Effects of different levels of dietary lipids on growth performance, liver histology and cold tolerance of Nile tilapia (Oreochromis niloticus).

This study evaluated the effects of different levels of dietary lipids on the growth performance, feed utilization, body composition and cold tolerance of Nile tilapia (Oreochromis niloticus) fingerlings (7.33 ± 0.12 g fish-1). Four isonitrogenous (275 g kg-1 crude protein), isocaloric (18.5 MJ kg-1) diets containing a mixture of fish oil and corn oil (1:1 ratio) at different levels (70, 85, 110 and 130 g kg-1) were prepared and fed to Nile tilapia reared at a fixed water temperature 25 ± 1 °C for two months. After the feeding trial, the fish were exposed to a cold challenge. The best growth rates and feed utilization were achieved at 70 and 85 g kg-1 dietary lipid, whereas the lowest results were recorded at higher lipid levels (110 and 130 g kg-1). The ability of Nile tilapia to survive the acute cold stress was significantly improved as the lipid level increased from 70 to 110 g kg-1 and decreased with further increase in lipid levels. During the cold stress, saturated fatty acids (SFA) significantly decreased, while unsaturated fatty acids (UFA) tended to increase. Thus, this study demonstrates, to a certain level, that high dietary lipid levels have a positive effect on the cold tolerance of Nile tilapia fingerlings.

Influences of dietary lipid and temperature on growth, fat deposition and lipoprotein lipase expression in darkbarbel catfish (Pelteobagrus vachellii).

Darkbarbel catfish (Pelteobagrus vachellii) is an important freshwater fish in China. Water temperature greatly influences the absorption and utilization of dietary lipid by fish. Response values (including growth, hepatic fat deposition, and gene expression) for darkbarbel catfish mediated by two factors (water temperature 20-34°C; dietary lipid level 2-17%) were the focus of this study. The relationship between the two factors and the response values was evaluated by the response surface method using the central composite design. The experiment was conducted under laboratory conditions and lasted for seven weeks. A total of 975 experimental fish (average weight 11.75 ± 0.17g) were selected and placed in 39 plastic tanks. The results showed that the linear effects of lipid level on feed conversion rate (FCR), hepatopancreas somatic index (HSI), hepatic triglycerides (TG), cholesterol (TC), and lipoprotein lipase (LPL) gene expression were significant (P < 0.05). The linear effects of water temperature on specific growth rate (SGR), HSI, TC level, and LPL mRNA expression were significant (P < 0.05). The quadratic effects of water temperature and lipid level on SGR and FCR were significant (P < 0.05). Low water temperature and low lipid diets significantly inhibited growth, increased HSI, and reduced hepatic TG and TC levels, and LPL mRNA expression. The adjusted R2 values for the SGR, FCR, HSI, TC, TG, and LPL mRNA regression models were 0.77, 0.85, 0.62, 0.73, 0.85, and 0.91, respectively. The optimal combination of water temperature and dietary lipid level was 27.5°C and 9.2%, at which the greatest growth and FCR were 2.13%.d-1 and 1.31 respectively, with desirability of 0.904. These results indicated that water temperature may mediate the requirement and utilization of dietary lipid, and intervene in hepatic fat deposition. The results of this study can be used to help optimize the culture conditions of darkbarbel catfish.

Docosahexaenoic acid biosynthesis via fatty acyl elongase and Δ4-desaturase and its modulation by dietary lipid level and fatty acid composition in a marine vertebrate.

The present study presents the first "in vivo" evidence of enzymatic activity and nutritional regulation of a Δ4-desaturase-dependent DHA synthesis pathway in the teleost Solea senegalensis. Juvenile fish were fed diets containing 2 lipid levels (8 and 18%, LL and HL) with either 100% fish oil (FO) or 75% of the FO replaced by vegetable oils (VOs). Fatty acyl elongation (Elovl5) and desaturation (Δ4Fad) activities were measured in isolated enterocytes and hepatocytes incubated with radiolabeled α-linolenic acid (ALA; 18:3n-3) and eicosapentaenoic acid (EPA; 20:5n-3). Tissue distributions of elovl5 and Δ4fad transcripts were also determined, and the transcriptional regulation of these genes in liver and intestine was assessed at fasting and postprandially. DHA biosynthesis from EPA occurred in both cell types, although Elovl5 and Δ4Fad activities tended to be higher in hepatocytes. In contrast, no Δ6Fad activity was detected on (14)C-ALA, which was only elongated to 20:3n-3. Enzymatic activities and gene transcription were modulated by dietary lipid level (LL>HL) and fatty acid (FA) composition (VO>FO), more significantly in the liver than in the intestine, which was reflected in tissue FA compositions. Dietary VO induced a significant up-regulation of Δ4fad transcripts in the liver 6h after feeding, whereas in fasting conditions the effect of lipid level possibly prevailed over or interacted with FA composition in regulating the expression of elovl5 and Δ4fad, which were down-regulated in the liver of fish fed the HL diets. Results indicated functionality and biological relevance of the Δ4 LC-PUFA biosynthesis pathway in S. senegalensis.

