Obesity Moderates the Effects of Motivational Interviewing Treatment Outcomes in Fibromyalgia

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Obesity is a common comorbid condition among patients with fibromyalgia (FM). Our objective was to assess if obesity moderates the treatment benefits of exercise-based motivational interviewing (MI) for FM.

Materials and Methods:

This is a secondary data analysis of a completed clinical trial of 198 FM patients who were randomized to receive either MI or attention control (AC). Using body mass index (BMI) to divide participants into obese (BMI≥30 kg/m2) and nonobese (BMI<30 kg m2) groups, mixed linear models were used to determine interaction between treatment arms and obesity status with regards to the primary outcome of global FM symptom severity (Fibromyalgia Impact Questionnaire, FIQ). Secondary measures included pain intensity (Brief Pain Inventory), 6-Minute Walk Test, and self-reported physical activity (Community Health Activities Model Program for Seniors).


Of the 198 participants, 91 (46%) were nonobese and 107 (54%) were obese. On global FM symptom severity (FIQ), the interaction between treatment arms and obesity status was significant (P=0.02). In the nonobese group, MI was associated with a greater improvement in FIQ than AC. In the obese group, MI participants reported less improvement in FIQ compared with AC. The interaction analysis was also significant for Brief Pain Inventory pain intensity (P=0.01), but not for the walk test and self-reported physical activity.


This is the first study to show that obesity negatively affects the treatment efficacy of MI in patients with FM. Our findings suggest that exercise-based MI may be more effective if initiated after weight loss is achieved.

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