IMPORTANCE: Knee osteoarthritis (OA), a common cause of chronic pain and disability, has biomechanical and inflammatory origins and is exacerbated by obesity. OBJECTIVE: To determine whether a >= 10% reduction in body weight induced by diet, with or without exercise, would improve mechanistic and clinical outcomes more than exercise alone. DESIGN, SETTING, AND PARTICIPANTS: Single-blind, 18-month, randomized clinical trial at Wake Forest University between July 2006 and April 2011. The diet and exercise interventions were center-based with options for the exercise groups to transition to a home-based program. Participants were 454 overweight and obese older community-dwelling adults (age >= 55 years with body mass index of 27 to 41) with pain and radiographic knee OA. INTERVENTIONS: Intensive diet-induced weight loss plus exercise, intensive diet-induced weight loss, or exercise. MAIN OUTCOMES AND MEASURES: Mechanistic primary outcomes: knee joint compressive force and plasma IL-6 levels; secondary clinical outcomes: self-reported pain (range 0 to 20), function (range 0 to 68), mobility, and health-related quality of life (range 0 to 100). RESULTS: Three hundred ninety-nine participants (88%) completed the study. Compared with exercise participants, knee compressive forces were lower in diet participants and IL-6 levels were lower in diet and diet+exercise participants. Mean weight loss for diet+exercise participants was 10.6 kg (11.4%); for the diet group 8.9 kg (9.5%); and for the exercise group 1.8 kg (2.0%). After 18 months, knee compressive forces were lower in diet participants (mean 2,487 N; 95% CI 2,393 to 2,581) compared with exercise participants (2,687 N; 95% CI 2,590 to 2,784, pairwise difference (delta) exercise versus diet = 200 N; 95% CI 55 to 345; p = 0.007). Concentrations of IL-6 were lower in diet+exercise (2.7 pg/mL; 95% CI 2.5 to 3.0) and diet participants (2.7 pg/mL; 95% CI 2.4 to 3.0) compared with exercise participants (3.1 pg/mL; 95% CI 2.9 to 3.4; delta exercise versus diet+exercise = 0.39 pg/mL; 95% CI -0.03 to 0.81; p = 0.007; delta exercise versus diet = 0.43 pg/mL; 95% CI 0.01 to 0.85, p = 0.006). The diet+exercise group had less pain (3.6; 95% CI 3.2 to 4.1) and better function (14.1; 95% CI 12.6 to 15.6) than both the diet group (4.8; 95% CI 4.3 to 5.2) and exercise group (4.7; 95% CI 4.2 to 5.1, delta exercise versus diet+exercise = 1.02; 95% CI 0.33 to 1.71; p (pain) = 0.004; 18.4; 95% CI 16.9 to 19.9; delta exercise versus diet+exercise), 4.29; 95% CI 2.07 to 6.50; p (function) < 0.001). The diet+exercise group (44.7; 95% CI 43.4 to 46.0) also had better physical health-related quality of life scores than the exercise group (41.9; 95% CI 40.5 to 43.2; delta exercise versus diet+exercise = -2.81; 95% CI -4.76 to -0.86; p = 0.005). CONCLUSIONS AND RELEVANCE: Among overweight and obese adults with knee OA, after 18 months, participants in the diet+exercise and diet groups had more weight loss and greater reductions in IL-6 levels than those in the exercise group; those in the diet group had greater reductions in knee compressive force than those in the exercise group. TRIAL REGISTRATION: ClinicalTrials.gov identifier NCT00381290.

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