DIETARY FISH-OIL AND DIGESTIBLE PROTEIN MODIFY SUSCEPTIBILITY TO LIPID-PEROXIDATION IN THE MUSCLE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) AND SEA BASS (DICENTRARCHUS-LABRAX)

The effects of dietary fish oil and digestible protein (DP) levels on muscle fatty acid composition and susceptibility to lipid peroxidation were studied in two representative fish species for human nutrition, from fresh and seawater, rainbow trout (Oncorhynchus mykiss) and Europ ean sea bass (Dicentrarchus labrax). In rainbow trout, higher concentr ations of dietary fat and DP led to higher weight gain (gld) (P=0.001 and P=0.043 respectively). Additionally, an interaction effect was obs erved in this species, since the effect of DP was only evident when th e dietary fat concentration was low (P = 0.043). A similar tendency wa s also observed in European sea bass, although with less marked differ ences among nutritional treatments. Trout fed on diets with a higher c oncentration of dietary fat had higher concentrations of intramuscular total and neutral lipids in the dorsal muscle (P=0.005). Increased le vels of dietary DP led to significantly lower concentrations of polar lipids in the dorsal muscle of both rainbow trout (P = 0.005) and Euro pean sea bass (P=0.006). In the neutral fraction of intramuscular lipi ds of dorsal muscle the concentration of n-3 fatty acids was positivel y affected by the dietary fat concentration in both rainbow trout (P = 0.04) and sea bass (P = 0.001). Muscle homogenates from trout and sea bass fed on diets rich in fish oil showed a significantly,higher susce ptibility to oxidation than muscle homogenates from fish fed on low-fa t diets (P=0.001). The higher DP concentration also increased suscepti bility to oxidation. Moreover, in rainbow trout an interaction effect was observed where the pro-oxidant effect was of higher magnitude when the dietary concentration of both nutrients, fat and protein, was hig h (P=0.004).