Incorporation of dietary fatty acid in European sea bass (Dicentrarchus labrax)--A methodological approach evidencing losses of highly unsaturated fatty acids

A quantitative approach is presented to evaluate fatty acid incorporation in fish. Fatty acid composition of European sea bass juvenile was studied during an experiment using 6 isoproteic (54%) and isolipidic (18%) diets containing 0.23, 0.56, 0.72, 0.86, 1.01 and 1.86% DM n-3 highly unsaturated fatty acids (n-3 HUFA). Whole body fatty acid compositions were studied at the beginning and after 52 and 81 days feeding, fatty acid profiles evolved during time under dietary influences. Incorporations of individual fatty acids into total lipids were calculated as increases in individual fatty acids as percentages of the increase in total fatty acids during growth, on a per animal basis. Relative incorporations (RI) so defined generated similar profiles for the two periods for each dietary treatment, consistent with fatty acids being incorporated in a stable way during the experiment. For most fatty acids, linear regressions could be drawn between RIs in whole fish and % of fatty acids in dietary lipid. RIs for DHA, EPA and AA demonstrated these HUFAs were incorporated in the fish in lower proportion that diet contents, in this experiment. Fractional retentions (FRs) of individual fatty acids were also calculated by dividing the quantities of given fatty acids present in lipid accumulated by the fish by the quantities of corresponding fatty acids ingested in the diet. Lipogenic activity was evidenced by 16:0 and 18:0 having FRs greater than unity. FR values greater than unity for 18:3n-6 and 20:3n-6, and 18:4n-3 in diets containing low levels of HUFA demonstrated some bioconversion capacity from 18:2n-6 and 18:3n-3 precursors, respectively. FRs for n-3 HUFA were negative in fish fed 0.2% n-3 HUFA. In other treatments FRs of n-3 HUFA were 0.5 to 0.6, lower than other dietary fatty acids as linoleic (0.75) or linolenic (0.70) acids. Results indicated a basal loss of DHA estimated around 14 μg g- 1 ABW d- 1 during the experiment. We conclude that dietary requirements of HUFA by marine fish comprise not only quantities required for production of polar lipids during growth, but also quantities required to replace losses probably induced by the active roles of HUFA. The results also indicate that RI and FR transformations are useful tools for better understanding fatty acid incorporations in juvenile fish.