Investigation of fatty acid metabolism of juvenile gilthead sea bream (Sparus aurata) fed diets containing vegetable oil

This study evaluated the effects of total (camelina seed oil, CSO and chia oil, CO) or partial (60%, 60CSO and 60CO) replacement of fish oil with different vegetable oils on growth performance and fatty acid metabolism in juvenile gilthead sea bream (Sparus aurata, 61.5 ± 4.9 g). Five iso-nitrogenous (48% crude protein) and iso-lipidic (18% crude lipid) experimental diets were prepared with different oils and fed to juvenile gilthead sea bream for 90 days. The Whole Body Fatty Acid Balance Method was used to investigate the in vivo fatty acid metabolism in fish. Further, gene expression studies was carried out to determine the effects of the diets on some of the major genes involved in fatty acid metabolism in sea bream. Histological examination of digestive system was performed to assess the effects of the dietary treatments on welfare and health of gilthead sea bream. Growth did not differ (P > 0.05) when juvenile gilthead sea bream were fed the experimental diets. Similarly, condition factor, hepatosomatic index and viscerosomatic index did not differ among experimental groups (P > 0.05). In addition, fish fed CSO and CO diets recorded lower feed intake and feed conversion ratios (P < 0.05). Slight variations in proximate composition of whole body of juvenile gilthead sea bream were recorded. Feeding vegetable oil-based diets reduced body content of linoleic acid (LA, 18:2n-6) whereas this fatty acid increased in liver in comparison to the initial fish. Body and liver contents of linolenic acid (ALA, 18:3n-3) fatty acid and n-3/n-6 ratio were increased when fish were fed CSO and CO-based diets. However, reduced (P < 0.05) body and liver contents of the highly unsaturated fatty acids (HUFA) eicosapentaenoic acid (EPA, 20:5n-3), docosahexaenoic acid (DHA, 22:6n-3) and arachidonic acid (ARA, 20:4n-6) were associated with feeding CSO and CO-based diets. Fish fed CSO diets recorded the highest β-oxidation (P < 0.05). Elongase, ∆-6 desaturase and ∆-9 desaturase activities were highest in CO fed fish (P < 0.05). However, ∆-5 desaturase activity could not be determined when the whole body fatty acid balance method was applied. Bioconversion of dietary LA to 18:3n-6, 20:2n-6 and 20:3n-6 and ALA to 18:4n-3, 20:3n-3 and 20:4n-3 fatty acids was recorded in fish fed CSO and CO-based diets. Expression of the various genes involved in fatty acid synthesis, oxidation, lipolysis, elongation and desaturation (∆6 and ∆9) were analyzed at the end of the experimental feeding. Fish fed CSO, CO and 60CO diets recorded higher expression (P < 0.05) of fatty acyl desaturase 2 (FADS2, ∆-6 desaturase). For stearoyl-CoA desaturase 1b (SCD1b, ∆-9 desaturase), all fish fed CSO and CO-based diets had higher expression (P < 0.05). FO fed fish expressed the lowest expression of the elongases, ELOVL 6, ELOVL 5, ELOVL 4 and ELOVL 1b genes (P < 0.05). CO fed fish had the highest expression for carboxyl ester lipase (CEL), although it did not differ from the levels of expression observed in fish fed the other VO-based diets. Expression of hepatic lipase (HL) gene was highest (P < 0.05) in 60CSO fed fish whereas CO fed fish exhibited the highest (P < 0.05) expression in lipoprotein lipase (LPL) gene. Gene expressions for hormone sensitive lipase (HSL), carnitine palmitoyltransferases (CPT) 1a and cytochrome C oxidase 2 (COX2) were highest in CO fed fish (P < 0.05). Higher expression for CPT 1b gene was observed in fish fed CO and 60CSO diets. For the peroxisome proliferatoractivated receptors (PPARs), PPARα expression was comparatively higher (P < 0.05) in 60CSO, CO and 60CO fed fish. Fish fed CSO and CO-based diets showed higher (P < 0.05) levels of PPARβ expression. For PPARγ, highest level (P < 0.05) of expression was recorded in 60CO fed fish. Gene expression for fatty acid synthase (FAS) and lysophosphatidylcholine acyltransferase 1 (LPCAT1) were highest (P < 0.05) in fish fed 60CO and 60CSO diets, respectively. Based on our results, the total (CSO and CO) or partial (60CSO and 60CO) replacement of dietary FO with CSO or CO did not negatively affect growth in juvenile gilthead sea bream. Further, feed intake and feed conversion ratio decreased when fish were fed FO-devoid diets. This suggests that fish were able to use such diets efficiently. Feeding CSO and CO-based diets led to increased β-oxidation, with dietary ALA being the preferential substrate for β-oxidation. Feeding sea bream diets low in HUFA led to increased in vivo elongase and desaturase (∆-6 and ∆-9) activities. Consequently, this resulted in the appearance of products associated with elongase and desaturase activities in whole body and liver of fish. The expression of genes involved in fatty acid metabolism was higher in fish fed CSO and CO-based diets, corroborating the results of the whole body fatty acid balance method. With limited supply in HUFA, the presence of products of elongation and desaturation of dietary ALA and LA signify attempts by the juvenile gilthead sea bream to alleviate EFA deficiencies in their diets. However, this was not enough to alleviate these deficiencies.