Background and ObjectivesThe aim of this study was to investigate the effect of isolated soy protein on prevention of rheumatoid arthritis in collagen-induced rats. MethodsA total of 24 female Sprague-Dawley rats (body weights: 180-200g) were randomly divided into four equal groups including normal; control of rheumatoid arthritis; rheumatoid arthritis + isolated soy protein (7g/kg); rheumatoid arthritis + dexamethasone (1mg/kg). Experimentally rheumatoid arthritis in rats was induced by subcutaneous injection of collagen ΙΙ emolsified in Incomplete Frund’s Adjuvant in days 1 and 7. Treatments were gavaged to animals for 30 days, once a day. The severity of rheumatoid arthritis symptoms in paws were daily measured in double blind format by standard method. The statistical comparison between the experimental groups was performed by Independent -samples t- test and One-Way ANOVA using SPSS software. ResultsThe clinical symptoms of rheumatoid arthritis started from day 17.4±2.4, 20±4 and 14.1±1.9 in isolated soy protein, dexamethasone and control groups, respectively (P<0.05). The severity of clinical symptoms of rheumatoid arthritis on day 30 was 3.84±2.04, 2±1.78 and 6.5±1.76 in isolated soy protein, dexamethasone and control groups, respectively (P<0.05). At the end of the study, the incidence rates of rheumatoid arthritis in isolated soy protein and control groups was %83.33 and %100, respectively. ConclusionIsolated soy protein decreased the severity of clinical symptoms of rheumatoid arthritis. Therefore, it could be a key factor in prevention and even treatment of rheumatoid arthritis.Keywords: Soybean Proteins; Arthritis, Rheumatoid; Isoflavones.