Plant-Derived Feed Additives Modulate the Microbiota–Gut–Immune Axis in Broiler Chickens: From Intestinal Homeostasis to Physiological Resilience

The withdrawal of antibiotic growth promoters has fundamentally changed the priorities of poultry nutrition, shifting attention from growth promotion towards maintenance of intestinal homeostasis and physiological resilience, defined here as the capacity of the organism to maintain or restore functional homeostasis when exposed to biological, nutritional, or environmental challenges. Although plant-derived feed additives have attracted considerable interest as natural alternatives, their biological activity is still commonly interpreted as a collection of independent antimicrobial, antioxidant, or immunomodulatory effects. This fragmented perspective may not fully capture the interconnected and context-dependent nature of their effects across experimental models. This review develops an integrated conceptual framework in which phytogenic feed additives are considered as potential systems-level modulators of the microbiota–gut–immune axis rather than as isolated sources of bioactive compounds. We synthesise experimental evidence from 155 broiler chickens identified through searches of PubMed, Web of Science, Scopus, and Google Scholar from database inception to June 2026, covering microbial ecology, intestinal morphology and barrier function, microbial metabolites, redox balance, inflammatory signalling, and innate and adaptive immunity across infectious, nutritional, and environmental challenge models. Particular attention is given to the different levels of evidence provided by culture-based and targeted analyses, microbiome profiling, integrated host–microbiome studies, and multi-omics approaches. The available evidence suggests that structurally diverse phytochemicals can influence overlapping regulatory networks linking microbial ecology, epithelial barrier integrity, redox balance, inflammatory signalling, and immune function. However, the magnitude and direction of these responses vary substantially according to botanical source, phytochemical composition, dose, formulation, duration of supplementation, animal genotype, basal diet, and experimental challenge. Collectively, the available evidence supports a paradigm shift from compound-centred interpretation towards network-based regulation of host homeostasis. Within this context, physiological resilience refers to the capacity to maintain or restore functional homeostasis in response to biological, nutritional, or environmental challenges. It may emerge from coordinated interactions among the intestinal microbiota, the epithelial barrier, and the immune system. Phytogenic supplementation may contribute to resilience by simultaneously modulating several components of this network, although causal relationships remain incompletely established for many proposed mechanisms. This systems-level perspective provides a framework for integrating heterogeneous evidence and for identifying priorities for future research, including dose–response relationships, phytochemical combinations, longitudinal multi-omics approaches, and validation under commercially relevant production conditions, with implications for precision poultry nutrition.