Optimal dietary lipid levels alleviated adverse effects of high temperature on growth, lipid metabolism, antioxidant and immune responses in juvenile turbot (Scophthalmus maximus L.).

Fish physiological health is often negatively impacted by high-temperature environments and there are few studies on how dietary lipids affect fish growth and physiology when exposed to heat stress. The main objective of this research was to examine the impact of dietary lipid levels on growth and physiological status of juvenile turbot (Scophthalmus maximus L.) and determine if dietary lipid concentration could alleviate the possible adverse effects of heat stress. Five diets containing 6.81%, 9.35%, 12.03%, 14.74%, and 17.08% lipid, respectively, were formulated and fed to turbot (initial weight 5.13 ± 0.02 g) under high-temperature conditions (24.0-25.0 °C). Meanwhile, the diet with 12.03% lipid (considered by prior work to be an optimal dietary lipid level) was fed to turbot of the same size at normal temperature. Results suggested that, among the different dietary lipid levels under high-temperature conditions, fish fed the optimal lipid (12.03%) exhibited better growth compared to non-optimal lipid groups, as evidenced by higher weight gain and specific growth rate. Simultaneously, the optimal lipid diet may better maintain lipid homeostasis, as attested by lower liver and serum lipid, along with higher liver mRNA levels of lipolysis-related genes (pgc1α, lipin1, pparα, lpl and hl) and lower levels of synthesis-related genes (lxr, fas, scd1, pparγ, dgat1 and dgat2). Also, the optimal lipid diet might mitigate oxidative damage by improving antioxidant enzyme activity, decreasing malondialdehyde levels, and up-regulating oxidation-related genes (sod1, sod2, cat, gpx and ho-1). Furthermore, the optimal lipid may enhance fish immunity, as suggested by the decrease in serum glutamic-oxalacetic/pyruvic transaminase activities, down-regulation of pro-inflammatory genes and up-regulation of anti-inflammation genes. Correspondingly, the optimal lipid level suppressed MAPK signaling pathway via decreased phosphorylation levels of p38, JNK and ERK proteins in liver. In summary, the optimal dietary lipid level facilitated better growth and physiological status in turbot under thermal stress.

Systemic adaptation of lipid metabolism in response to low- and high-fat diet in Nile tilapia (Oreochromis niloticus).

Natural selection endows animals with the abilities to store lipid when food is abundant and to synthesize lipid when it is limited. However, the relevant adaptive strategy of lipid metabolism has not been clearly elucidated in fish. This study examined the systemic metabolic strategies of Nile tilapia to maintain lipid homeostasis when fed with low- or high-fat diets. Three diets with different lipid contents (1%, 7%, and 13%) were formulated and fed to tilapias for 10 weeks. At the end of the feeding trial, the growth rate, hepatic somatic index, and the triglyceride (TG) contents of serum, liver, muscle, and adipose tissue were comparable among three groups, whereas the total body lipid contents and the mass of adipose tissue increased with the increased dietary lipid levels. Overall quantitative PCR, western blotting and transcriptomic assays indicated that the liver was the primary responding organ to low-fat (LF) diet feeding, and the elevated glycolysis and accelerated biosynthesis of fatty acids (FA) in the liver is likely to be the main strategies of tilapia toward LF intake. In contrast, excess ingested lipid was preferentially stored in adipose tissue through increasing the capability of FA uptake and TG synthesis. Increasing numbers, but not enlarging size, of adipocytes may be the main strategy of Nile tilapia responding to continuous high-fat (HF) diet feeding. This is the first study illuminating the systemic adaptation of lipid metabolism responding to LF or HF diet in fish, and our results shed new light on fish physiology.