Research Progress on Effects of Vitamin A on Antioxidant Indicators in Pigs

Oxidative stress constitutes a key restrictive factor that undermines growth performance, reproductive efficiency and meat quality throughout the entire production cycle of commercial pig herds. Vitamin A (VA), an essential fat-soluble nutrient for pigs, fulfills dual antioxidant regulatory functions: it directly scavenges free radicals, and indirectly mediates transcriptional regulation of antioxidant processes via retinoic acid receptor-associated signaling pathways. By integrating recent in vivo feeding trial data and in vitro molecular experimental evidence, this review demonstrates that the antioxidant effect of VA in pigs is characterized by significant stress dependence, dose responsiveness and physiological stage specificity. Under stress challenge conditions, VA can significantly upregulate the body's antioxidant response, while its regulatory effect is relatively weak under non-stress basal physiological conditions. From the mechanistic perspective, all-trans retinoic acid (ATRA) bound to RAR/RXR receptors can exert synergistic effects with the Nrf2 antioxidant pathway when VA is supplied at physiological nutritional doses, whereas high pharmacological doses of ATRA may induce competitive inhibitory crosstalk. It should be noted that most existing direct promoter-binding evidence is derived from cell line models rather than in vivo porcine tissue samples. For weaned piglets, an optimal dietary VA level can alleviate oxidative damage and intestinal barrier impairment induced by weaning stress, but excessive VA supplementation will trigger lipotoxic oxidative disturbance. In growing-finishing pigs, appropriate VA supplementation can increase glutathione (GSH) content in skeletal muscle and improve the lipid oxidation stability and color stability of pork during post-mortem storage; however, supra-nutritional VA will compete with vitamin E for tissue deposition, which may in turn impair the storage quality of pork. For sows, most existing studies on the reproductive antioxidant effect of VA are derived from intramuscular injection trials, and data from practical dietary VA intervention studies are still insufficient. Existing evidence indicates that maternal VA can regulate the transfer of retinol to offspring via the placenta and colostrum-milk, but no consistent dose-response relationship has been confirmed to date. Meanwhile, relevant research in breeding boars remains extremely insufficient. In terms of nutrient interaction, VA shows antioxidant synergistic effects with vitamin E, vitamin C and selenium, while excessive intake of dietary vitamin D, copper or zinc will reduce the bioavailability of VA and weaken its antioxidant protective effect. Current research in this field is constrained by prominent inter-study heterogeneity, insufficient verification of in vivo molecular mechanisms in pigs, and the absence of antioxidant-targeted VA feeding standards adapted to diverse stress scenarios. The systematic summary of the above findings provides an important reference for the formulation of precision antioxidant nutritional strategies for commercial pig herds. To address the current knowledge gaps, future research should focus on constructing standardized porcine experimental stress models, applying multi-omics technical means to dissect tissue-specific VA antioxidant regulatory networks, clarifying the mechanism of maternal-offspring VA transfer under graded dietary VA intake, and optimizing compatible dosage forms and nutrient combinations adapted to on-farm application scenarios. The advancement of related research will help alleviate production losses caused by oxidative stress and promote the construction of healthy pig production systems.