Effect of plant-based diets with varying ratios of ω6 to ω3 fatty acids on growth performance, tissue composition, fatty acid biosynthesis and lipid-related gene expression in Atlantic salmon (Salmo solar)

Little is known about how variation in omega-6 to omega-3 (omega 6:omega 3) fatty acid (FA) ratios affects lipid metabolism and eicosanoid synthesis in salmon, and the potential underlying molecular mechanisms. The current study examined the impact of five plant-based diets (12-week exposure) with varying omega 6:omega 3 (0.3-2.7) on the growth, tissue lipid composition (muscle and liver), and hepatic transcript expression of lipid metabolism and eicosanoid synthesis-related genes in Atlantic salmon. Growth performance and organ indices were not affected by dietary omega 6:omega 3. The liver and muscle FA composition was highly reflective of the diet (omega 6:omega 3 of 0.2-0.8 and 0.3-1.9, respectively) and suggested elongation and desaturation of the omega 3 and omega 6 precursors 18:3 omega 3 and 18:2 omega 6. Furthermore, proportions of omega 6 and omega 3 PUFA in both tissues showed significant positive correlations with dietary inclusion (% of diet) of soy and linseed oils, respectively. Compound-specific stable isotope analysis (CSIA) further demonstrated that liver long-chain polyunsaturated fatty acid (LC-PUFA) synthesis (specifically 20:5 omega 3 and 20:4 omega 6) was largely driven by dietary 18:3 omega 3 and 18:2 omega 6, even when 20:5 omega 3 and 22:6 omega 3 were supplied at levels above minimum requirements. In addition, significant positive and negative correlations were identified between the transcript expression of LC-PUFA synthesis-related genes and liver omega 6 and omega 3 LC-PUFA, respectively, further supporting FA biosynthesis. Liver omega 3 LC-PUFA also correlated negatively with the eicosanoid synthesis-related transcripts pgds and cox1. This is the first study to use CSIA, hepatic transcriptome, and tissue lipid composition analyses concurrently to demonstrate the impact of plant-based diets with varying omega 6:omega 3 on farmed Atlantic salmon.