Tannic acid reduced the growth performance, antioxidant, and immune functions of the Nile tilapia (Oreochromis niloticus)

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The Nile tilapia (Oreochromis niloticus) is highly regarded for its delicious taste and abundant nutritional value. With the growing demand for sustainable aquaculture, plant proteins have become crucial in fish feeds due to their cost-effectiveness and environmental benefits. Proteins such as soy protein, rapeseed meal, and faba bean protein have partially replaced traditional animal-derived proteins. However, these plant proteins often contain significant amounts of antinutritional factors, such as tannic acid (TA), and the effects of high doses on Nile tilapia remain not fully understood. The objective of this 60-day experiment was to evaluate how varying levels of TA affect the growth performance, intestinal structure, physiological indices, and immune functions of Nile tilapia. Five groups were designed for this experiment: TA0 (0 g/kg), TA1 (5 g/kg), TA2 (10 g/kg), TA3 (15 g/kg), and TA4 (20 g/kg). The results of the study showed that the body weight of Nile tilapia in the other TA groups was significantly lower compared to the TA0 groups (P < 0.05). The liver somatic index and viscerosomatic index of the TA0 group were significantly lower than those of the other TA groups (P < 0.05). Observations of intestine tissues indicated varying degrees of structural damage in the treatment groups. The muscle thickness of the TA0 was significantly greater than that of the TA4 group (P < 0.05). The villus width was significantly reduced in the TA3 and TA4 groups (P < 0.05), while the villus height in the TA0 group was significantly greater than that of the other groups (P < 0.05). Regarding physiological indicators, there were no notable variations in the levels of total antioxidant capacity and superoxide dismutase in the serum, liver, and intestine. However, catalase activity was significantly reduced in serum and liver (P < 0.05), with a decreasing trend in the levels of glucose-6-phosphate dehydrogenase observed in the serum, liver, and muscle tissues of the TA1, TA2, and TA3 groups. Also, the expression levels of Hsp70 and IL-1β were significantly upregulated in the liver, intestine, and muscle tissues of the treatment groups (P < 0.05). These results will help to clarify the effects of high concentrations of TA on Nile tilapia and provide valuable insights for selection of appropriate dosage of TA.