There is a burgeoning literature supporting the efficacy of MI for obesity management [11–14]. A number of studies that have examined the use of MI for obesity management in paediatric and adult populations are described below.

Evidence supporting MI for paediatric obesity management. Recent research on the use of MI in paediatric populations with obesity has yielded promising results. For example, a recent randomized controlled trial (RCT) demonstrated that obese adolescents (mean age = 13 years, mean BMI = 29.57 kg/m²) who were exposed to a standard weight loss management programme of physical activity plus a 6 session MI intervention demonstrated greater weight loss, enhanced physical activity, and increased motivation compared to those in the standard weight loss management programme [15]. However, the significant group differences with respect to weight were not observed 6 months later, and it was speculated that this might have been due to small sample sizes in both groups (n = 28 and 26) [15]. Another pilot RCT suggested that MI is a feasible intervention to enhance healthy eating behaviours in African-American adolescents (aged 13–17 years) with obesity [16]. The group that received the MI intervention reduced their consumption of fast food and soft drinks. They also reported an increase in intrinsic motivation regarding physical activity; however, there were no significant group differences in physical activity levels or BMI. However, given the brevity of the intervention (four sessions) the authors indicated that they did not anticipate differences in BMI [16]. Another study with adolescents (aged 13–17 years) reported that a single session MI intervention lead to reductions in the proportion of calories from fat (as a percentage of the total energy intake) and the amount of dietary cholesterol consumed 3 months following the intervention [17]. Overall, these studies suggest that MI can impact motivation, physical activity, and eating behaviours in youth. The studies conducted with adolescents to date do not demonstrate significant BMI changes following MI interventions; however, a limitation of this literature is that most MI interventions are brief in nature and the potential impact of increased dose (e.g. through additional MI sessions or follow-up booster sessions) remains to be tested.

The use of MI for adolescents (ages 14–18 years) with obesity has been found to be more effective when parental support is incorporated as an active ingredient [18]. A prospective RCT compared participants across three conditions: (1) MI plus parental involvement, (2) MI without parental involvement, and (3) a control group. Both MI interventions focused on healthy eating and physical activity. In the parental involvement condition, adolescents received an extra session of MI with their parents in order to aid parental promotion of the adolescents’ weight-related goals and focus on their attitudes towards their adolescents with respect to eating behaviours and physical activity. Significant differences between groups were found on a variety of measures at 12-month follow-up. The MI plus parental involvement group demonstrated greater improvements in BMI, exercise, and nutritional habits as compared with the MI without parental involvement group and control group [18]. Furthermore, a recent study that focused solely on BMI outcomes within a paediatric primary care setting found that a brief MI intervention (i.e. four sessions) delivered to parents of overweight children (ages 2–8 years) yielded statistically significant reductions in BMI as compared to a group that only received usual care [19]. Within this study, parents received: (1) usual care from their primary care physician, (2) MI counselling from their primary care physician, or (3) MI counselling from their primary care physician supplemented by dietetic counselling. This latter group was the only intervention that differed significantly from usual care [19]. It was hypothesized that adding more sessions to the primary care MI intervention alone group might have led to significant differences. Once again, dose of treatment and means of administration of MI intervention techniques are queried as possible factors to be manipulated in producing longer term impact on weight management.

In a recent meta-analysis, the use of parental involvement with MI strategies has also been shown to be beneficial for adolescents presenting with other health-related behaviours, including obesity [20]. The interested reader is referred to this meta-analysis for an overview of 12 studies that examined the use of MI for obesity in paediatric healthcare [20]. According to the meta-analysis, several studies suggested that MI as a stand-alone intervention was more successful than when it was combined with another intervention. We now turn to an overview of the recent empirical evidence for MI interventions for adult obesity management .

Evidence supporting MI for adult obesity management. The literature supporting the use of MI in the treatment of adult obesity has grown rapidly over the past decade. Burke and colleagues [21] were the first to publish a meta-analysis investigating the efficacy of MI in controlled trials across health-behaviour domains, including diet and exercise. They included 30 studies examining MI in their analyses. The MI interventions were, on average, 180 min shorter in duration than other active treatments to which MI was compared, and yet it showed comparative efficacy in addressing dietary and exercise problems. Rubak, Sandboek, Lauritzen, and Christensen [22] have since conducted the most comprehensive review of MI, with the inclusion of 72 RCTs of MI in the treatment of several disease indicators , including weight. MI produced a statistically significant effect in almost three quarters (74 %) of the studies reviewed, with a large combined estimate effect size for BMI reduction in particular. These large meta-analyses speak to the potential role of MI in managing obesity for adults.

A more recent systematic review of the literature examined the use of MI interventions in primary care for individuals with obesity. Twenty-four RCTs were examined and most patients were reported to be “obese” or “overweight” although specific BMIs were not reported for every study [23] Results suggested that in over one-third of the studies analysed, patients who received MI interventions experienced greater weight loss than those who received usual care, often consisting of what was referred to as “standard dietary care ” [23]. Half of the studies reported that participants lost 5 % of their initial weight. This review suggests that MI strategies can be used effectively within a primary care setting and may begin to aid in dietary changes that could provide initial weight loss [23].

MI has not only shown promise for improving dietary patterns and physical activity levels, but also for affecting change in obesity-associated medical conditions. For example, a recent study examined the long-term impact of a 6-month MI intervention on both behavioural and biomedical risk factors for cardiovascular disease [7]. A physical activity specialist as well as a registered dietitian delivered the intervention and this group was compared to those receiving standard information. At 18-month follow-up, significant improvements were noted in physical activity (walking) and cholesterol levels although initial BMI reductions were not maintained [7]. MI has also been found effective in improving patients’ glucose management and physical activity levels both alone and in combination with other interventions [24]. Thus, the use of MI as a clinical intervention appears to have broader applications for obesity management, extending beyond weight and dietary changes.

MI interventions can be flexibly used across health care settings. In this regard, it has been empirically studied not only as a stand-alone intervention for improving BMI, eating patterns, and self-efficacy [12], but also an adjunctive intervention intended to promote adherence to another treatment. For example, adding MI to a standard behavioural programme for individuals with obesity and type 2 diabetes enhanced adherence to programme recommendations, including higher attendance, greater completion of food diaries, and more frequent blood glucose level recordings [8]. In a larger study examining the same outcomes, increased adherence and engagement in a behavioural programme accounted for a significant portion of the enhanced weight loss seen in the MI group at both the 6- and 18-month follow-ups [25]. Taken together, these studies suggest that MI can be used as an adjunct to behavioural weight loss programmes to increase treatment adherence and optimize weight loss outcomes.