Link between lipid metabolism and voluntary food intake in rainbow trout fed coconut oil rich in medium-chain TAG

Effect of Dietary Supplementation of Glycerol Monolaurate on Growth Performance, Digestive Enzymes, Serum Immune and Antioxidant Parameters, and Intestinal Morphology in Black Sea Bream

An eight-week feeding trial was conducted to examine the impact of dietary supplementation with glycerol monolaurate (GML) on juvenile black sea bream. A basal diet was formulated containing 24% fish meal, while five additional diets were prepared, each supplemented with varying levels of GML: GML1 (0.01%), GML2 (0.02%), GML3 (0.04%), GML4 (0.08%), and GML5 (0.16%). Triplicate tanks were randomly allocated to each diet, each containing 20 fish with an initial weight of 1.55 ± 0.05 g. By the trial's end, the GML3 group displayed a notably higher final body weight (FBW), weight gain (WG), specific growth rate (SGR), and protein efficiency ratio (PER) compared to the other groups (p < 0.05), but the FCR was significantly higher in the control group. However, no significant differences were observed in the MFI, PPV, CF, HSI, IPF, VSI, or SR among the groups (p > 0.05). Regarding the proximate compositions of the dorsal muscle and whole body, no substantial differences were observed across the groups (p > 0.05). Additionally, there were no significant variations in digestive enzyme activity (p > 0.05), serum immune, or biochemical parameters in the midgut and hindgut among the treatment groups. But in the serum immune response IgM, C3 and C4 were significantly higher in the GML3 group as compared to the other groups (p < 0.05). However, the GML3 group exhibited significantly greater fore-intestinal villus height, crypt depth, villus height per crypt depth, and the number of goblet cells per villus compared to the other groups (p < 0.05). Overall, GML supplementation, particularly GML3, significantly improved growth indicators like the final body weight and intestinal morphology. While certain parameters remained unaffected, these findings suggest GML's potential as a beneficial dietary supplement in fish diets.

Dietary High Levels of Coconut Oil Replacing Fish Oil Did Not Affect Growth, but Promoted Liver Lipid Deposition of Orange-Spotted Groupers (Epinephelus coioides)

In this study, we conducted an 8-week feeding trial to investigate the effects of replacing fish oil (FO) with coconut oil (CO) on the growth performance, blood components, tissue fatty acid (FA) profile, and mRNA levels of genes related to lipid metabolism in the liver of the orange-spotted grouper (Epinephelus coioides). Five isolipidic and isoproteic diets were formulated through increasing the CO levels (0, 25%, 50%, 75%, and 100%, respectively). Triplicate groups of twenty-five fish (initial wet weight of about 22.4 g/fish) were fed one of the diets twice daily to apparent satiety. The 25% CO diet had the highest growth rate and feed utilization, and the 100% CO diet exhibited a comparable growth and feed utilization with that of the control diet, indicating a suitable FO substitute. Moreover, the hepatosomatic index, intraperitoneal fat rate, liver lipid content, as well as the serum HDL-C content and ALT activity had positive linear and/or quadratic responses, but the serum TC and LDL-C contents exhibited the opposite trend, with an increasing CO inclusion level. The FA profile in the liver and muscle generally mirrored the FA profile in the feed. Furthermore, the mRNA levels of the fas, acc, g6pd, srebp-1c, and δ6fad genes in the liver had positive linear and/or quadratic responses, but the mRNA levels of elovl 4 and elovl 5 had the opposite trend, with increasing dietary CO inclusion levels. When compared with the control diet, 25% and 50% CO diets up-regulated the mRNA levels of cpt 1, while the 75% and 100% CO diets down-regulated its mRNA levels. The hsl and atgl were down-regulated through the addition of dietary CO. The mRNA level of lpl was not affected by dietary treatments. Results showed that CO could completely replace FO without affecting growth performance, but high CO will lead to the significant liver lipid deposition and lower LC-PUFAs contents of fish flesh.

Long-term regulation of fat sensing in rainbow trout (Oncorhynchus mykiss) fed a vegetable diet from the first feeding: focus on free fatty acid receptors and their signalling

Taste plays a fundamental role in an animal’s ability to detect nutrients and transmits key dietary information to the brain, which is crucial for its growth and survival. Providing alternative terrestrial ingredients early in feeding influences the growth of rainbow trout (RT, Oncorhynchus mykiss). Thus, the present study aimed to assess the influence, via long-term feeding (from the first feeding to 8 months), of alternative plant ingredients (V diet for vegetable diet v. C diet for a control diet) in RT on the mechanism of fat sensing at the gustatory level. After the feeding trial, we studied the pathways of the fat-sensing mechanism in tongue tissue and the integrated response in the brain. To this end, we analysed the expression pattern of free fatty acid receptors (ffar) 1 and 2, markers of calcium-signalling pathways (phospholipase Cβ, Orai, Stim or Serca), the serotonin level (a key neurotransmitter in taste buds) and the expression pattern of appetite-regulating neuropeptides in the hypothalamus (central area of appetite regulation). The results revealed that the V diet modified the expression pattern of ffar1 and paralogs of ffar2 genes in tongue tissue, along with differential regulation of calcium-signalling pathways and a defect in serotonin level and brain turnover, without influencing neuropeptide expression. This study is the first to support that changes in feeding behaviour of RT fed a V diet could be due to the difference in nutrient sensing and a decrease in hedonic sensation. We revealed that RT have similar fat-detection mechanisms as mammals.

Underestimated health risks: Dietary restriction magnify the intestinal barrier dysfunction and liver injury in mice induced by polystyrene microplastics

Microplastics (MPs) have gained significant attention due to their widespread presence in the environment. While studies have been conducted to investigate the risks associated with MPs, the potential effects of MPs on populations with varying dietary habits, such as dietary restriction (DR), remain largely undefined. The sensitivity of the body to invasive contaminants may increase due to insufficient food intake. Here, we aimed to investigate whether dietary restriction could affect the toxicity of MPs in mice. Following a 5-week exposure to 200 μg/L polystyrene microplastics (PSMPs), DR-PSMPs treatment group exhibited significant intestinal barrier dysfunction compared to ND-PSMPs treatment group, as determined by histopathological and biochemical analysis. Dietary restriction worsened liver oxidative stress and bile acid disorder in mice exposed to PSMPs. 16S rRNA sequencing analysis revealed that DR-PSMPs treatment caused alterations in gut microbiota composition, including the downregulation of probiotics abundance and upregulation of pathogenic bacteria abundance. The negative effects caused by PSMPs in mice with dietary restriction could attribute to increased MPs bioaccumulation, declined water intake, reduced probiotics abundance, and elevated pathogenic bacteria abundance, as well as the susceptibility of the dietary restriction individual. Our findings hint that the biological effects of contaminants could be affected by dietary habits.

