Effects of dietary soy isoflavones on Rainbow trout, Oncorhynchus mykiss

A balanced diet and an appropriate feeding are the two most important requirements of aquaculture. Research on feed, quality control and biological evaluation are important to formulate correct diets because without correct and suitable feeding, fish are unable to stay healthy and productive. Furthermore, quality and composition of feeds deeply affect the nutritional and organoleptic characteristics of the final product. In the last years, fish meal has been gradually and partially substituted by plant-based products, because of their improved sustainability and lower costs. Soybean meal is the principal vegetable protein source, due to its high protein content, good amino acid profile, high digestibility, low cost, consistency and availability. Soybean meal contains isoflavones which are polyphenolic compounds (genistein, daidzein, glycitein) that, as phytoestrogens, may act as estrogen receptor agonists or antagonists causing biological activity on fish in addiction to possible changes on the fillet quality. The potential effects of isoflavones may affect the soybean meal utilization as an alternative protein source in aquaculture feed. In the meantime, it is important, for consumer safety, to evaluate the potential amount of the isoflavones in edible tissue and their antioxidant effect that could enhance the product shelf life. Our objective was to examine, on the one hand, the effect of dietary isoflavones on growth, reproduction and health in rainbow trout (Oncorhynchus mykiss), and on the other hand, on the quality of fish food. Rainbow trout were fed three experimental diets containing different concentrations of isoflavones (0, 500, 1500 ppm) for 70 days. Growth, estrogenic activity expressed by vitellogenin (VTG) protein levels in plasma and liver, plasma 17β-estradiol (E2) levels, gonadal development, state of stress and histological changes in various tissues were evaluated in a subsample of the animals at the end of feeding trial. Complementary analyses on growth performance, proximate composition, lipid oxidation and isoflavone deposition were carried out on the remaining part of the animals for each treatment, after being stored at 4 °C for 1 and 7 days. Neither growth performance nor the relative mRNA levels of the liver Insulin Growh Factor I (Igf-I) were influenced by different levels of dietary isoflavones. Plasma and liver VTG protein levels and plasma E2 were unaffected by the treatments and the correlation between E2 plasma levels and VTG densitometry values was significant (p < 0.05). The fish gonadosomatic index (GSI) did not significantly differ among the three experimental groups but correlated with plasma VTG densitometry values (p < 0.05). Plasma, muscle and fin cortisol concentrations fell within the normal welfare range and were not correlated to isoflavone levels. Histologically, the distal intestine showed a normal morphology with well-differentiated enterocytes, as well as the liver showed normal hepatocytes. A supranuclear accumulation of lipid droplets in enterocytes and some lipid droplets in hepatocytes were observed in all the tested groups, suggesting an impact of basal dietary lipid on transport/metabolism of fat in the fish. Inclusion and different levels of isoflavones in diets did not affect the performance of trout used for final product analysis either. Indeed, trout showed similar body weight at caught, 337 g on average, and an average gutted body weight of 299 g. Biometric indexes, skin and fillet colour, rheological characteristics, proximate composition and fatty acids profile of fillet were not affected by the different dietary treatments. Otherwise, time of storage reduced fillet yield (56.4 vs. 53.6%; p < 0.01), and skin lightness (59.2 vs. 51.5; p < 0.001); red index moved to more negative values (p < 0.001) and yellow index decreased (6.99 vs. 5.07; p < 0.001). Fillet pH (6.22 vs. 6.34; p < 0.001) and lightness increased (38 vs. 43.6; p < 0.001) while yellow index (6.20 vs. 4.52; p < 0.001) and shear force decreased (0.94 vs. 0.80 g kg-1; p < 0.001). According to the time of storage, trout fillet showed an increase of water, a loss of crude protein and an increase of total volatile basic nitrogen content (19.3 vs. 21.2 mg 100 g-1; p < 0.001). As a result of PUFAs omega-6 decrease (13.1% vs. 12.7%; p < 0.05), omega-3/omega-6 ratio increased (1.05 vs. 1.20; p < 0.05) during storage. The results evidenced an accumulation of isoflavones on trout fillets, even if isoflavone concentration in fillets was not significantly influenced by the content of dietary isoflavones in both storage times. Lipid peroxidation expressed as thiobarbituric acid reactive substances (TBARS) levels in fillet on day 1 of refrigeration was significantly higher than that at day 7 (p < 0.05) but no difference resulted among groups fed different isoflavone levels, evidencing no effect of feed soy isoflavones on fillet lipid oxidation. Overall, these results suggest that the isoflavone tested doses do not compromise rainbow trout reproduction, growth and health; although a moderate transfer of isoflavones from diet to fillet was observed, quality, technological and nutritional characteristics and lipid oxidation, were not affected by dietary treatment. Our data demonstrate that, with regard to isoflavone contents and their potential effects, soybean meal can replace fish meal in rainbow trout diets at a high level with no negative effects on fish performance and final product quality.