in muscle of fish fed high starch content diet. Variations in metabolome were generally in agreement with the increase in the glycogen synthesis through direct pathway, however, this metabolic shift seemed to be insufficient to keep the growth rate as ensured by the diet with high protein content. Although liver glycogen does not make up a substantial quantity of total stored dietary energy in carnivorous fish, it is a key regulatory intermediate in dietary energy utilization.
Highlights
Aquaculture production of carnivorous fish continues to increase worldwide, with barramundi Lates calcarifer one such species that is widely cultured in Southeast Asia and Australia (Glencross et al, 2013)
The hepatosomatic index (HSI) was lower in fish fed with the Starch diet (Protein 1.88 ± 0.05 vs. Starch 1.30 ± 0.07; t-test p < 0.001), while total hepatic glycogen content did not differ between diets (Protein: 7.5 ± 0.5 vs. Starch: 6.9 ± 0.3 g 100 g−1 liver; t-test p > 0.05)
This decrease in indirect pathway contributions was proportionally distributed between Krebs cycle sources, which fell from 42% to 15%, and Triose-P sources such as glycerol, which decreased from 9% to 4%
Summary
Aquaculture production of carnivorous fish continues to increase worldwide, with barramundi Lates calcarifer one such species that is widely cultured in Southeast Asia and Australia (Glencross et al, 2013). 2017), and a range of terrestrial animal and plant-derived raw ingredients have been evaluated as a partial replacement for fishmeal protein in aquafeeds (Gatlin et al, 2007; Glencross et al, 2012; Glencross B. et al, 2017; Turchini et al, 2019). Digestible starch has been demonstrated to be redirected into lipogenesis (Viegas et al, 2019; Williams et al, 2001) the precise ratelimiting steps remain uncharacterized Without this knowledge, the practical replacement of dietary protein will be limited to indigestible starch sources such as barley, wheat, oats, sorghum, and triticale among others (Glencross et al, 2012). The purpose of this study was to assess the metabolic limitations of barramundi that underlie poor utilization of highly digestible starch
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