Chemical and Physical Properties of African Catfish (Clarias gariepinus) Fillet Following Prolonged Feeding with Insect Meal-Based Diets

A 25-week experiment was undertaken to explore the effect of partial replacement of dietary fishmeal (FM) with black soldier fly meal (Hermetia illucens) (BS), mealworm meal (Tenebrio molitor) (MW), and a 1 : 1 mixture of both insect meals (BSMW) on fillet quality in African catfish (Clarias gariepinus). A total of 96 fish with an average initial body weight of 248 ± 28 g were stocked into a recirculating aquaculture system and fed in four different dietary groups (control, BS, MW, and BSMW). No mortality was recorded in any of the groups. At the end of the feeding period, 24 fish (n = 6 for each treatment, weight between 690 and 822 g) were used for analysis. There was no alteration in filleting yield or other slaughter indices within experimental groups, except the hepatosomatic index. Among quality attributes, pH 24 hr postmortem exhibited a significant difference (p < 0.05). In respect of the fatty acid profile, the n-6/n-3 ratio ranged between 1.17 and 1.40 but was not significantly modified by the partial replacement of FM. Similarly, the proximate composition of the fillets was not significantly different between the control and experimental diet groups. The ratio of polyunsaturated fatty acid to saturated fatty acids ranged between 0.67 and 0.79 in the fillets, without significant differences between groups. The atherogenic index was increased in the BS group, as compared to the others; however, the thrombogenicity index of fillets was not significantly affected. Similarly, the conventional quality traits of the fillet, such as cooking, drip, and thawing losses, did not differ within treatments. This study demonstrates that the dietary inclusion of black soldier fly and/or mealworm meals used for African catfish at the tested inclusion level has negligible impact on fillet properties.

A new perspective on endocrine disrupting effects of triphenyltin on marine medaka: From brain transcriptome, gut content metabolome and behavior

Triphenyltin (TPT) is an endocrine contaminant that is often detected in the environment. However, the mechanism of the effects of TPT on biological systems is not fully understood. Here we exposed marine medaka (Oryzias melastigma) to TPT for 21 days. Brain transcriptome, intestinal content metabolism group, and behavior analysis were carried out. Through the comprehensive analysis of multiomics for the in-depth understanding of the ways related to health improvement, we determined that the glycine-serine-threonine metabolic axis was most perturbed by TPT. Through behavioral analysis, it was found that there was behavioral hyperactivity in the exposed group; behavioral hyperactivity may be caused by the interference of TPT with the neuroendocrine system. In order to gain a full understanding of the impacts of TPT on human health, transcriptomic screening of differential genes and an impartial attitude based on bioinformatics were used. Gene-disease interaction analysis using the Comparative Toxicogenomics Database (CTD) revealed the possible effects of TPT on human health. Finally, based on these findings, the relevant adverse outcome pathway (AOP), which is the "epigenetic modification of PPARG leading to adipogenesis," was identified from AOP Wiki. Further research is required to validate the potential AOP of TPT.

Production of rainbow trout (Oncorhynchus mykiss) using black soldier fly (Hermetia illucens) prepupae-based formulations with differentiated fatty acid profiles

The aquaculture sector is expanding rapidly and needs an increasing supply of fishery products. To ensure an ecological transition of this sector, alternative feed ingredients are required for fish nutrition. Potential alternatives include insects, particularly the black soldier fly (BSF, Hermetia illucens L. 1758), which is being increasingly targeted for their nutritional qualities and their sustainable production practices. BSF have a well-balanced amino acid profile; however, their fatty acid profile is not sufficiently balanced for most aquafeed formulations but can be modulated through their feed. In this study, two different batches of BSF prepupae (BSFP) were firstly produced: BSFP with a standard ω3 content (C18:3n-3 ≈ 1.36%) and ω3-enriched BSFP (C18:3n-3 ≈ 9.67%). Then, three isoproteic, -lipidic and -energetic trout feeds were formulated and produced: one control and two feeds containing 75% BSF meal as a substitute for fish meal (standard vs ω3-enriched-BSF). Finally, a trout feeding trial (n = 3 for each feed batch) in a recirculating aquaculture system was carried out for 75 days. BSFP meal inclusion in trout diets did not impact most nutritional and growth parameters of trout compared to the control; however, the coefficient of fatness increased, weight gain decreased and fatty acid profiles of fillets were altered. In conclusion, this study presents a more sustainable model of trout production by including insects from bioconversion of local byproducts in aquafeed.

The effects of dietary saturated fat source on weight gain and adiposity are influenced by both sex and total dietary lipid intake in zebrafish

The effects of saturated fat intake on obesity and cardiovascular health remain inconclusive, likely due in part to their varied nature and interactions with other nutrients. Investigating the synergistic effects of different saturated fat sources with other dietary lipid components will help establish more accurate nutritional guidelines for dietary fat intake. Over the past two decades, zebrafish (Danio rerio) have been established as an attractive model system to address questions regarding contributions of dietary lipid intake to diet-induced obesity in humans. The goal of the present study was to assess interactions of three different saturated fat sources (milk fat, palm oil, and coconut oil) with sex and total dietary lipid intake on weight gain and body composition in adult zebrafish. Larvae were raised on live feeds until 28 days post fertilization, and then fed a formulated maintenance diet until three months of age. An eight-week feeding trial was then initiated, in which zebrafish were fed nine experimental low- and high-fat diets varying in saturated fatty acid and long-chain polyunsaturated fatty acid content, in addition to a low-fat and high-fat control diet. At termination of the feeding trial, each treatment was evaluated according to body mass, moisture content, and adiposity. Sex and diet significantly interacted in their effects on body mass (P = 0.026), moisture content (P = 0.044), and adiposity (P = 0.035). The influence of saturated fat source on body mass was observed to be dependent on intake of total dietary lipid. In females, all three saturated fat sources had similar effects on adiposity. From these observations, we hypothesize that impacts of saturated fat intake on energy allocation and obesity-related phenotypes are influenced by both sex and intake of other dietary lipid components. Our results suggest that current nutritional guidelines for saturated fat intake may need to be re-evaluated and take sex-specific recommendations into consideration.

Growth of Yellowtail (Seriola quinqueradiata) Fed on a Diet Including Partially or Completely Defatted Black Soldier Fly (Hermetia illucens) Larvae Meal

Simple Summary

Insects which can be reared artificially, such as the black soldier fly, housefly or yellow mealworm are considered as promising feed sources for sustainable aquaculture. The present study is the first to reveal the potential of diets containing insect meal for juvenile yellowtail. The growth of fish fed diets in which fish meal was replaced by 10–30% partially defatted black soldier fly larvae meal was decreased in accordance with the content of the larvae meal. On the other hand, growth of fish with a diet including 20% completely defatted larvae meal was equivalent to that with a diet of the partially defatted larvae meal. Thus, the fat fraction of black solider fly larvae could cause growth retardation of yellowtail, and the defatting process of the insect meal may be important in the manufacture of black soldier fly larvae meal for yellowtail.

