The Gut–Lung Axis in Allergic Asthma: A Narrative Review of Microbial Dysbiosis, Immune Regulation, and Nutritional Modulation

Allergic asthma is a prevalent chronic inflammatory disease of the airways whose pathogenesis has traditionally been attributed to localized immune dysfunction within the lung. However, accumulating evidence from microbiome research supports a broader system-level perspective in which cross-organ interactions contribute to disease susceptibility and progression. In particular, the gut–lung axis has emerged as a key regulatory pathway linking intestinal microbial ecology, immune development, and respiratory health. This review synthesizes current epidemiological, mechanistic, and experimental evidence supporting the role of gut microbiota dysbiosis in allergic asthma. We examine how early-life environmental and nutritional exposures and gut microbiota establishment during critical developmental windows shape long-term immune tolerance and asthma susceptibility. We then summarize characteristic features of asthma-associated gut dysbiosis and discuss how microbial-derived metabolites, including short-chain fatty acids, tryptophan metabolites, pro-allergic lipid mediators such as 12,13-dihydroxy-9Z-octadecenoic acid, and bacterial-derived histamine, modulate distal airway immune responses through epigenetic, receptor-mediated, and immune trafficking mechanisms. Particular emphasis is placed on the role of diet as a key upstream regulator of gut microbiota composition and metabolic function. Finally, we evaluate experimental and translational studies targeting the gut–lung axis, including dietary modulation, microbiome-targeted interventions such as fecal microbiota transplantation, and emerging postbiotic approaches. Collectively, current evidence indicates that gut microbial composition and metabolic function are critical determinants of respiratory immune homeostasis. Targeting the gut–lung axis through nutrition- and microbiome-based strategies offers a promising avenue for the prevention and precision treatment of allergic asthma.