This study was approved by the Institutional Review Board of the University of Tokyo (2022061NI). The research protocol was registered with the University Hospital Medical Information Network Clinical Trials Registry (UMIN000048667). All the participants provided written informed consent. All data were anonymized, and access was limited to authorized research team members only. Participants received up to JP¥ 20,000 (approximately US $150 based on the exchange rate at the time of study start) via QUO Card.

Figure 1 shows screenshots of the smartphone app we developed for obesity management. Users launch the app upon receiving a notification at the end of the day, typically in the evening at an individually preset time, and select behaviors to perform the following day (Figure 1A). The selected behaviors can be further modified for specificity (Figure 1B). The user receives reminder notifications several times the following day regarding the behaviors they have set. Upon receiving a notification at the end of the following day, users launch the app to record whether they performed the selected behaviors and log any additional behaviors (Figure 1C). Users then select the behaviors to perform the following day. The researchers prepared a behavioral list based on the cognitive behavioral therapy manual [9] and the Japanese guidelines for obesity management [4]. Each participant can also create original items individually. The app displays a complimentary message and a graphical presentation of the number of behaviors performed immediately after recording them.

The presentation order was optimized to display easily achievable, low-effort items earlier on the screen (Figure 1A). This optimization was based on the theory of cognitive behavioral therapy [9,10] and the Fogg behavior model [25], similar to a study on a smartphone app using reinforcement learning to promote health-related activities [15]. A multiarmed bandit algorithm was used, which simultaneously performs exploration (collecting data to determine which items are achievable) and exploitation (presenting achievable items based on the collected data). This has the advantage that it can be applied to even a small number of users while ensuring a diversity of suggestions [14].

We used Thompson sampling, a multiarmed bandit algorithm that does not require prior hyperparameter adjustments [14]. In this study, each behavior was treated as an independent arm and scored by a randomly sampled value of θ from the following beta distribution, where N is the number of times the user selects the item and a is the number of executions:

When N is small, samples of θ from the beta distribution exhibit a large variance, and the model assigns random scores to behaviors with few selections, which promotes exploration. As N increases, the samples converge toward the actual achievable rate of each behavior, which promotes exploitation by assigning higher scores to behaviors with higher completion rates than to those with lower completion rates. In this study, each participant had an independent model trained using only their own behavior data. Each behavior was initialized with a uniform prior over the interval (0, 1).

The app was developed using Flutter (Google) and Firebase (Google). Reinforcement learning was implemented using Python (version 3.9; Python Software Foundation) on the Google Cloud Platform. The app was distributed exclusively to participants’ smartphones (iPhone or Android) through DeployGate (DeployGate, Inc) [26] rather than being publicly released on the Apple App Store or Google Play Store.

This study included individuals diagnosed with “obesity disease” according to criteria established by the Japan Society for the Study of Obesity. This disease is defined as a BMI of ≥25 kg/m² with one or more obesity-related disorders such as type 2 diabetes or visceral fat accumulation that likely causes health problems [4,27]. Individuals aged ≥18 years attending the University of Tokyo Hospital were eligible for this study. Exclusion criteria were (1) extremely severe physical or psychiatric diseases, (2) conditions for which the burden of the study would be harmful, and (3) lack of decision-making ability.

A previous study examining the feasibility of an app using a multiarmed bandit algorithm included approximately 20 participants over 3 weeks [15]. Following this, we set the study period to 4 weeks (28 days) and the sample size to 20. Eligible individuals were recruited continuously until the target sample size was achieved.

Figure 2 provides the study schedule. At the beginning of the study, the Japanese version of the International Physical Activity Questionnaire (IPAQ) long form, last–7-day version, was used to assess physical activity [28]. The brief-type self-administered diet history questionnaire (BDHQ) was used to assess nutritional intake [29]. The BDHQ consists of questions assessing the consumption frequency of 58 food and beverage items over the preceding month and estimates average daily energy intake and various nutrient intakes.

During the study, daily behaviors were recorded using the app. Participants received a notification daily at a random time within a prearranged 1-hour interval and recorded their mood status (Figure 3). Nine items from the Depression and Anxiety Mood Scale were used to calculate scores for anxiety, positive affect, and depression [30]. This was recorded on a 101-point (0‐100) visual analog scale. Additionally, we provided the participants with a body scale (HN300T2; OMRON) connected via Bluetooth to a smartphone (iPhone or Android) to record weight automatically [31]. Their weight was recorded daily upon waking.

