Soy Isoflavones in Poultry Production: Research Advances on Metabolism, Functions, and Applications

Soy isoflavones are a class of natural secondary metabolites with phytoestrogenic activity found in soybeans, primarily classified into free-type and conjugated-type, with the conjugated form accounting for 97%–98% of the total. This review summarizes the absorption, transformation, and metabolic pathways of soy isoflavones in the small intestine, liver, and large intestine of chickens. Conjugated isoflavones must be hydrolyzed by gut microbiota-secreted β-glucosidase into free aglycones before absorption. Free isoflavones undergo phase I and phase II metabolism in the liver, with some metabolites excreted into the intestine via bile, forming enterohepatic circulation. The unabsorbed fraction enters the large intestine and is further converted into highly active metabolites such as equol. Gut microbiota-mediated hydrolysis and conversion are key to producing these bioactive metabolites. Application studies in poultry production demonstrate that soy isoflavones can effectively improve laying and growth performance, enhance meat quality, and boost antioxidant capacity, immune regulation, and intestinal health by modulating endocrine function, activating antioxidant and immune-related signaling pathways (e.g., NF-κB and MAPK), and regulating gut microbiota. This review systematically summarizes the multifaceted biological effects and underlying mechanisms of soy isoflavones, providing a theoretical basis for their rational and standardized application in livestock and poultry farming.