High dietary intake of alpha-tocopherol acetate enhances the non-specific immune response of gilthead seabream (Sparus aurata L.)

To determine the effects of three high levels of dietary intake of alpha-tocopherol acetate (vitamin E) on the non-specific immune response of gilthead seabream (Sparus aurata L.), specimens were fed a commercial diet (100 mg alpha-tocopherol kg-1) as control, or vitamin E supplemented diets (600, 1200 or 1800 mg alpha-tocopherol acetate kg-1) for 15, 30 or 45 days. Growth, serum alpha-tocopherol levels, natural haemolytic complement activity and head-kidney leucocyte migratory, respiratory burst and phagocytic activities were studied at each of the assay times. A positive correlation between alpha-tocopherol acetate intake and serum alpha-tocopherol levels was observed, the increase being linked to both the dosage and length of treatment. Specimens fed the diet supplemented with 600 mg vitamin E kg-1 showed no enhancement in any of their immune parameters, while those fed the diet supplemented with 1200 mg vitamin E kg-1 presented a slightly higher (but not statistically significant) specific growth rate than fish fed the other diets. In addition, serum haemolytic activity and the phagocytosis of head-kidney leucocytes were enhanced by the dietary intake of 1200 mg vitamin E kg-1 after 30 and 45 days of treatment, although leucocyte migration and respiratory burst activity remained unaffected. The highest vitamin E dietary dose used, 1800 mg kg-1, unexpectedly provoked no immunostimulation. These results indicate that a moderate level of vitamin E in the diet (1200 mg kg-1) stimulates the seabream's non-specific immune system after 30 days of administration. Lower or higher vitamin E concentrations may not be so effective, because of an imbalance in the vitamin E ratio with other antioxidants. The proposed dietary levels of vitamin together with the indicated administration time could be useful for reducing the susceptibility of farmed fish to infectious diseases.