Effect of high dietary intakes of vitamin E and n-3 HUFA on immune responses and resistance to Edwardsiella tarda challenge in Japanese flounder (Paralichthys olivaceus, Temminck and Schlegel)

A feeding experiment was conducted to investigate the effects of high dietary intake of vitamin E (supplied as dl-α-tocopheryl acetate) and n-3 highly unsaturated fatty acid (n-3 HUFA) on the non-specific immune response and disease resistance in Japanese flounder Paralichthys olivaceus. Nine practical diets were formulated to contain one of three levels of vitamin E namely, 0, 80 or 200 mg kg^-1 (the total α-tocopherol contents in the diets were 21, 97 and 213 mg kg^-1 based on analysis), and at each vitamin E level with one of three n-3 HUFA levels i.e. 0.5%, 1.5% or 2.0%. Each diet was randomly assigned to triplicate groups of Japanese flounder (initial body weight: 40.5±1.0 g, mean±SD) in a re-circulation rearing system. Fish were fed twice daily to apparent satiation at 07:00 and 18:00 hours for 12 weeks. During the experimental period, water temperature was maintained at 18±1°C, salinity 31-35 g L^-1, and pH 7.8-8.2. Dissolved oxygen was not less than 6 mg L^-1, and there were negligible levels of free ammonia and nitrite. The results showed that the increase in dietary n-3 HUFA from 0.5% to 1.0% significantly decreased muscle α-tocopherol contents in fish-fed diets with 21 and 97 mg α-tocopherol kg^-1 diet (P<0.05). In 1.0% HUFA groups, alternative complement pathway activity (ACH₅₀) of fish fed the diet containing the 213 mg α-tocopherol kg^-1 diet was significantly higher than noted for fish fed the diet containing 97 mg α-tocopherol kg^-1 diet (P<0.05). Fish fed the diet with 213 mg α-tocopherol kg^-1 and 2.0% n-3 HUFA had the highest lysozyme activity (131.7 U mL^-1) among all the dietary treatments. Fish fed the diets containing 97 and 213 mg α-tocopherol kg^-1 with 1.0% n-3 HUFA had significantly higher respiratory burst activity than those fed the diets containing 21 mg α-tocopherol kg^-1 with 0.5 and 1.0% n-3 HUFA (P<0.05). In the disease resistance experiment, high intake of dietary vitamin E with 213 mg α-tocopherol kg^-1 significantly decreased cumulative mortality and delayed the days to first mortality after a 7-day Edwardsiella tarda challenge (P<0.05). In addition, under the experimental conditions, dietary vitamin E and n-3 HUFA had a synergistic effect on the non-specific immune responses and disease resistance in Japanese flounder (P<0.05).