A 12-week feeding trial was conducted to evaluate the effects of cholesterol supplementation to different ratios of soybean oil (SO) and linseed oil (LO) on growth performance, fillet fatty acid composition, and muscle gene expression in rainbow trout, Oncorhynchus mykiss. Twelve isonitrogenous (48%, crude protein), isolipidic (21%, crude lipid) and isocaloric (22.5 MJ/kg, energy) experimental diets were produced: Two control diets (fishmeal/fish oil based (FM/FO) and plant protein/fish oil based (PM/FO)) and ten experimental diets with graded ratios of SO and LO (100/0, 75/25, 50/50, 25/75 and 0/100), with or without added cholesterol (C). Rainbow trout (initial body weight :18.8 ± 0.3g) of the USDA-UI strain crossed with Donaldson strain, selected for improved growth on an all-plant protein diet, were stocked into 145-L tanks at 25 fish per tank.
Compared with fish fed other diets, the weight gain of fish fed diet FM/FO, PM/FO or L100+C was greatest (P<0.05). Different ratios of SO/LO did not affect trout weight gain; however, the addition of cholesterol to the LO100 diet (LO100+C) significantly increased fish weight gain. The interaction between the two main factors (cholesterol and SO/LO ratio) significantly impacted feed intake (P<0.05); but had no significant effects on growth performance or feed utilization (P>0.05). A hypocholesterolemic effect was observed in plasma of fish fed plant-based diets without cholesterol supplementation compared with the fish fed control treatments and diets supplemented with cholesterol. Plasma cholesterol of fish fed L100+C diet was significantly higher than fish fed SO100 and SO75/LO25 diets. Regarding fatty acid profile, the interaction of the two main factors (cholesterol x SO/LO ratio) significantly increased DHA content in fish fillet (P<0.05). Eicosapentaenoic acid (EPA) content in fish fillet was also affected (P<0.05). Supplementation of cholesterol did not affect the expression of liver and muscle genes involved in fatty acid elongation, desaturation, and β-oxidation (P>0.05); but there was a non-significant trend toward increased gene expression for elongase and desaturase with cholesterol supplementation when linseed oil was provided above 50%.
In conclusion, results of the present study demonstrated that: 1) A plant-based diet without added cholesterol resulted in growth reduction and plasma hypocholesterolemia in juvenile rainbow trout, and 2) Fish fed linseed oil, as 100% of dietary lipid, with cholesterol supplementation had significantly higher weight gain and feed intake compared with other plant-based diets. Furthermore, the observation that cholesterol supplementation showed increased EPA and DHA levels in fish fillet when linseed oil was provided above 50% supports further research, over an entire production period, to evaluate the long-term effects at harvest.