Abstract

Against a background of increased demand for fish meal (FM), black soldier fly larva is a promising alternative feed source for sustainable aquaculture. Yellowtail, the most popular farmed fish in Japan, is a carnivorous fish; therefore, it requires a high proportion of FM in its diet. This study represents the first example of yellowtail fed on a diet including insect meal as a replacement for FM. Partially defatted black soldier fly meal (PDBM) comprised 49.0% crude protein and 23.2% crude fat, while completely defatted black soldier fly meal (CDBM) contained less than 10% crude fat, as the same level as FM was achieved with defatting PDBM using hexane. In feeding trials, growth of the fish was reduced in accordance with PDBM content: 10%, 20%, and 30% in their diet. Although a diet including 8% CDBM (with the same protein composition as 10% PDBM) also resulted in decreased fish growth, growth with a diet including 16% CDBM (with the same protein composition as 20% PDBM) was significantly higher than that of 20% PDBM, and equivalent to that of 10% PDBM. Therefore, even 10% of partially or completely black soldier fly larvae meal in diets inhibited growth in juvenile yellowtail, and we found that removal of the fat fraction could improve fish growth.

The influence of coconut oil on the growth, immune, and antioxidative responses and the intestinal digestive enzymes and histomorphometry features of Nile tilapia (Oreochromis niloticus)

The trials of finding non-conventional and alternative aquafeed ingredients are increasing. In this sense, this study evaluated the influence of coconut oil on the growth, feed utilization, immune, and antioxidative responses of Nile tilapia. Five test diets were formulated by mixing coconut oil with the other ingredients at 0, 1, 2, 3, and 4% of the total ration and presented for tilapia for 60 successive days. The final weight, SGR, weight gain (WG), and feed intake were superior in fish delivered 2% of coconut oil (P < 0.05). Concurrently, fish that received 2% coconut oil had lower FCR and higher PER than fish of the control and 4% groups (P < 0.05). Higher lipase activity was observed in fish of 2% and 3% levels than the remaining groups (P < 0.05). Besides, the amylase and protease activities of fish in 1%, 2%, and 3% groups were higher than the 0% level (P < 0.05). The total blood cholesterol, RBCs, and PCV showed higher values in Nile tilapia fed 2% and 3% coconut oil (P < 0.05). The lysozyme and phagocytic activities were higher in fish fed 2% and 3% levels than the control (P < 0.05), while the phagocytic index in 2% and 3% levels was higher than 0% and 4% levels. Furthermore, SOD and CAT were higher in fish fed 1%, 2%, and 3% than fish fed 0% and 4% levels while GSH was higher in fish of 1%, 2%, and 3% than fish fed 0% level (P < 0.05). However, the MDA level was markedly lower in fish fed 25, 3%, and 4% coconut oil than the 0% level (P < 0.05). The intestine's histological structure in all groups appeared normal, forming of intestinal villi projecting from the intestinal wall. Also, the structure of the hepatopancreas had a normal architecture in all groups. To sum up, the inclusion of coconut oil at 2 to 3% is recommended as a replacer for fish oil in Nile tilapia diets.

Transcriptome sequencing and metabolite analysis reveal the toxic effects of nanoplastics on tilapia after exposure to polystyrene

Plastic particles, which are formed from routinely used plastics and their fragments, have become a new pollutant raising widespread concern about their potential effects. Several studies have been conducted to examine their toxicity, but the effects of nano-sized plastic fragments on freshwater organisms remain largely unclear and need to be further investigated. In this study, larval tilapia were first exposed to 100 nm polystyrene nanoparticles (PS-NPs, 20 mg/L) for seven days and then returned to freshwater without PS-NPs for another seven days in order to determine the toxic effects of PS-NPs at both transcriptomic and metabolomic levels. A total of 203 significantly changed metabolites, and 2,152 differentially expressed unigenes were identified between control and PS-NP treatment groups, control and recovery groups, as well as treatment and recovery groups. Our data suggested that PS-NPs induced abnormal metabolism of glycolipids, energy, and amino acids in tilapia after short-term exposure. Additionally, PS-NPs caused disturbed signaling, as suggested by the transcriptomic results. Different transcriptomic and metabolomic levels between the treatment group and recovery group indicated a persistent impact of PS-NPs on tilapia. The presence of adhesion molecule-related differentially expressed genes (DEGs) suggested that PS-NPs might cause early inflammatory responses. Notably, the detection of chemical stimulus involved in the sensory perception of smell was the most severely impacted biological process. Our work systemically studied the ecotoxicity of nano-sized plastics in aquatic creatures at the molecular and genetic levels, serving as a basis for future investigations on the prevention and treatment of such pollutants.

Schizochytrium sp. (T18) Oil as a Fish Oil Replacement in Diets for Juvenile Rainbow Trout (Oncorhynchus mykiss): Effects on Growth Performance, Tissue Fatty Acid Content, and Lipid-Related Transcript Expression

In this study, we evaluated whether oil extracted from the marine microbe, Schizochytrium sp. (strain T18), with high levels of docosahexaenoic acid (DHA), could replace fish oil (FO) in diets for rainbow trout (Oncorhynchus mykiss). Three experimental diets were tested: (1) a control diet with fish oil (FO diet), (2) a microbial oil (MO) diet with a blend of camelina oil (CO) referred to as MO/CO diet, and (3) a MO diet (at a higher inclusion level). Rainbow trout (18.8 ± 2.9 g fish-1 initial weight ± SD) were fed for 8 weeks and evaluated for growth performance, fatty acid content and transcript expression of lipid-related genes in liver and muscle. There were no differences in growth performance measurements among treatments. In liver and muscle, eicosapentaenoic acid (EPA) was highest in trout fed the FO diet compared to the MO/CO and MO diets. Liver DHA was highest in trout fed the MO/CO diet compared to the FO and MO diets. Muscle DHA was highest in trout fed the MO and MO/CO diets compared to the FO diet. In trout fed the MO/CO diet, compared to the MO diet, fadsd6b was higher in both liver and muscle. In trout fed the FO or MO/CO diets, compared to the MO diet, cox1a was higher in both liver and muscle, cpt1b1a was higher in liver and cpt1a1a, cpt1a1b and cpt1a2a were higher in muscle. Schizochytrium sp. (T18) oil was an effective source of DHA for rainbow trout.

Low-Dose Coconut Oil Supplementation Induces Hypothalamic Inflammation, Behavioral Dysfunction, and Metabolic Damage in Healthy Mice

SCOPE

Coconut oil (CO) diets remain controversial due to the possible association with metabolic disorder and obesity. This study investigates the metabolic effects of a low amount of CO supplementation.

METHODS AND RESULTS

Swiss male mice are assigned to be supplemented orally during 8 weeks with 300 µL of water for the control group (CV), 100 or 300 µL of CO (CO100 and CO300) and 100 or 300 µL of soybean oil (SO; SO100 and SO300). CO led to anxious behavior, increase in body weight gain, and adiposity. In the hypothalamus, CO and SO increase cytokines expression and pJNK, pNFKB, and TLR4 levels. Nevertheless, the adipose tissue presented increases macrophage infiltration, TNF-α and IL-6 after CO and SO consumption. IL-1B and CCL2 expression, pJNK and pNFKB levels increase only in CO300. In the hepatic tissue, CO increases TNF-α and chemokines expression. Neuronal cell line (mHypoA-2/29) exposed to serum from CO and SO mice shows increased NFKB migration to the nucleus, TNF-α, and NFKBia expression, but are prevented by inhibitor of TLR4 (TAK-242).

