Exploring the Receptivity and Feasibility of Just-In-Time Support for Healthy Food Choices: Mixed-Method Insights for Adaptive Intervention Development.

Background: A suboptimal diet is a major health risk, yet adopting and maintaining healthy eating habits remains challenging. Just-in-Time (JIT) digital support may help, however, little is known about when and where users are most receptive to such support. Objectives: This study aimed to evaluate receptivity to a researcher-developed JIT-based app that prompts healthy food choices near food outlets and at preset times. The primary outcome was receptivity to JIT prompts, defined as users' willingness and ability to receive, process, and act upon prompts in daily life. Exploratory outcomes included usability, perceived privacy and perceived effectiveness. Methods: In a single-arm mixed-methods study, 14 adults (M = 27 y) used the app for 1 wk after selecting a nutritional health goal. Data were collected via poststudy questionnaires, in-app feedback, and interviews with 8 participants. Receptivity was assessed based on participants' willingness and ability to engage with prompts in real-world contexts. Quantitative data were analyzed descriptively and using exploratory statistical tests; qualitative data were analyzed thematically. Results: Receptivity to JIT prompts was highest when prompts were delivered at meaningful times and locations, particularly at home and in supermarkets, and appeared higher during more positive and calmer emotional states. Exploratory feasibility findings indicated that usability was affected by technical issues and battery drain. Participants generally expressed willingness to share personal data when used transparently for personalization. Users reported improvements in self-rated diet quality and goal-related dietary behavior, and emotional states were associated with perceived momentary effectiveness. Lower-educated participants reported more installation difficulties, less favorable perceptions of prompt tone, and smaller improvements in food choices. Conclusions: Our findings provide initial insight into when and where individuals are most receptive to JIT support for healthy eating, informing the design of future adaptive dietary interventions tailored to users' contexts and momentary states.This trial was registered at clinicaltrials.gov as NCT05773625 (https://clinicaltrials.gov/ct2/show/NCT05773625).