Results of the present study shown that, based on weight gain, FI, FER and survival, vitamin E requirement of Nile tilapia was not affected by dietary lipid levels and supplementation of 50 mg of vitamin E/kg diet (analyzed values of 112 mg/kg) was sufficient for tilapia fed purified diets containing an equal mixture of corn oil and menhaden fish oil ranging from 6 to 14%.
This value is higher than those determined for O. aureus (10 and 25 mg/kg of diet at 3 and 6% dietary lipid, respectively, or 3 to 4 mg vitamin E per percent of corn oil) and Shiau and Shiau for hybrid tilapia (O. niloticus×O. aureus) (42–44 mg/kg and 60–66 mg/kg in diets containing 5% and 12% lipid, respectively).
These values, however, are considerably lower than the requirement values of 50 to 100 mg/kg for a diet containing 5% lipid (pollock liver oil) and 500 mg/kg for a diet containing 10 to 15% lipid reported for Nile tilapia.
Whether the differences in the degree of fatty acid unsaturation between the lipid source used in this study and that of contributed to the differences in the vitamin E requirements cannot be ascertained.
However, it has been reported that, in fish, high levels of dietary highly unsaturated fatty acids induced higher vitamin E requirements.
It should be noted that our diets were also supplemented with the antioxidant, ethoxyquin, at 200 mg/kg diet.
Ethoxyquin, although it has no biological activity of vitamin E, can partially spare vitamin E in the diet for growth and other physiological functions.