CONCLUSIONS

These results show that a low-dose CO changes the behavioral pattern, induces inflammatory pathway activation, TLR4 expression in healthy mice, and stimulates the pro-inflammatory response through a TLR4-mediated mechanism.

Plasma biochemistry, gene expression and liver histomorphology in common carp (Cyprinus carpio) fed with different dietary fat sources

Demand for omega-3 long chain polyunsaturated fatty acids has become global challenge for aquaculture and different components have been used to increase nutritional value of fillets. The aim of this study was to evaluate influences of feeds on zootechnical parameters, biochemical plasma parameters, expression of lipid-dependent genes, hepatocyte histomorphologies, and fatty acid profiles in common carp fillets. We compared a control diet (CTRL), mimicking a commercial feed formulation for common carp, with three diets containing blends of vegetable oils and a DHA-rich alga (Schizochytrium sp.) included at 3.125% (CB1) or 1.563% (CB2), and 2.1% salmon oil (CB3). The study revealed no differences in final body weight of fish fed CB1-3 diets in comparison with significantly lower CTRL. Concentrations of all biochemical parameters in plasma increased gradually in fish fed CB1-3 diets when compared to CTRL diet, with exception of triacylglycerol levels. Expression of hepatic fas, elovl-5a and pparα genes increased significantly in fish fed CB1 and CB2. Additionally, eicosapentaenoic (EPA) and docosahexaenoic acid (DHA) accumulation in muscle tissue was directly proportional to the amounts supplied in the diets. Our study revealed that carp fillet profiles can be manipulated for DHA and EPA-contents using enriched diets, depending on the source of fat.

Dose-response relationship between dietary choline and lipid accumulation in pyloric enterocytes of Atlantic salmon (Salmo salar L.) in seawater

Foamy, whitish appearance of the pyloric caeca, reflecting elevated lipid content, histologically visible as hypervacuolation, is frequently observed in Atlantic salmon fed high-plant diets. Lipid malabsorption syndrome (LMS) is suggested as term for the phenomenon. Earlier studies have shown that insufficient supply of phospholipids may cause similar symptoms. The objective of the present study was to strengthen knowledge on the role of choline, the key component of phosphatidylcholine, in development of LMS as well as finding the dietary required choline level in Atlantic salmon. A regression design was chosen to be able to estimate the dietary requirement level of choline, if found essential for the prevention of LMS. Atlantic salmon (456 g) were fed diets supplemented with 0, 392, 785, 1177, 1569, 1962, 2354, 2746 and 3139 mg/kg choline chloride. Fish fed the lowest-choline diet had pyloric caeca with whitish foamy surface, elevated relative weight, and the enterocytes were hypervacuolated. These characteristics diminished with increasing choline level and levelled off at levels of 2850, 3593 and 2310 mg/kg, respectively. The concomitant alterations in expression of genes related to phosphatidylcholine synthesis, cholesterol biosynthesis, lipid transport and storage confirmed the importance of choline in lipid turnover in the intestine and ability to prevent LMS. Based on the observations of the present study, the lowest level of choline which prevents LMS and intestinal lipid hypervacuolation in post-smolt Atlantic salmon is 3·4 g/kg. However, the optimal level most likely depends on the feed intake and dietary lipid level.

Choline supplementation prevents diet induced gut mucosa lipid accumulation in post-smolt Atlantic salmon (Salmo salar L.)

Background

Various intestinal morphological alterations have been reported in cultured fish fed diets with high contents of plant ingredients. Since 2000, salmon farmers have reported symptoms indicating an intestinal problem, which we suggest calling lipid malabsorption syndrome (LMS), characterized by pale and foamy appearance of the enterocytes of the pyloric caeca, the result of lipid accumulation. The objective of the present study was to investigate if insufficient dietary choline may be a key component in development of the LMS.

Results

The results showed that Atlantic salmon (Salmo salar), average weight 362 g, fed a plant based diet for 79 days developed signs of LMS. In fish fed a similar diet supplemented with 0.4% choline chloride no signs of LMS were seen. The relative weight of the pyloric caeca was 40% lower, reflecting 65% less triacylglycerol content and histologically normal gut mucosa. Choline supplementation further increased specific fish growth by 18%. The concomitant alterations in intestinal gene expression related to phosphatidylcholine synthesis (chk and pcyt1a), cholesterol transport (abcg5 and npc1l1), lipid metabolism and transport (mgat2a and fabp2) and lipoprotein formation (apoA1 and apoAIV) confirmed the importance of choline in lipid turnover in the intestine and its ability to prevent LMS. Another important observation was the apparent correlation between plin2 expression and degree of enterocyte hyper-vacuolation observed in the current study, which suggests that plin2 may serve as a marker for intestinal lipid accumulation and steatosis in fish. Future research should be conducted to strengthen the knowledge of choline’s critical role in lipid transport, phospholipid synthesis and lipoprotein secretion to improve formulations of plant based diets for larger fish and to prevent LMS.

Conclusions

Choline prevents excessive lipid accumulation in the proximal intestine and is essential for Atlantic salmon in seawater.

Central regulation of food intake in fish: an evolutionary perspective

Evidence indicates that central regulation of food intake is well conserved along the vertebrate lineage, at least between teleost fish and mammals. However, several differences arise in the comparison between both groups. In this review, we describe similarities and differences between teleost fish and mammals on an evolutionary perspective. We focussed on the existing knowledge of specific fish features conditioning food intake, anatomical homologies and analogies between both groups as well as the main signalling pathways of neuroendocrine and metabolic nature involved in the homeostatic and hedonic central regulation of food intake.

The composition of dietary fat alters the transcriptional profile of pathways associated with lipid metabolism in the liver and adipose tissue in the pig

