In this randomized controlled trial, we demonstrate that a mHBCSS is effective as stand-alone treatment for obesity. During the 6-month intervention, the group with immediate access to mHBCSS achieved greater weight reduction than the wait-list control group. Importantly, the weight loss achieved was maintained until 12 months. Moreover, a quarter of the mHBCSS group achieved clinically significant weight loss of 5% or more after six months’ use of mHBCSS.

Without any other intervention component, mHBCSS facilitated weight loss in the group with immediate access to mHBCSS. The weight loss result in the group was similar to the results with other smartphone and stand-alone web-based interventions [22]. Better weight loss results than in our trial have been demonstrated in eHealth intervention studies, where the study settings have had typical limitations for eHealth studies such as low sample size [23,24,25] or short intervention without follow-up period [24]. For example, six months intervention with MMM app which applies goal setting, self-monitoring (dietary and physical activity) and feedback features built mainly around the calorie counting resulted in weight loss of –4.6 kg (95% CI –6.2 to –3.0) versus –2.3 kg (95% CI –3.1 to –1.5) achieved in this study [23,24,25]. Weight loss results (reported mean ± SE) were also greater in the 6-month app intervention implemented by diet and step self-monitoring and podcast including behavioural content (–6.8 kg ± 0.8) [23,24,25]. Both of these studies lacked a no-intervention control group, had no follow-up after the 6-month intervention to explore effects on weight maintenance and had small sample sizes in the app groups (n = 43 and n = 42, respectively). A more intensive CBT intervention with 19 group meetings delivered in-person during the 6-month intervention demonstrated a greater weight loss result (mean −8.1 kg ± SD 6.8) [26]. This study had no follow-up after intervention and had smaller number of participants (n = 95) than our trial.

There is a lack of quality research comparing stand-alone eHealth (application based) interventions to in-person interventions. A non-significant trend on the superiority of the face-to-face counselling over the eHealth interventions has been reported [27, 28]. However, and perhaps most importantly, the most efficient interventions seem to be the ones combining the face-to-face counselling with the eHealth interventions [12, 29, 30]. This was also the result in our previous study with the web-based HBCSS in combination with face-to-face counselling [19]. Thus, for the most optimal weight loss intervention, the future research needs to find the most efficient way to combine these intervention approaches. However, our current study shows that a mHBCSS can be a relevant obesity treatment tool even as a stand-alone therapy, e.g., when the resources limit the availability of face-to-face counselling.

After 12 months, the maintained weight loss result of this study (−2.1%, 95% CI –3.3 to –0.9) achieved by mobile app delivered intervention is comparable with the result with face-to-face counselling observed in one arm of our previous HBCSS study [19] where 6-month face-to-face CBT counselling, including eight clinical nutritionist-delivered sessions and utilizing strategies similar to mHBCSS (building self-efficacy, self-monitoring and feedback), resulted in a weight loss of −1.8% (95% CI −2.9 to −0.6) at the 12-months follow up. The 12-month weight loss result in the group with immediate access to mHBCSS was also comparable to the result of the study arm with the web-based HBCSS as a stand-alone treatment in our previous study (−2.1% vs. −1.4%) [19]. It should be noted that our previous trial involved participants with a lower BMI range (27–35 kg/m2) than our current trial (30–40 kg/m2).

The mHBCSS aims at long-term changes in body weight by inducing lifestyle changes beneficial for weight loss maintenance [6]. To accomplish these lifestyle changes, mHBCSS utilized principles of CBT, ACT and PSD, all of which have been demonstrated to be effective in the adoption of the behavioural changes needed for sustainable weight loss [7, 31, 32], especially when they are combined in a digital intervention [32]. The behavioural and information systems science approach to weight loss possibly facilitated the sustainable weight loss achieved with mHBCSS in our trial.

