Soybean meal (SBM) has become an essential ingredient in fish feed formulations, reducing the aquafeed industry reliance on fishmeal (FM). However, challenges remain when using SBM as the major source of protein, especially for carnivorous species. Reduced growth, and the occurrence of intestinal inflammation, referred to as soybean meal-induced enteritis (SBMIE), are signs of limited tolerance to SBM. Formulation of functional feeds using vitamins or amino acids could help increase SBM inclusion levels in fish diets. Vitamin D (VitD), well known for its role in calcium and phosphorus homeostasis, has more recently been suggested to play a role in intestinal immunity and barrier integrity. Therefore, we hypothesized its potential role as feed additive towards SBMIE attenuation. In addition, possible synergistic effects of VitD and the non-essential amino acid glutamine (Gln), known for its protective action during intestinal inflammation, were also examined in the present study.
A 10-week feeding trial was designed to investigate these hypotheses in rainbow trout (Oncorhynchus mykiss). A total of 378 fish initially weighing ~ 9.5 ± 1.0 g, were randomly distributed into 21, 65-L tanks (18 fish/tank). Seven experimental diets (42% isonitrogenous and 20% isolipidic) including a FM diet (control), a SBM30 and SBM40 diet (30 g/Kg and 40 g/Kg inclusion level, respectively); two SBM30 and two SBM40, each supplemented with either VitD or VitD-Gln (5 µg/Kg VitD or 5 µg/Kg VitD + 2 g/Kg Gln) were formulated and fed to apparent satiation.
Feed intake was recorded daily, and fish weight and tissue samples were taken at 5 and 10 weeks. Samples were collected from the distal intestine for histology and gene expression analyses of the inflammatory markers: TNF-α, IL-1β, IL-10; barrier function markers: MLCK, occludin, and claudin-12; as well as the VitD receptor. Vertebra, kidney, and muscle samples were also obtained for mineral analysis and calcium determination.
Fig. 1., shows that by week 10, growth performance was improved in terms of weight gain in the SBM30-VitD-Gln group, which also showed slightly lower FCR (p < 0.05). Gene expression analysis showed changes in VitD receptor and MLCK expression at 5 and 10 weeks as well as IL-1β at 10 weeks. Further analysis will help reach conclusions on the use of VitD or VitD-Gln in SBM functional feeds.