The objective was to investigate the of effect chemical composition of dietary fat on transcription of genes involved in lipid metabolism in adipose tissue and the liver via transcriptional profiling in growing pigs. A total of 48 Genetiporc 6.0 × Genetiporc F25 (PIC, Inc., Hendersonville, TN) barrows (initial BW of 44.1 ± 1.2 kg) were randomly allotted to 1 of 6 dietary treatments. Each experimental diet included 95% of a corn-soybean meal basal diet and 5% cornstarch (control; CNTR), animal-vegetable blend (AV), coconut oil (COCO), corn oil (COIL), fish oil (FO), or tallow (TAL). Pigs were sacrificed on d 10 (final BW of 51.2 ± 1.7 kg) to collect tissues. Expression normalization across samples was performed by calculating a delta cycle threshold (ΔCt) value using . Delta delta cycle threshold (ΔΔCt) values were expressed relative to the CNTR treatment. In adipose tissue, adding dietary fat, regardless of the source, decreased the mRNA abundance of compared with the CNTR ( = 0.014). Pigs fed a COIL-based diet tended to have greater adipose tissue expression of ( = 0.071) than pigs fed the other dietary fat sources tested. Abundance of mRNA was greater in adipose tissue of barrows a fed COIL-based diet than barrows fed CNTR or FO-based diets ( = 0.047). In the liver, adding dietary fat, regardless of source, increased the mRNA abundance of , , , , , and ( ≤ 0.020) and tended to increase the abundance of ( = 0.071) and ( = 0.086) compared with the CNTR. Pigs fed a TAL-based diet had greater hepatic transcription of than pigs fed CNTR-, COCO-, or FO-based diets ( = 0.013). Hepatic transcription of tended to be greater in pigs fed COCO than in pigs fed other dietary fat sources ( = 0.074). Dietary omega-3 fatty acid content tended to negatively correlate with mRNA abundance of ( = 0.065) in adipose tissue and ( = 0.063) in the liver. Dietary fat SFA content was negatively correlated with in the liver ( ≤ 0.039). Dietary fat MUFA content tended to be positively correlated with , , and mRNA abundance in the liver ( ≤ 0.100). To conclude, the intake of omega-3 fatty acids suppressed the mRNA abundance of genes involved in lipolysis in both adipose tissue and the liver. Dietary SFA are greater inhibitors of lipogenesis in adipose tissue than omega-6 fatty acids. Intake of medium-chain fatty acids alters hepatic lipid metabolism differently than intake of long-chain fatty acids.

Sodium salt medium-chain fatty acids and Bacillus-based probiotic strategies to improve growth and intestinal health of gilthead sea bream (Sparus aurata)

Background

The increased demand for fish protein has led to the intensification of aquaculture practices which are hampered by nutritional and health factors affecting growth performance. To solve these problems, antibiotics have been used for many years in the prevention, control and treatment against disease as well as growth promoters to improve animal performance. Nowadays, the use of antibiotics in the European Union and other countries has been completely or partially banned as a result of the existence of antibiotic cross-resistance. Therefore, a number of alternatives, including enzymes, prebiotics, probiotics, phytonutrients and organic acids used alone or in combination have been proposed for the improvement of immunological state, growth performance and production in livestock animals. The aim of the present study was to evaluate two commercially available feed additives, one based on medium-chain fatty acids (MCFAs) from coconut oil and another with a Bacillus-based probiotic, in gilthead sea bream (GSB, Sparus aurata), a marine farmed fish of high value in the Mediterranean aquaculture.

Methods

The potential benefits of adding two commercial feed additives on fish growth performance and intestinal health were assessed in a 100-days feeding trial. The experimental diets (D2 and D3) were prepared by supplementing a basal diet (D1) with MCFAs in the form of a sodium salt of coconut fatty acid distillate (DICOSAN^®; Norel, Madrid, Spain), rich on C-12, added at 0.3% (D2) or with the probiotic Bacillus amyloliquefaciens CECT 5940, added at 0.1% (D3). The study integrated data on growth performance, blood biochemistry, histology and intestinal gene expression patterns of selected markers of intestinal function and architecture.

Results

MCFAs in the form of a coconut oil increased feed intake, growth rates and the surface of nutrient absorption, promoting the anabolic action of the somatotropic axis. The probiotic (D3) induced anti-inflammatory and anti-oxidant effects with changes in circulating cortisol, immunoglobulin M, leukocyte respiratory burst, and mucosal expression levels of cytokines, lymphocyte markers and immunoglobulin T.

Discussion

MCFA supplementation showed positive effects on GSB growth and intestinal architecture acting mainly in the anterior intestine, where absorption takes place. The probiotic B. amyloliquefaciens CECT 5940 exhibited key effects in the regulation of the immune status inducing anti-inflammatory and anti-oxidant effects which can be potentially advantageous upon infection or exposure to other stressors. The potential effects of these feed additives in GSB are very promising to improve health and disease resistance in aquaculture.

Evaluation of the suitability of a partially defatted black soldier fly (Hermetia illucens L.) larvae meal as ingredient for rainbow trout (Oncorhynchus mykiss Walbaum) diets

BACKGROUND

Two trials were performed to evaluate a partially defatted Hermetia illucens (HI) larvae meal as potential feed ingredient in rainbow trout (Oncorhynchus mykiss Walbaum) diets. In the first trial, 360 trout (178.9 ± 9.8 g of mean initial body weight) were randomly divided into three experimental groups (4 tanks/treatment, 30 fish/tank). The fish were fed for 78 days with isonitrogenous, isolipidic and isoenergetic diets containing increasing levels of HI, on as fed basis: 0% (HI0, control diet), 25% (HI25) and 50% (HI50) of fish meal substitution, corresponding to dietary inclusion levels of 0, 20% and 40%. In the second trial, 36 trout (4 tanks/treatment, 3 fish/tank) were used to evaluate the in vivo apparent digestibility coefficients (ADC) of the same diets used in the first trial.

RESULTS

Survival, growth performance, condition factor, somatic indexes, and dorsal fillet physical quality parameters were not affected by diet. The highest dietary inclusion of HI larvae meal increased dry matter and ether extract contents of trout dorsal fillet. The use of HI larvae meal induced a decrease of valuable polyunsaturated fatty acids (PUFA) even if differences were only reported at the highest level of HI inclusion. The insect meal worsened the lipids health indexes of the same muscle. Dietary inclusion of insect meal did not alter the villus height of the fish. No differences were found among treatments in relation to ADC of ether extract and gross energy, while ADC of dry matter and crude protein were higher in HI25 if compared to HI50.

CONCLUSIONS

The obtained results showed that a partially defatted HI larvae meal can be used as feed ingredient in trout diets up to 40% of inclusion level without impacting survival, growth performance, condition factor, somatic indexes, dorsal fillet physical quality parameters, and intestinal morphology of the fish. However, further investigations on specific feeding strategies and diet formulations are needed to limit the observed negative effects of the insect meal on the FA composition of dorsal muscle.

Hypothalamic Integration of Metabolic, Endocrine, and Circadian Signals in Fish: Involvement in the Control of Food Intake

The regulation of food intake in fish is a complex process carried out through several different mechanisms in the central nervous system (CNS) with hypothalamus being the main regulatory center. As in mammals, a complex hypothalamic circuit including two populations of neurons: one co-expressing neuropeptide Y (NPY) and Agouti-related peptide (AgRP) and the second one population co-expressing pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) is involved in the integration of information relating to food intake control. The production and release of these peptides control food intake, and the production results from the integration of information of different nature such as levels of nutrients and hormones as well as circadian signals. The present review summarizes the knowledge and recent findings about the presence and functioning of these mechanisms in fish and their differences vs. the known mammalian model.

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.