A weight loss (between 0 and 10%) achieved by an intervention lasting 6 to 12 months leads to significant improvements in several cardiovascular risk factors, as analysed within-intervention groups in a meta-analysis [33]. In the present study, 74% of the group with immediate access to mHBCSS lost weight during the 6-month mHBCSS intervention. At the 6-month time point, significant improvements were observed in blood pressure and plasma triglyceride levels. However, there was no significant difference in blood pressure and triglycerides when compared between the groups. A significant difference was observed between the groups in the change of waist circumference, which is associated with increased risk of CVDs [34]. In the Look AHEAD trial, a decrease in waist circumference (−9.66 cm ± 0.16), achieved by intensive lifestyle intervention including face-to-face counselling for one year, was associated with lower risk for cardiovascular outcomes compared with participants with increased waist circumference regardless of weight change [35]. Therefore, the sustained decrease in waist circumference achieved by mHBCSS is promising when considering the possible preventive effect of mHBCSS on CVDs.

The importance of patients’ engagement in the intervention for successful weight loss was observed as those in the group with immediate access to mHBCSS who read more articles provided by mHBCSS and recorded their weight more actively had significantly better weight loss results compared with participants with lower user activity. The most effective weight loss application is probably the one that succeeds in engaging patients for the longest time [14]. From this perspective, the mHBCSS had many strengths. The mHBCSS was delivered by an easily accessible mobile application that included engaging features designed by utilizing the principles of PSD [17] and methods of CBT and ACT. In addition to self-monitoring features, the mHBCSS included short and easily performed exercises such as multiple-choice questions that activated participants to reflect on their health behaviour and may have enhanced their engagement with the intervention. Simultaneously, it is remarkable that despite high user activity, 26% of the participants in the mHBCSS group ended up gaining weight (Fig. 2). It is possible that the persons gaining weight with mHBCSS intervention require in-person counselling and more social support to strengthen their self-efficacy towards weight loss process. The reasons behind the interindividual differences in the efficacy of the mHBCSS, and especially weight gain, require further study and are an important topic for our future analyses.

The same strengths of the mHBCSS described above can explain the exceptionally high retention rate in this study. Compared with other behavioural eHealth weight loss intervention studies, most delivered by website or e-mail and including also non-eHealth components (e.g. telephone contacts, face-to-face counselling and written material), this study achieved a remarkably high retention rate at the 6- and 12-month time points compared with the mean retention rate reported in a meta-analysis including 84 trials (98.5% and 89.0% in this study vs. 78% in a meta-analysis) [12]. Interestingly, the retention rate in this study seems to be higher than in other stand-alone digital intervention studies in which one meta-analysis, which excluded hybrid interventions, showed that 7 out of the 11 retrieved studies had attrition rates ≥20% [36]. The retention rate in this mHBCSS study was also higher compared with our previous study on web-based HBCSS and based on the same principles as the mHBCSS (91% vs 81% at the 12-month time-point) [10]. However, it should be noted that the enrolment to our current trial was targeted for people employed at proximity to our research centre which probably enhanced the retention. Smartphones are considered as the most effective platform for eHealth interventions, with higher adherence than in web-based or personal digital assistant interventions [22]. In addition to higher engagement, mobile interventions are easily accessible, with no time-limits for use. Self-management of obesity treatment with the help of mobile interventions could not only help ease the burden on healthcare providers but also be more suitable and pleasant for some patients when compared with traditional intervention modalities, such as face-to-face counselling, which are usually offered only during office hours.

Our study has some limitations. We were not able to have a no-treatment control group for the whole duration of the study as access to mHBCSS was given to the wait-list control group after a 6-month control period for motivational and ethical reasons. We assumed that if the control group had not been given access to mHBCSS, recruitment would have been more difficult and the dropout rate higher. In addition, as in most eHealth studies, blinding was considered unfeasible. Furthermore, the majority of the participants were women employed in the local University and the University Hospital affecting the generalizability of the results.

This randomized controlled trial demonstrates that after a 6-month intervention, a stand-alone mHBCSS that includes evidence-based behavioural change strategies induces a significant weight loss which is maintained up to 12 months. The weight loss was accomplished with minimal resources as the participants did not receive any other type of counselling. Therefore, the stand-alone mHBCSS is a useful, resource-efficient and scalable method to treat obesity when resources to offer face-to-face counselling for obesity are limited or when remote treatment suits better to the patient’s lifestyle. There is a need for studies on the implementation of interventions such as mHBCSS in the standard care of obesity in order to find the most cost-effective ways to achieve maximal long-term weight loss results in the population at large.