Aim The present study aimed at investigating the effects of Lactobacillus casei 01 supplementation on symptoms and inflammatory biomarkers of rheumatoid arthritis (RA) in women.

Method In this randomized double‐blind clinical trial, female patients with established RA for more than 1 year, 20–80 years of age and body mass index (BMI) lower than 40, who followed stable medication for 3 months prior to the supplementation, were randomly allocated to receive either one capsule containing 108 colony forming units (CFU) of L. casei 01, or a placebo for 8 weeks; allocation was stratified by BMI and menopausal status. Disease activity score‐28 (DAS28) was calculated, European League Against Rheumatism (EULAR) response was evaluated and the cytokines, interleukin (IL)‐1β, IL‐6, IL‐10, IL‐12 and tumor necrosis factor (TNF)‐α were measured.

Results Thirty patients were recruited in each group; 22 and 24 patients were analyzed in the probiotic and placebo groups, respectively. L. casei 01 supplementation decreased serum high‐sensitivity C‐reactive protein (hs‐CRP) levels, tender and swollen joint counts, global health (GH) score and DAS28 (P < 0.05). More patients in the L. casei 01 group had moderate response to the treatment, based on the EULAR criteria, at the end of the study (P < 0.01). At the end of the study, a significant difference was observed between the two groups for IL‐10, IL‐12 and TNF‐α changes through the study course (P < 0.05), in favor of the probiotic group. No adverse effects were reported for the intervention.