Thinking different strategies for indexing fatty acids in fish fillets Part I: Rainbow trout as a case-study

Fish is a source of important nutrients, such as polyunsaturated fatty acids (PUFA), particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Rainbow trout (Oncorhynchus mykiss) is the most important farmed finfish in EU, mainly raised in freshwater. As with other species, its fillet FA profile highly depends on the one of the administered feeds, since trout has a moderate ability to synthesize long chain FAs. Several nutritional indexes, such as Atherogenicity (IA) and Thrombogenicity (IT), can be calculated based on the fillet FA profile (% of total fatty acid methyl esters), whose aim is to characterize lipid quality on the basis of pro- and anti- atherogenic and thrombogenic potential of some FAs. Recently, two Healthy Fatty Indexes (HFI1 and 2) have been set considering the various classes of FAs by increasing or decreasing their relative content expressed in weight according to their health impact. The aim of the present study was to compare the effectiveness of the HFIs to the IA and IT in evaluating the nutritional differences of rainbow trout fed commercial (COM), marine ingredient based (FISH), vegetable-based (VEG), or insect-based diets (INS). EPA+DHA content was also calculated. As expected, diet affected all indexes however, if consumers choose the best fillet based on the indexes, some inconsistencies would emerge. For instance, COM and INS had the lowest AI (0.22 and 0.24, respectively); INS and VEG had the lowest TI (0.18 and 0.19, respectively). The HFI1 calculated for VEG fish was 1.53, higher than COM (p<0.05), but it did not differ from INS and FISH values (1.38 and 1.43, respectively). The FISH group showed the highest EPA+DHA content, followed by VEG fish, while COM and INS had the lowest amounts of these healthy FAs. In conclusion, caution is required when using the lipid related indexes to compare fish quality, especially AI.