Thymol, with antimicrobial, antioxidative, and anti-inflammatory properties, shows promise in improving epithelial barrier function and reducing inflammation. This study investigated whether thymol could alleviate the detrimental impacts of severe feed restriction (SFR) on barrier function by its effects on inflammatory signaling pathways and tight junction (TJ) protein expression. Thirty Baluchi ram lambs (45.2 ± 6.2 kg; 8 months of age) were randomly allocated to three groups: control (CON), SFR, and SFR plus thymol (SFRT), with 10 lambs per group serving as experimental replicates. The experiment consisted of four weeks of feed restriction (FR) followed by four weeks of ad libitum feeding (Ad-L). Thymol (300 mg/d per lamb) was orally administered to SFRT group during the FR period. Growth performance, ruminal fermentation parameters, plasma metabolites, and inflammatory biomarkers were assessed. Histological measurements of the rumen tissue, as well as mRNA expressions of TJ proteins and proinflammatory cytokines in the rumen epithelium were analyzed. Compared with CON, the final body weight of lambs significantly decreased during FR in SFRT and SFR groups (P < 0.001). During the Ad-L, FR lambs showed greater average daily gain and feed efficiency compared to CON (P < 0.05). Total volatile fatty acid (VFA) (P = 0.033), acetate (P = 0.025), and propionate (P = 0.014) concentrations were significantly higher during FR compared to Ad-L, while ruminal pH was greater during the Ad-L period (P = 0.017). SFRT lambs tended to have higher concentrations of total VFA (P = 0.102), acetate (P = 0.073), isovalerate (P = 0.071), and isobutyrate (P = 0.080) than SFR. During the Ad-L period, the plasma concentration of haptoglobin in SFRT lambs was significantly lower than that in both CON and SFR (P = 0.005). Significant differences were observed in papillae length, muscle layer thickness, and connective tissue width among the experimental groups (P < 0.05). Thymol upregulated the expression of claudin-1 (CLDN-1) (P = 0.015), occludin (OCLDN) (P < 0.001), and zonula occludens 1 (ZO1) (P < 0.001) in the ruminal epithelium and mitigated the upregulation of interleukin-6 (IL-6) mRNA expression induced by feed restriction (P < 0.001). In conclusion, thymol administration during SFR has the potential to maintain rumen epithelial barrier integrity and modulate inflammatory responses in lambs.
