Relative expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) in liver of common carp fed experimental diets. Control, the control diet; HF, the high-fat diet; NaBT, the high-fat diet added with 0.1% sodium butyrate. Bars marked with different letters are significantly different (P<0.05).