Abstract
An 8-week feeding trial was conducted to investigate the effects of dietary docosahexaenoic to eicosapentaenoic acid ratio (DHA/EPA) on growth performance, fatty acid profiles, antioxidant capacity, hematological characteristics and expression of some lipid metabolism related genes of juvenile black seabream (Acanthopagrus schlegelii) of initial weight 9.47 ± 0.03 g. Five isonitrogenous and isolipidic diets (45% crude protein and 14% crude lipid) were formulated to contain graded DHA/EPA ratios of 0.65, 1.16, 1.60, 2.03 and 2.67. There were no differences in growth performance and feed utilization among treatments. Fish fed higher DHA/EPA ratios had higher malondialdehyde (MDA) contents in serum than lower ratios. Serum triacylglycerol (TAG) content was significantly higher in fish fed the lowest DHA/EPA ratio. Tissue fatty acid profiles reflected the diets despite down-regulation of LC-PUFA biosynthesis genes, fatty acyl desaturase 2 (fads2) and elongase of very long-chain fatty acids 5 (elovl5), by high DHA/EPA ratios. Expression of acetyl-CoA carboxylase alpha (accα) and carnitine palmitoyl transferase 1A (cpt1a) were up-regulated by high DHA/EPA ratio, whereas sterol regulatory element-binding protein-1 (srebp-1) and hormone-sensitive lipase (hsl) were down-regulated. Fatty acid synthase (fas), 6-phosphogluconate dehydrogenase (6pgd) and peroxisome proliferator-activated receptor alpha (pparα) showed highest expression in fish fed intermediate (1.16) DHA/EPA ratio. Overall, this study indicated that dietary DHA/EPA ratio affected fatty acid profiles and significantly influenced lipid metabolism including LC-PUFA biosynthesis and other anabolic and catabolic pathways, and also had impacts on antioxidant capacity and hematological characteristics.
Highlights
The C18 polyunsaturated fatty acids (PUFA) linoleic acid (LA, 18:2n-6) and α-linolenic acid (ALA, 18:3n-3) cannot be synthesized de novo in vertebrates and they are regarded as dietary essential fatty acids (EFA) [1,2]
Dietary docosahexaenoic to eicosapentaenoic acid ratio (DHA/EPA) ratio had no effect on condition factor (CF) or intraperitoneal fat ratio (IPF) (P > 0.05)
Several studies have demonstrated a higher biological value for DHA than for EPA during first feeding of marine fish species such as red seabream (Pagrus major), gilthead seabream (Sparus aurata L.) and turbot (Scophthalmus maximus) [51,52,53,54,55]. These results suggested that n-3 long-chain (C20-24) PUFA (LC-PUFA) requirements might be a function of the total amount of these fatty acids in the diet, and of the relative proportions of essential LC-PUFA like DHA and EPA [56]
Summary
The C18 polyunsaturated fatty acids (PUFA) linoleic acid (LA, 18:2n-6) and α-linolenic acid (ALA, 18:3n-3) cannot be synthesized de novo in vertebrates and they are regarded as dietary essential fatty acids (EFA) [1,2]. Marine carnivorous fish have limited ability to convert LA and ALA into the physiologically important long-chain (C20-24) PUFA (LC-PUFA) such as eicosapentaenoic acid (EPA, 20:5n-3), arachidonic acid (ARA, 20:4n-6) and docosahexaenoic acid (DHA, 22:6n-3), and these compounds must be supplied in their diet to ensure normal growth and development [3]. Specific deficiencies in one or more enzymes, namely fatty acyl desaturases (Fads) with and elongases of very long-chain fatty acids (Elovl), involved in LC-PUFA biosynthesis underpin the abovementioned limitation in the biosynthetic capability of marine fish [1]. Previous studies reported that the absolute requirement for n-3 LC-PUFA decreased with increased dietary DHA/EPA ratio [16,17]. The requirements of dietary DHA/EPA ratios for marine fish has been reported to range from 0.5 to 2.0 according to NRC [3]
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