HP and rheumatoid arthritis: While a cross-sectional study by Mucientes et al. [31] found significant correlations between serum zonulin concentrations and TNF-α concentration in patients with rheumatoid arthritis (r = 0.266; p < 0.05), a study evaluating intestinal barrier markers in RA patients before and after DMARD treatment found that zonulin-related proteins did not differ significantly from controls and showed no change following DMARD therapy, suggesting that zonulin may be a less reliable barrier marker in well-established disease than in the preclinical stage [30].