Uptake and Accumulation of Polystyrene Microplastics in Zebrafish (Danio rerio) and Toxic Effects in Liver

Microplastics have become emerging contaminants, causing widespread concern about their potential toxic effects. In this study, the uptake and tissue accumulation of polystyrene microplastics (PS-MPs) in zebrafish were detected, and the toxic effects in liver were investigated. The results showed that after 7 days of exposure, 5 μm diameter MPs accumulated in fish gills, liver, and gut, while 20 μm diameter MPs accumulated only in fish gills and gut. Histopathological analysis showed that both 5 μm and 70 nm PS-MPs caused inflammation and lipid accumulation in fish liver. PS-MPs also induced significantly increased activities of superoxide dismutase and catalase, indicating that oxidative stress was induced after treatment with MPs. In addition, metabolomic analysis suggested that exposure to MPs induced alterations of metabolic profiles in fish liver and disturbed the lipid and energy metabolism. These findings provide new insights into the toxic effects of MPs on fish.

Dietary Fatty Acid Metabolism is Affected More by Lipid Level than Source in Senegalese Sole Juveniles: Interactions for Optimal Dietary Formulation

This study analyses the effects of dietary lipid level and source on lipid absorption and metabolism in Senegalese sole (Solea senegalensis). Juvenile fish were fed 4 experimental diets containing either 100 % fish oil (FO) or 25 % FO and 75 % vegetable oil (VO; rapeseed, linseed and soybean oils) at two lipid levels (~8 or ~18 %). Effects were assessed on fish performance, body proximate composition and lipid accumulation, activity of hepatic lipogenic and fatty acid oxidative enzymes and, finally, on the expression of genes related to lipid metabolism in liver and intestine, and to intestinal absorption, both pre- and postprandially. Increased dietary lipid level had no major effects on growth and feeding performance (FCR), although fish fed FO had marginally better growth. Nevertheless, diets induced significant changes in lipid accumulation and metabolism. Hepatic lipid deposits were higher in fish fed VO, associated to increased hepatic ATP citrate lyase activity and up-regulated carnitine palmitoyltransferase 1 (cpt1) mRNA levels post-prandially. However, lipid level had a larger effect on gene expression of metabolic (lipogenesis and β-oxidation) genes than lipid source, mostly at fasting. High dietary lipid level down-regulated fatty acid synthase expression in liver and intestine, and increased cpt1 mRNA in liver. Large lipid accumulations were observed in the enterocytes of fish fed high lipid diets. This was possibly a result of a poor capacity to adapt to high dietary lipid level, as most genes involved in intestinal absorption were not regulated in response to the diet.

Metabolic response in liver and Brockmann bodies of rainbow trout to inhibition of lipolysis; possible involvement of the hypothalamus-pituitary-interrenal (HPI) axis

We previously demonstrated in rainbow trout that the decrease in circulating levels of fatty acid (FA) induced by treating fish with SDZ WAG 994 (SDZ) induced a counter-regulatory response in which the activation of the hypothalamus-pituitary-interrenal (HPI, equivalent to mammalian hypothalamus-pituitary-adrenal) axis was likely involved. This activation, probably not related to the control of food intake through FA sensor systems but to the modulation of lipolysis in peripheral tissues, liver and Brockmann bodies (BB, the main site of pancreatic endocrine cells in fish), would target the restoration of FA levels in plasma. To assess this hypothesis, we lowered circulating FA levels by treating fish with SDZ alone, or SDZ in the presence of metyrapone (an inhibitor of cortisol synthesis). In liver, the changes observed were not compatible with a direct FA-sensing response but with a stress response, which allows us to suggest that the detection of a FA decrease in the hypothalamus elicits a counter-regulatory response in liver, resulting in an activation of lipolysis to restore FA levels in plasma. The activation of these metabolic changes in liver could be attributable to the activation of the HPI axis and/or to the action of sympathetic pathways. In contrast, in BB, changes in circulating FA levels induce changes in several parameters compatible with the function of FA-sensing systems informing about the decrease in circulating FA levels.

Feeding rainbow trout with a lipid-enriched diet: effects on fatty acid sensing, regulation of food intake, and cellular signaling pathways

Using rainbow trout fed with low fat (LF) or high-fat (HF) diets we aimed to determine if the response of food intake, mRNA abundance of hypothalamic neuropeptides involved in the metabolic regulation of food intake, and fatty acid (FA) sensing systems in hypothalamus and liver is similar to that previously observed when levels of specific FA were raised by injection. Moreover, we also aimed to determine if the phosphorylation state of intracellular energy sensor 5′-AMP-activated protein kinase (AMPK), and proteins involved in cellular signalling such as protein kinase B (Akt) and target of rapamycin (mTOR) display changes that could be related to FA-sensing and the control of food intake. The increased levels of FA in hypothalamus and liver of rainbow trout fed the HF diet only partially activated FA-sensing systems and did not elicit changes in food intake suggesting that FA-sensing response in fish to increased levels of FA is more dependent on the presence of specific FA such as oleate or octanoate rather than to the global increase in FA. We also obtained, for the first time in fish, evidence for the presence and function of energy sensors like AMPK and proteins involved in cellular signaling like mTOR and Akt in hypothalamus. These proteins in hypothalamus and liver were generally activated in fish fed the HF vs LF diet suggesting the activation of the cellular signaling pathways in response to the increased availability of FA.

Counter-regulatory response to a fall in circulating fatty acid levels in rainbow trout. Possible involvement of the hypothalamus-pituitary-interrenal axis

We hypothesize that a decrease in circulating levels of fatty acid (FA) in rainbow trout Oncorhynchus mykiss would result in the inhibition of putative hypothalamic FA sensing systems with concomitant changes in the expression of orexigenic and anorexigenic factors ultimately leading to a stimulation of food intake. To assess this hypothesis, we lowered circulating FA levels treating fish with SDZ WAG 994 (SDZ), a selective A1 adenosine receptor agonist that inhibits lipolysis. In additional groups, we also evaluated if the presence of intralipid was able to counteract changes induced by SDZ treatment, and the possible involvement of the hypothalamus-pituitary-interrenal (HPI) axis by treating fish with SDZ in the presence of metyrapone, which decreases cortisol synthesis in fish. The decrease in circulating levels of FA in rainbow trout induced a clear increase in food intake that was associated with the decrease of the anorexigenic potential in hypothalamus (decreased POMC-A1 and CART mRNA abundance), and with changes in several parameters related to putative FA-sensing mechanisms in hypothalamus. Intralipid treatment counteracted these changes. SDZ treatment also induced increased cortisol levels and the activation of different components of the HPI axis whereas these changes disappeared in the presence of intralipid or metyrapone. These results suggest that the HPI axis is involved in a counter-regulatory response in rainbow trout to restore FA levels in plasma.

Contribution of glucose- and fatty acid sensing systems to the regulation of food intake in fish. A review

Food intake in fish is a complex process regulated through many different factors including abundance of energy and nutrients. In recent years, evidence have been obtained in several fishes, mainly in rainbow trout, regarding the presence and functioning in brain areas of metabolic sensors informing about changes in the levels of nutrients like glucose and fatty acids. The activity of these sensors relate to the control of food intake through changes in the expression of anorexigenic and orexigenic neuropeptides. The present review will provide a picture of the main results obtained to date in these studies, as well as perspectives for future research in the field.

