Abstract
Increasing the proportion of concentrate in diets can effectively improve ruminant production, and is therefore widely used. However, high-concentrate diets (HCD) enriched with rapidly fermentable carbohydrates can accelerate the production of lactate and short-chain fatty acids (SCFAs). The accumulation of lactate and SCFAs in the rumen leads to a reduction in rumen fluid pH, potentially resulting in subacute rumen acidosis (SARA), which can decrease dry matter intake (DMI), induce local and systemic inflammation, and cause other negative impacts on the host. The substantial prevalence of SARA attributable to long-term HCD causes considerable economic losses, as it can decrease DMI by up to 20%. Understanding its mechanisms and pathogenesis is essential. The rumen epithelium (RE), which is in direct contact with rumen fluid, is an important tissue in the rumen due to its roles in absorption, transport, and barrier functions. The changes that occur in RE under HCD and the subsequent impacts of these changes are worth exploring. In the short term, HCD feeding promotes RE cell proliferation and upregulates the activity of various transporter proteins, enhancing RE absorption and metabolism. However, with prolonged feeding, these functions of RE are negatively affected, accompanied by the development of inflammation. This review elucidates the structure, the functions, and the responses of RE under HCD, providing a detailed analysis of SARA pathogenesis at the cellular and molecular levels.