The melting point of dietary fatty acids is a key regulator of omega-3 fatty acid metabolism in Atlantic salmon

Current research on lipid nutrition in farmed fish has primarily focused on the nutritional, biochemical, biomolecular, and physiological aspects of n-3 LC-PUFA, with limited knowledge available on the interrelationships amongst different dietary fatty acids. This study aimed to evaluate the effects of individual dietary fatty acids (FA) on the metabolism of omega-3 fatty acids in juvenile Atlantic salmon. Specifically, the study investigated the relationships between omega-3 fatty acids and other individual dietary fatty acids characterised by different chain lengths but the same level of unsaturation (lauric acid, 12:0; palmitic acid, 16:0; and stearic acid, 18:0), and different degrees of unsaturation but the same chain length (stearic acid, 18:0; oleic acid, 18:1n-9; and linoleic acid, 18:2n-6). Diets rich in n-3 LC-PUFA or alpha-linolenic acid (ALA, 18:3n-3) were used to explore possible interactions and nutritional relationships. The experiment followed a five by two factorial design, with a total of 10 experimental dietary treatments fed to triplicate groups of fish (n = 3, N = 30). Five diets were formulated with minimal n-3 LC-PUFA content and abundant ALA content (referred to as "ALA diets"), while the other five diets had minimal ALA content and abundant n-3 LC-PUFA content (referred to as "n-3 LC-PUFA diets"). Within each dietary group, the five experimental diets were isoproteic, isolipidic, and isoenergetic. They were formulated to spike the concentration of a single FA while maintaining a constant supply of all other FA across all diets. The selected FA used to formulate the experimental diets were 12:0, 16:0, 18:0, 18:1n-9, or 18:2n-6. The feeding trial lasted 92 days, during which all treatments resulted in fish growth exceeding 300% of their initial body weight. Chemical composition analysis, fatty acid apparent metabolism, and selected gene transcription were performed on the samples to monitor the main nutritional, biochemical, biomolecular, and physiological outcomes. The most interesting and unexpected finding was the identification of a direct link between the physical properties of dietary fatty acids, particularly their melting point, and the resulting changes in omega-3 fatty acid metabolic patterns. This study demonstrated, for the first time, that the melting point of dietary fatty acids plays a crucial role in regulating omega-3 fatty acid metabolism in Atlantic salmon. It is envisioned that this discovery could open new possibilities for future advancements in lipid nutrition for aquaculture species.