Animal stress is a complex physiological state characterized by a suite of non-specific responses (e.g., lethargy, anorexia, and impaired growth) to various internal or external threats, collectively known as stressors. It is the synthesis of non-specific responses produced by the body to various external or internal stimuli. Animals in livestock production are often stressed by breeding density, inappropriate temperature and humidity, harmful gases, noise and complicated immunization. Consequently, the nutritional requirements and underlying metabolic mechanisms in stressed animals are critical and growing research hotspots. Emerging evidence has shown that nutritional intervention can maintain animal health and performance under stresses. In addition, the regulation of metabolic pathways and targets under different stress states is also a potential way to alleviate the stress response of animals. With the rapid development of intensive farming and the aggravation of environmental changes, animals are facing increasing challenges such as heat stress, transportation stress, and pathogen infection. The resulting metabolic disorders and health problems seriously restrict the production efficiency of animals. This review aims to systematically analyze the variation in animal nutritional requirements under stress, clarify the molecular mechanism of metabolic imbalance, and summarize the regulatory targets and effects of nutritional intervention strategies, providing theoretical basis and technical support for solving stress-related problems in livestock production.

During swine gestation, the placenta serves as the sole interface for nutrient, gas, and signaling molecule exchange between the dam and fetuses, and its function critically determines pregnancy outcomes. Branched-chain amino acids (BCAAs)—leucine, isoleucine, and valine—are substrates for protein synthesis, precursors for glutamate and glutathione, and signaling molecules that activate the mechanistic target of rapamycin (mTOR) pathway. This review synthesizes current evidence on BCAA transport, catabolism, and signaling in the porcine placenta, organized around four mechanistic axes: transporter-mediated transplacental transfer, local catabolism supporting energy production and antioxidant defense, mTORC1-dependent regulation of growth and differentiation, and modulation of endocrine and immune homeostasis. Both deficiency and excess of BCAAs compromise placental function, contributing to intrauterine growth restriction and metabolic programming abnormalities through a U-shaped dose–response relationship. Critical knowledge gaps remain, including the reliance on rodent and in vitro models, the absence of standardized supplementation protocols and dose–response data, unaddressed confounders (parity, litter size, environment), and the lack of formal power analyses or meta-analytic synthesis. We propose precision dietary strategies to optimize BCAA supplementation during gestation for improved reproductive performance in swine.