Sustainable Protein Transitions: A Comprehensive Review of Insect Meal in Broiler Diets—Nutritional Value, Immunomodulatory Effects, Gut Health Interactions, and Future Perspectives

Broiler production keeps intensifying, and the environmental cost of the protein sources that feed it, principally soybean meal and fishmeal, has become harder to justify. Insect meal, derived chiefly from black soldier fly larvae (Hermetia illucens), yellow mealworm (Tenebrio molitor), and housefly larvae (Musca domestica), has emerged as a candidate able to address nutritional, immunological, and ecological goals at once. This review evaluates the evidence on the nutritional composition, digestibility, growth performance, immunomodulatory mechanisms, and gut health effects of insect meal in broiler diets, and weighs these findings against sustainability and economic considerations. The amino acid profile of insect meal is favorable, the fatty acid composition is distinctive, and bioactive compounds, chitin, antimicrobial peptides, and lauric acid among them, shape both innate and adaptive immune responses, reshape the gut microbiota, and reinforce mucosal integrity. Growth performance at partial replacement levels of soybean meal or fishmeal (5–15%) is broadly encouraging, although results differ across insect species, inclusion rate, and processing method. The immunomodulatory pathways involved, Toll-like receptor signaling, cytokine regulation, macrophage polarization, and immunoglobulin synthesis, converge on chitin acting simultaneously as a prebiotic substrate and an immune adjuvant. Critical knowledge gaps persist, including substantial variability in biological outcomes across insect species and rearing substrates, the absence of standardized processing protocols, limited mechanistic data from avian in vivo challenge models, and insufficient long-term validation under commercial production conditions. Cost and regulatory fragmentation across jurisdictions still constrain commercial uptake. Realizing the full potential of insect meal in sustainable broiler production will depend on multi-omics research, precision nutrition tools, and rigorous life-cycle assessment.