At the end of the 4-week study, the IPAQ and the BDHQ were administered, as at the start of the study. The top 5 behaviors displayed on the app screen were evaluated on paper using a visual analog scale regarding their subjective likelihood of achievement. In addition, qualitative interviews were conducted to identify potential improvements to the app.

Feasibility was assessed via app use rate, defined as the percentage of days on which the behavior was recorded during the study period. Relevant EMI studies using smartphone apps for obesity or type 2 diabetes have reported recording rates of approximately 80% to 95% [20,32,33], which we set as the benchmark for feasibility.

BMI, physical activity measured using the IPAQ, and calorie and salt intake measured using the BDHQ were compared between the start and end of the study using the Wilcoxon signed rank test, with statistical significance set at .05.

These analyses were performed using R (version 4.3.1; R Foundation for Statistical Computing).

In this post hoc analysis, we combined data on daily behaviors with mood status recorded within the preceding 24 hours. Only behavioral records reviewed more than 12 hours and less than 36 hours after setting were included. A linear mixed-effects model was developed using this nested longitudinal dataset, with the daily number of behaviors performed as the outcome. The outcome was defined as the sum of the preselected behaviors that were actually performed and any additional behaviors.

Level 1 variables included (1) preceding mood status (anxiety, positive affect, and depression) measured using the Depression and Anxiety Mood Scale, which was scaled from 0 to 1; (2) the difference between the weight recorded immediately before the behavior review and the weight at the start of the study; (3) the difference between the weight recorded immediately before the behavior review and the weight recorded at one earlier time point; and (4) the number of weeks, ranging from 1 to 4. Level 2 variables included (1) sex; (2) BMI at the start, centered by subtracting 35; and (3) the presence of comorbid psychiatric disorders. The inclusion of these variables and random effects was determined based on the Akaike information criterion.

R (version 4.3.1) and the package lmerTest (version 3.1-3) were used for this analysis.

Participants were recruited from August 2022 to July 2023, and all 20 individuals with obesity completed the 4-week study. Table 1 provides the descriptive statistics. The median app use rate, defined as the percentage of days on which behaviors were recorded, was 98.3% (range 76.9%‐100%), with a mean of 94.8% (SD 7.1%).

Table 2 shows treatment-related results. BMI decreased significantly (median at start 34.9 kg/m², range 27.4‐52.9; median at end 34.1 kg/m², range 26.7‐51.0; P=.01). In total, 15% (3/20) of the participants gained weight during the study period, 2 of whom had undergone bariatric surgery. Significant changes were also observed in calorie intake and weekend sitting time.

The median number of behaviors performed over the 4 weeks, including individually created behaviors, was 11 (range 6‐27). The subjective evaluation of the achievability of the items presented at the top of the app screen, which ranged from 0 to 100, had a median score of 85 (range 58‐100).

In the qualitative interviews, the most frequently raised concern was the following (8/20, 40%):

This implies that the algorithm converged on a small subset of items with higher completion rates toward the end of the study, resulting in these items being repeatedly displayed at the top and predominantly selected by users.

The next most frequently raised concern was the following (6/20, 30%):

This means that, although each behavior could be further specified, as exemplified in Figure 1B, users had difficulty doing so for several items.

A total of 432 records collected during the study were included in the analysis. The final model developed based on the Akaike information criterion is presented below, where subscript i denotes each participant and subscript j denotes the time point. Yi,j is the daily number of behaviors performed, Depressioni,j is the preceding depressive mood, and Psychiatryi indicates the presence of psychiatric comorbidities. Fixed and random effects are represented by γ and ζ, respectively.

Level 1:

Level 2:

The coefficients of each variable are listed in Table 3. The following can be interpreted from each coefficient: (1) a higher preceding depressive mood was associated with fewer behaviors, and (2) individuals with comorbid psychiatric disorders had smaller coefficients for preceding depressive mood.

The smartphone app developed in this study for individuals with obesity was used by 20 recruited participants over 4 weeks, with a median use rate of 98.3%. Participants’ BMI decreased significantly during the study period. Furthermore, a linear mixed-effects model based on data obtained through the app showed that a higher preceding depressive mood was associated with a reduced number of completed behaviors related to weight loss.

The median use rate was 98.3%, which exceeded the adherence rates of approximately 80% to 95% reported in relevant studies [20,32,33], indicating sufficient feasibility. Future studies should address the concerns identified in the qualitative interviews. To prevent the selection of behaviors from being monotonous, items that are consistently completed may be retired from the list. In addition, improving the clarity of item explanations may effectively address the challenges in specifying item content.