Fatty acid-specific alterations in leptin, PPARα, and CPT-1 gene expression in the rainbow trout

It is known that fatty acids (FA) regulate lipid metabolism by modulating the expression of numerous genes. In order to gain a better understanding of the effect of individual FA on lipid metabolism related genes in rainbow trout (Oncorhynchus mykiss), an in vitro time-course study was implemented where twelve individual FA (butyric 4:0; caprylic 8:0; palmitic (PAM) 16:0; stearic (STA) 18:0; palmitoleic16:1n-7; oleic 18:1n-9; 11-cis-eicosenoic 20:1n-9; linoleic (LNA) 18:2n-6; α-linolenic (ALA) 18:3n-3; eicosapentenoic (EPA) 20:5n-3; docosahexaenoic (DHA) 22:6n-3; arachidonic (ARA) 20:4n-6) were incubated in rainbow trout liver slices. The effect of FA administration over time was evaluated on the expression of leptin, PPARα and CPT-1 (lipid oxidative related genes). Leptin mRNA expression was down regulated by saturated fatty acids (SFA) and LNA, and was up regulated by monounsaturated fatty acids (MUFA) and long chain PUFA, whilst STA and ALA had no effect. PPARα and CPT-1mRNA expression were up regulated by SFA, MUFA, ALA, ARA and DHA; and down regulated by LNA and EPA. These results suggest that there are individual and specific FA induced modifications of leptin, PPARα and CPT-1 gene expression in rainbow trout, and it is envisaged that such results may provide highly valuable information for future practical applications in fish nutrition.

Central administration of oleate or octanoate activates hypothalamic fatty acid sensing and inhibits food intake in rainbow trout

If levels of fatty acids like oleate and octanoate are directly sensed through different fatty acid (FA) sensing systems in hypothalamus of rainbow trout, intracerebroventricular (ICV) administration of FA should elicit effects similar to those previously observed after intraperitoneal (IP) treatment. Accordingly, we observed after ICV treatment with oleate or octanoate decreased food intake accompanied in hypothalamus by reduced potential of lipogenesis and FA oxidation, and decreased potential of ATP-dependent inward rectifier potassium channel (K(+)ATP). Those changes support direct FA sensing through mechanisms related to FA metabolism and mitochondrial activity. The FA sensing through binding to FAT/CD36 and subsequent expression of transcription factors appears to be also direct but an interaction with peripheral hormones cannot be rejected. Moreover, decreased expression of NPY and increased expression of POMC were observed in parallel with the activation of FA sensing systems and decreased food intake. These results allow us to suggest the involvement of at least these peptides in controlling the decreased food intake noted after oleate and octanoate treatment in rainbow trout.

Voluntary feed intake in rainbow trout is regulated by diet-induced differences in oxygen use

This study investigated the hypothesis that the voluntary feed intake in fish is regulated by diet-induced differences in oxygen use. Four diets were prepared with a similar digestible protein:digestible energy ratio (18 mg/kJ), but which differed in the composition of nonprotein energy source. This replacement of fat (F) by starch (S) was intended to create a diet-induced difference in oxygen use (per unit of feed): diets F30-S70, F50-S50, F65-S35, and F80-S20 with digestible fat providing 28, 49, 65, and 81% of the nonprotein digestible energy (NPDE), respectively. Each diet was fed to satiation to triplicate groups of 20 rainbow trout for 6 wk. As expected, diet-induced oxygen use decreased linearly (R(2) = 0.89; P < 0.001) with increasing NPDE as fat. The digestible and metabolizable energy intakes of trout slightly increased with increasing NPDE as fat (i.e., decreasing starch content) (R(2) = 0.30, P = 0.08; and R(2) = 0.34, P = 0.05, respectively). Oxygen consumption of trout fed to satiation declined with increasing dietary NPDE as fat (R(2) = 0.48; P = 0.01). The inverse relation between digestible energy intake of trout and the diet-induced oxygen use (R(2) = 0.33; P = 0.05) suggests a possible role of diet-induced oxygen use in feed intake regulation as shown by the replacement of dietary fat by starch.

In vitro response of putative fatty acid-sensing systems in rainbow trout liver to increased levels of oleate or octanoate

In a previous study we provided evidence for the presence in liver of rainbow trout of fatty acid (FA) sensing systems responding to changes in levels of oleate (long-chain FA) or octanoate (medium-chain FA). Since those effects could be attributed to an indirect effect, we have evaluated in the present study in vitro (in the absence of extrahepatic regulatory mechanisms) whether or not liver responds to changes in FA concentration in a way similar to that previously observed in vivo. Accordingly, liver slices were exposed to increased oleate or octanoate concentrations to evaluate changes in parameters related to FA metabolism, FA transport, nuclear receptors and transcription factors, ROS effectors, and glucose metabolism. The responses observed in vitro in liver were in general not coincident with those previously observed in vivo allowing us to suggest that FA sensing capacity of liver in vivo is of indirect nature and could be related among other reasons to an interaction with other endocrine systems and/or to FA sensing in hypothalamus.

Oleic acid and octanoic acid sensing capacity in rainbow trout Oncorhynchus mykiss is direct in hypothalamus and Brockmann bodies

In a previous study, we provided evidence for the presence in hypothalamus and Brockmann bodies (BB) of rainbow trout Oncorhynchus mykiss of sensing systems responding to changes in levels of oleic acid (long-chain fatty acid, LCFA) or octanoic acid (medium-chain fatty acid, MCFA). Since those effects could be attributed to an indirect effect, in the present study, we evaluated in vitro if hypothalamus and BB respond to changes in FA in a way similar to that observed in vivo. In a first set of experiments, we evaluated in hypothalamus and BB exposed to increased oleic acic or octanoic acid concentrations changes in parameters related to FA metabolism, FA transport, nuclear receptors and transcription factors, reactive oxygen species (ROS) effectors, components of the KATP channel, and (in hypothalamus) neuropeptides related to food intake. In a second set of experiments, we evaluated in hypothalamus the response of those parameters to oleic acid or octanoic acid in the presence of inhibitors of fatty acid sensing components. The responses observed in vitro in hypothalamus are comparable to those previously observed in vivo and specific inhibitors counteracted in many cases the effects of FA. These results support the capacity of rainbow trout hypothalamus to directly sense changes in MCFA or LCFA levels. In BB increased concentrations of oleic acid or octanoic acid induced changes that in general were comparable to those observed in hypothalamus supporting direct FA sensing in this tissue. However, those changes were not coincident with those observed in vivo allowing us to suggest that the FA sensing capacity of BB previously characterized in vivo is influenced by other neuroendocrine systems.

