Twelve RCTs were included in the review. Eleven RCTs were included in the analysis (1,448 participants, range 22 to 599). One RCT was excluded as the interventionists were randomised rather than the participants. Eight RCTs scored 3 points for quality on the Jadad scale, two RCTs scored 2 points, and two RCTs scored 1 point. Eight RCTs reported randomisation and allocation concealment and blinding. Five RCTs reported an intention-to-treat analysis. All trials reported losses to follow-up ranging from 8% to 35%. The duration of follow-up ranged from three to 18 months)
Motivational interviewing was associated with a non statistically significant reduction in body mass compared with control treatments (SMD -0.51, 95% CI -1.04 to 0.01; 11 RCTs), but there was evidence of significant heterogeneity (Ι²=95%). There was no evidence of publication bias.
There was a significant reduction in body weight (WMD -1.47kg, 95% CI -2.05 to -0.88; eight RCTs) and a non statistically significant reduction in BMI (WMD -0.25 kg/m-2, 95% CI -0.50 to -0.01; seven RCTs) for motivational interviewing compared with control treatments. There was evidence of heterogeneity for both these outcomes (body weight Ι²=55%; BMI Ι²=24.5%).
Subgroup analysis found that weight as a primary outcome, duration of treatment longer than six months, a treatment fidelity measure and the use of a behavioural weight-loss programme were associated with an increased effect of motivational interviewing on body mass. Meta-regression found no significant sources of heterogeneity.