Subacute ruminal acidosis in high-producing dairy cows: an integrated diet-microbiota-epithelium-host framework for diagnosis and nutritional management.

High-producing dairy cows are commonly fed energy-dense diets to meet the nutrient demands of intensive milk production, but excessive rumen-degradable starch and insufficient physically effective fibre increase the risk of subacute ruminal acidosis (SARA). Rather than being defined only by low ruminal pH, SARA reflects a failure of acid-load adaptation, in which acid production exceeds the capacity for buffering, epithelial absorption, microbial adaptation, and host recovery. This review examines SARA within a diet-microbiota-epithelium-host framework, linking dietary acid-load pressure with microbial dysbiosis, epithelial barrier dysfunction, host inflammatory responses, diagnostic interpretation, and nutritional management. Particular emphasis is placed on interpreting ruminal pH as an indicator of acid-load exposure, integrating host-response and organ-level signals as supportive evidence for risk assessment, and matching nutritional strategies to disrupted points in acid-load control. By connecting mechanisms with diagnosis and nutritional decision-making, this review provides a structured basis for earlier SARA risk recognition and more targeted nutritional management in high-producing dairy cows.