The replacement of fish meal (FM) in aquafeeds by alternative sources of protein such as plant ingredients like soybean meal (SBM) has typically encountered a variety of challenges at high levels of inclusion, especially in carnivorous species like rainbow trout (Oncorhynchus mykiss). The development of intestinal inflammation, in this context referred to as soybean meal-induced enteritis, is a noteworthy sign of reduced tolerance to SBM, and there is an array of approaches that have been explored to alleviate inflammation. Bile salts (BS) have recently been suggested to have a role in immune signaling regulation , and we have therefore hypothesized their potential as feed supplements towards SBM diets improvement.
The goal of this study was to evaluate the potential mechanisms by which BS might exert a protective effect on reducing inflammation and restore barrier function in rainbow trout over a long-term experimental period. A total of 2, 000 fish initially weighing ~ 40.0 ± 1.0 g, were randomly distributed into 20 350-L tanks (100 fish/tank) . Five experimental diets (isonitrogenous: 42 % crude protein and isolipidic: 20% lipid) including a FM diet (control), a SBM30 diet (30% inclusion level), a SBM40 diet (40% inclusion level) and two SBM-BS supplemented diets ( 1.5% BS supplemented to each SBM diet) were fed to apparent satiation for 18 weeks . During the trial fish were sampled three times at 6, 12, and 18 weeks.
Feed intake was recorded daily, and fish weight was measured at each sampling point. Samples were collected from the distal intestine for gene expression analyses of the inflammatory markers: TNF-α, NF-κB, IL-8, IL-10; barrier function markers: MLCK, occludin and claudins ; as well as the bile brush border transporter TGR5 and basolateral transporter OSTα. Similarly, liver gene expression analysis was done on the bile acid synthesis enzyme . Digesta was taken from proximal and distal intestine 12 hrs. after feeding for bile acid quantification and resorption calculation. Distal intestine and liver samples were also collected for histology analysis.
No significant differences ( p > 0.05) in weight gain, feed intake and feed conversion ratio (FCR) were observed after 6 weeks, but fish fed the SBM40 diet showed slightly higher weight gain in the BS supplemented diet compared to the non-supplemented one (Fig. 1). Upon completion of the trial, growth performance resu lts will provide a further insight on the possible impact of BS on growth . Gene expression analysi s in the distal intestine and liver will help elucidate the molecular mechanism underlying the role of BS in inflammation and SBMIE mitigation.