Soybean meal (SBM) has become a common dietary replacement for fishmeal in aquafeed. However, at high inclusions, SBM has been shown to have negative impacts on fish such as reduced feed intake and intestinal inflammation. Medicinal plant extracts, namely essential oils, have been used to promote growth performance and immune response. One notable plant extract is the essential oil from oregano (Origanum vulgare) as it is composed of two health-promoting compounds: carvacrol and thymol. These compounds have shown antimicrobial, antioxidant, and anti-inflammatory properties. The objective of this study was to investigate the effects of oregano oil on utilization of a high-inclusion SBM diet using zebrafish as a model.
At 20 days post-hatch (dph), zebrafish were randomly separated into 15 tanks (3.0 L) with 11 fish per tank. Five diets were used in this study: reference – fishmeal based diet, control – SBM based diet, and three experimental SBM based diets OEO1, OEO2, and OEO3 that were supplemented with 1, 2, or 3% of oregano oil respectively. After 28 days, the FM group had overall better growth performance when compared to the other treatment groups. However, the OEO3 mean weight was significantly higher than that of SBM control (p = 0.0478) and not significantly different from the FM group (p > 0.1). Similarly, the FCR in the OEO3 was significantly lower than the FCR in the SBM group (p = 0.0490) and not significantly different from the FM group (p > 0.1). The OEO2 mean total length was significantly higher than that of the SBM group (p = 0.0478) but not significantly different than the FM group (p > 0.1). The OEO1 mean weight, FCR, and total length were not significantly different from those of the SBM group (p > 0.5).
Furthermore, the OEO2 group showed a numerical downregulation of inflammation related genes tnfa, il-1b, mmp9, and il-10 when compared to the other groups, but most notably in comparison to the SBM group. However, there was only significant downregulation in the il-10 gene expression for anti-inflammatory cytokines when compared to the FM group (p = 0.032). While the FM group had higher numerical expression of nutrient absorption related genes (pept1 and fabp2), the OEO1 group showed numerical upregulation of these genes when compared to the other SBM-based groups. The OEO1 group had significantly higher expression of fabp2 than the SBM and OEO2 groups (p = 0.055, p = 0.040). These results suggest that inclusion of oregano essential oil at or above 2% inclusion improved growth performance of zebrafish fed a high-inclusion SBM diet and might potentially have beneficial anti-inflammatory effects on the gut.