Molecular characterization, tissue distribution and kinetic analysis of carnitine palmitoyltransferase I in juvenile yellow catfish Pelteobagrus fulvidraco

Up to date, only limited information is available on genetically and functionally different isoforms of CPT I enzyme in fish. In the study, molecular characterization and their tissue expression profile of three CPT Iα isoforms (CPT Iα1a, CPT Iα1b and CPT Iα2a) and a CPT Iβ isoform from yellow catfish Pelteobagrus fulvidraco is determined. The activities and kinetic features of CPT I from several tissues have also been analyzed. The four CPT I isoforms in yellow catfish present distinct differences in amino acid sequences and structure. They are widely expressed in liver, heart, white muscle, spleen, intestine and mesenteric adipose tissue of yellow catfish at the mRNA level, but with the varying levels. CPT I activity and kinetics show tissue-specific differences stemming from co-expression of different isoforms, indicating more complex pathways of lipid utilization in fish than in mammals, allowing for precise control of lipid oxidation in individual tissue.

Response of hepatic lipid and glucose metabolism to a mixture or single fatty acids: Possible presence of fatty acid-sensing mechanisms

To assess the hypothesis that an acute dietary fatty acid (FA) supply may improve glucose tolerance in rainbow trout, we orally administered fish with fish oil (FO; 10mL.kg(-1), one time), which were then subjected to a glucose tolerance test and sampled 6h after injection. Parameters related to glucose and lipid metabolism were then assessed. The results suggest that when both nutrients were administered at the same time, an increased potential for lipogenesis occurred concomitantly with a lower level of glycaemia. In a second experiment we administered intraperitoneally a single FA present in the FO mixture such as oleic acid (60 or 300μg.kg(-1)) whereas octanoic acid (60 or 300μg.kg(-1)) was used as negative control (absent from the FO). However, the effects of both FA were similar in reducing the potential of lipid synthesis and oxidation, and in enhancing the potential of glucose synthesis and glycogenesis. Differences found between FO and single FA administration show that response to FA was dependent on the treatment (mixture vs. single FA) but also comply with the idea that an interaction between FA and glucose rather than FA alone are in the origin of the results reported. The administration of individual FA such as oleic and octanoic acid failed in enhancing lipogenesis and reducing plasma glucose levels and thus in explaining results obtained with FO. However, results provide evidence that FA even provided at a low dose play a key role in the regulation of several putative components of a FA sensing system present in rainbow trout liver.

A comparative study of the metabolic response in rainbow trout and Nile tilapia to changes in dietary macronutrient composition

Metabolic mechanisms underlying the divergent response of rainbow trout (Oncorhynchus mykiss) and Nile tilapia (Oreochromis niloticus) to changes in dietary macronutrient composition were assessed. Fish were fed one of four isoenergetic diets having a digestible protein-to-digestible energy (DP:DE) ratio above or below the optimal DP:DE ratio for both species. At each DP:DE ratio, fat was substituted by an isoenergetic amount of digestible starch as the non-protein energy source (NPE). Dietary DP:DE ratio did not affect growth and only slightly lowered protein gains in tilapia. In rainbow trout fed diets with low DP:DE ratios, particularly with starch as the major NPE source, growth and protein utilisation were highly reduced, underlining the importance of NPE source in this species. We also observed species-specific responses of enzymes involved in amino acid catabolism, lipogenesis and gluconeogenesis to dietary factors. Amino acid transdeamination enzyme activities were reduced by a low dietary DP:DE ratio in both species and in tilapia also by the substitution of fat by starch as the NPE source. Such decreased amino acid catabolism at high starch intakes, however, did not lead to improved protein retention. Our data further suggest that a combination of increased lipogenic and decreased gluconeogenic enzyme activities accounts for the better use of carbohydrates and to the improved glycaemia control in tilapia compared with rainbow tront fed starch-enriched diets with low DP:DE ratio.

Constraints on energy intake in fish: the link between diet composition, energy metabolism, and energy intake in rainbow trout

The hypothesis was tested that fish fed to satiation with iso-energetic diets differing in macronutrient composition will have different digestible energy intakes (DEI) but similar total heat production. Four iso-energetic diets (2 × 2 factorial design) were formulated having a contrast in i) the ratio of protein to energy (P/E): high (H(P/E)) vs. low (L(P/E)) and ii) the type of non-protein energy (NPE) source: fat vs. carbohydrate which were iso-energetically exchanged. Triplicate groups (35 fish/tank) of rainbow trout were hand-fed each diet twice daily to satiation for 6 weeks under non-limiting water oxygen conditions. Feed intake (FI), DEI (kJ kg(-0.8) d(-1)) and growth (g kg(-0.8) d(-1)) of trout were affected by the interaction between P/E ratio and NPE source of the diet (P<0.05). Regardless of dietary P/E ratio, the inclusion of carbohydrate compared to fat as main NPE source reduced DEI and growth of trout by ~20%. The diet-induced differences in FI and DEI show that trout did not compensate for the dietary differences in digestible energy or digestible protein contents. Further, changes in body fat store and plasma glucose did not seem to exert a homeostatic feedback control on DEI. Independent of the diet composition, heat production of trout did not differ (P>0.05). Our data suggest that the control of DEI in trout might be a function of heat production, which in turn might reflect a physiological limit related with oxidative metabolism.

Macronutrient-induced differences in food intake relate with hepatic oxidative metabolism and hypothalamic regulatory neuropeptides in rainbow trout (Oncorhynchus mykiss)

This study examines how dietary macronutrient-induced changes in voluntary food intake (FI) relate to changes in markers of hepatic oxidative metabolism and in the expression of FI regulatory neuropeptides in a teleost model, the rainbow trout. Rainbow trout were fed for 6weeks with one of four iso-energetic diets (2×2 factorial design), containing either a high (HP, ~500 g·kg(-1) DM) or a low (LP, ~250 g·kg(-1) DM) protein level (PL) with, at each PL, fat (diets HP-F and LP-F) being substituted by an iso-energetic amount of gelatinized corn starch (diets HP-St and LP-St) as non-protein energy source (ES). Irrespective of the dietary PL, FI (g·kg(-0.8)·d(-1)) and digestible energy intake (DEI, kJ·kg(-0.8)·d(-1)) were significantly (P<0.05) reduced by the iso-energetic replacement of fat by starch as non-protein ES. Interestingly, trout fed these St-diets had higher gene expression of markers of hepatic oxidative phosphorylation (OxPhos), i.e., ubiquinol-cytochrome c reductase subunit 2 (UCR2) and cytochrome oxidase subunit 4 (COX4) and of aerobic oxidative capacity (CS, citrate synthase), which paralleled glucokinase (GK) transcription. This positive relation suggests that glucose phosphorylation and markers of mitochondrial OxPhos are linked at the hepatic level and possibly triggered the observed reduction in FI. Moreover, trout displaying the reduced FI had higher cocaine amphetamine regulator transcript (CART) mRNA in hypothalamus, whereas neuropeptide Y (NPY) mRNA did not follow the macronutrient-induced changes in FI. Further studies are needed to unravel the mechanisms by which diet-induced changes in hepatic metabolism inform central feeding centers involved in the regulation of FI in fish.