Overweight and obesity affect a significant portion of the adult population in Germany and are associated with numerous chronic diseases and reduced life expectancy. Multimodal lifestyle interventions, combining dietary changes, increased physical activity, and behavioral modification, are the recommended first-line treatment. While smartphone apps are increasingly popular for weight management, evidence on their effectiveness for multimodal obesity treatment is limited. Existing apps often lack the comprehensive, multimodal approach recommended by guidelines and access limitations. The Digital Healthcare Act (2019) introduced Digital Health Applications (DiHA) in Germany, which are medical devices subject to regulatory approval and health insurance coverage. This study aimed to evaluate the efficacy of a DiHA-certified app-based multimodal lifestyle intervention for weight loss in adults with obesity. The primary objective was to assess weight loss after 12 weeks, and secondary objectives included evaluating weight change at 24 weeks, quality of life, and app usability.

A systematic review and meta-analysis showed that nutrition apps with features like goal setting and self-monitoring have a statistically significant effect on eating behavior and BMI. However, these apps often lacked the multimodal approach recommended for obesity treatment. This study aimed to address this gap by evaluating a comprehensive app-based program conforming to German obesity guidelines, addressing the limitations of previous research.

This single-center, randomized controlled trial recruited 168 adults (aged 18-70 years) with a BMI between 30.0 and 40.0 kg/m² from the Munich region. Participants were randomized (1:1 ratio) into two groups: ADHOC (immediate app intervention) and EXPECT (app intervention after a 12-week waiting period). The DiHA app "Oviva Direkt für Adipositas" provided a 12-week multimodal weight loss program comprising self-management, self-monitoring, and educational content. Anthropometric data (weight, height, body composition), quality of life (EQ-5D-5L), app usage data, and app usability/acceptance (TAM 3, SUS) were collected at baseline, 12 weeks, and 24 weeks. Completers analysis and last observation carried forward (LOCF) imputation were used to handle missing data. Statistical analysis included t-tests, Mann-Whitney U tests, chi-squared tests, and multiple linear regression.

The completers analysis (139 participants) showed a mean weight loss of 3.2 ± 3.2 kg (3.2 ± 3.0%) in the ADHOC group and 0.4 ± 2.6 kg (0.3 ± 2.6%) in the EXPECT group after 12 weeks (p < 0.001). The ADHOC group maintained this weight loss at 24 weeks. LOCF imputation yielded similar results: -2.6 ± 3.1 kg (2.6 ± 3.0%) in ADHOC vs. -0.3 ± 2.5 kg (0.3 ± 2.5%) in EXPECT (p < 0.001). Time spent on the app was significantly associated with weight reduction in both groups (p=0.03 and p=0.02 respectively). In the ADHOC group, higher Technology Acceptance Model (TAM) scores were associated with greater weight loss, suggesting that positive attitudes toward app usability and acceptance correlated with better outcomes. No significant differences in quality of life were observed between groups.

This study demonstrates the clinical effectiveness of a fully digital, multimodal weight loss program delivered through a DiHA app. The observed moderate weight loss is comparable to other studies using app interventions, highlighting the potential of this approach. The weight loss effect, even though moderate, may still offer clinical benefits by reducing blood pressure and improving metabolic parameters. The 12-week intervention period, while mandated by German regulations, may not be sufficient to achieve maximal weight loss; longer intervention periods warrant investigation. The positive association between app usage and weight loss underscores the importance of user engagement and acceptance. The findings support the use of the app as a readily accessible, cost-effective intervention for initial weight loss management in individuals with obesity within the German healthcare system.

This study demonstrates that a fully digital, multimodal lifestyle intervention delivered via a DiHA-certified app leads to clinically meaningful weight loss in adults with obesity, which is maintained over a 12-week follow-up period. The app's efficacy is positively associated with user engagement and acceptance. Further research should investigate the long-term effectiveness of this app and explore strategies to optimize user engagement and adherence.

The study's single-center design and the recruitment primarily from the Munich region limit the generalizability of the findings. The high proportion of women in the study sample (64.3%) might affect the generalizability to other populations. The reliance on self-reported data for some variables introduces potential biases. Future studies could benefit from broader geographical representation and a more balanced gender distribution to strengthen the study's external validity.