A Cochrane review included 14 RCTs involving a total of 707 subjects and 1 controlled trial with 130 patients. A single trial found that fasting, followed by 13 months on a vegetarian diet, reduced pain (mean difference (MD) on a 0 to 10 scale -1.89, 95% CI -3.62 to -0.16), but not physical function or morning stiffness immediately after intervention. In another trial a 12-week Cretan Mediterranean diet reduced pain (MD on a 0 to 100 scale -14.00, 95% CI -23.6 to -4.37), but not physical function or morning stiffness immediately after intervention. 2 trials compared a 4-week elemental diet with an ordinary diet and reported no significant differences in pain, function or stiffness. The effects of vegan and elimination diets were uncertain. When comparing any dietary manipulation with an ordinary diet a significantly higher total drop-out of 10% (risk difference (RD) 0.10, 95% CI 0.02 to 0.18), higher treatment-related drop-out of 5% (RD 0.05, 95% CI -0.03 to 0.14) and a significantly higher weight loss (weighted mean difference -3.23, 95% CI -4.79 to -1.67 kg) were found in the diet groups compared to the control groups.
Comment: The quality of evidence is downgraded by limitations in study quality, and by imprecise results (limited study size for each comparison).