Dietary Tannic Acid Improves Hepatic Health and Capacity to Deal with Temperature Fluctuation in the Chinese Soft-Shelled Turtle (Pelodiscus sinensis)

To assess the impact of tannic acid (TA) on the hepatic health of the Chinese soft-shelled turtle, the individuals were fed diets containing 0 (CG), 0.5, 1, 2, and 4 g/kg TA diets for 60 days (0 hps). Afterwards, the turtles were challenged with 15 °C cold stress for 24 h (24 hps) and then recovered to 28 °C for 24 h (48 hps). The results indicated that 4 g/kg TA has a significant toxic effect on the turtles after 60-day administration. The hepatic T-SOD, CAT, GSH-Px, and T-AOC activities in the TA2 were increased at 0 hps and 24 hps (<i>p</i> < 0.05) compared with CG. Similarly, <i>Sod1</i>, <i>Sod2</i>, <i>Cat</i>, <i>Gsh-px3</i>, and <i>Gsh-px4</i> mRNA levels in the TA2 were higher than in the other four groups at 0 hps and 24 hps (<i>p</i> < 0.05). The histology data showed that 4 g/kg TA induced injuries in liver at 0 hps and after temperature fluctuation, whereas the 2 g/kg TA effectively attenuated the hepatic injuries. A total of 202 differentially expressed metabolites (DEMs) in the CG vs. TA2 and 115 DEMs in the LTCG vs. LTTA2 were separately detected by the metabolome. The cephalosporin C, protoporphyrin, bis(2-ethylhexyl) phthalate, 2-acetamidoethylphosphonat, verbasosid, and norcocain, were obvious DEMs in the CG vs. TA2. Furthermore, valienone 7-phosphate, 5-HETE, pregnanetriolone, 4-keto-anhydrotetracyclin, dephospho-CoA, and lysoPC(18:4(6Z,9Z,12Z,15Z)/0:0) were top DEMs in the LTCG vs. LTTA2. The "adipocytokine signaling pathway" and "AMPK signaling pathway" were the key pathways enriched in the CG vs. TA2, while "fatty acid biosynthesis", "steroid biosynthesis", and "linoleic acid metabolism" were most abundant in the LTCG vs. LTTA2. Generally, this research indicated that 2 g/kg TA could protect hepatic health from temperature fluctuations by improving antioxidant capacity, reducing histological injuries, and regulating lipid-related signaling pathways.