The median decrease in BMI was 2.3% over 4 weeks. The Japanese clinical guidelines recommend a 3% weight loss for individuals with a BMI of <35 kg/m² and a 5% to 10% weight loss for those with a BMI of ≥35 kg/m² over 3 to 6 months [4]. Accordingly, the weight loss achieved in this study appeared sufficient. Calorie intake and weekend sitting time improved, and these behavior changes may have contributed to weight loss. Thus, the developed app may be effective for weight loss.

The effect size should be interpreted with caution due to several features of this study design. Daily weight measurements were conducted throughout the study, which may have influenced weight loss [34]. In total, 15% (3/20) of the participants gained weight during the study, 2 of whom had undergone bariatric surgery, a population known to frequently experience weight regain [35]. A previous umbrella review of smartphone app and web application interventions for obesity showed a mean difference of −0.12 kg compared with face-to-face treatment and −4.32 kg compared with no treatment [16]. However, our study was a single-arm trial with a follow-up duration different from those of previous studies, making direct comparisons difficult. Therefore, further studies are warranted to more accurately determine the effect size of the app developed in this study.

Given the high ratings for the subjective likelihood of achievement of the top-suggested items, with a median of 85 on a scale from 0 to 100, the multiarmed bandit algorithm seemingly approached convergence. Qualitative interviews revealed that the participants tended to choose only a few types of items presented at the top and reported a sense of monotony, further suggesting algorithm convergence. Future studies with more accumulated data should evaluate the convergence of the algorithm and may consider applying other optimization approaches such as collaborative filtering [14]. On the basis of the Fogg behavior model [25], this study prioritized behaviors based solely on completion rates; however, future studies should also consider the behaviors’ contributions to clinical outcomes such as weight loss, along with the effort and duration required for achievement.

In the post hoc analysis, the linear mixed-effects model showed that preceding depressive mood was related to weight loss behaviors. This aligns with a previous study showing that baseline depression is associated with emotional eating and binge eating [22]. One study showed that delay discounting mediates the association between depressive symptoms and the onset of diabetes [36], which may help explain these findings. Another study suggested that individuals with higher levels of depression exhibited unhealthy behaviors such as physical inactivity [37]. On the basis of these findings, future studies may benefit from providing suggestions on the number of behaviors to select on the app and prioritizing lower-effort behaviors according to daily depression levels. The interaction coefficient of depressive mood and psychiatric comorbidity was positive, indicating that, when adjusted for depressive mood as a covariate, the number of behaviors performed was large for individuals with psychiatric diseases. This may suggest that treating psychiatric disorders positively influences weight loss behaviors; however, further studies are needed to interpret these findings.

This study has several limitations. First, the study period was short, and long-term use of the app remains unexplored. Second, the dietary and physical activity questionnaires were administered only at baseline and after the intervention, which limited assessment of temporal changes and intervention feasibility. Third, energy intake estimated using the BDHQ appeared low. Alternative assessment tools such as the Automated Self-Administered 24-hour dietary recall [38] may be preferable in future studies. Fourth, this study used weight alone without distinguishing between fat and muscle mass; this is particularly relevant for participants receiving glucagon-like peptide-1 receptor agonists or who underwent bariatric surgery. Future studies should assess body composition using tools such as bioelectrical impedance analysis. Fifth, in the optimization algorithm, the behaviors were evaluated only with regard to achievability. Their contributions to weight loss may vary and should ideally be incorporated into the algorithm. Sixth, the impact of the multiarmed bandit algorithm on optimization remains unevaluated. Seventh, the effect size for weight loss was inconclusive in this single-arm study. Finally, the analysis of the relationship between preceding depressive mood and daily behaviors used only the subjective input of behavioral execution.

Future research should improve the app based on the findings of this study and assess the treatment efficacy through a randomized controlled trial with a longer follow-up period. Additionally, future studies should assess the contribution of optimization to weight loss using a randomized design that compares optimized and nonoptimized interventions. Furthermore, although this study used an item-fixed measurement based on classic test theory, future research could use computer-adaptive tests [39,40] to measure mood status more efficiently.

The smartphone app for obesity developed in this study demonstrated sufficient feasibility, and its potential effectiveness for weight loss was suggested. Data obtained through this app indicate that a preceding depressive mood may influence daily behaviors related to weight loss.