**Background:** The urgent need to shift current dietary patterns toward sustainable healthy diets—defined as nutritionally balanced, low environmental impact, economically fair, and culturally acceptable—is well recognized. Most existing interventions have focused on health or environmental dimensions separately, often neglecting socioeconomic aspects and the interplay between food groups. This protocol addresses these gaps by evaluating a smartphone-based intervention that targets the whole dietary pattern, food waste, and fair sourcing, using intensive longitudinal methods to capture behavior change dynamics.
**Methods:** This pilot study employs a series of 1-year ABA hybrid n-of-1 trials. The first A phase is a 2-week baseline, the B phase is a 22-week intervention, and the second A phase is a 24-week post-intervention follow-up. Twenty-one participants (7 from each of low, moderate, and high subjective socioeconomic status groups) will be recruited from Barcelona. Inclusion criteria: adults aged 18-65, fluent Spanish speakers, omnivorous, no eating disorders or chronic illnesses affecting diet, and scoring ≥5 on a dietary questionnaire. The intervention includes text messages (educational, motivational, recipes) sent twice weekly via the m-Path app, plus brief (≈15 min) web-based feedback sessions at baseline and every 6 weeks during the intervention. Quantitative data are collected via daily food frequency questionnaires (12 items assessing food groups and food waste) during 15 evaluation weeks (105 potential measurement days per participant). A weekly environmentally sustainable healthy diet index (0-10 points) is computed from 10 items based on EAT-Lancet, FAO, and WHO recommendations. Mechanisms of action (health, environmental, social, and price motives; ecoanxiety; habit strength; self-efficacy) are measured weekly. Qualitative data come from three semistructured interviews (baseline, end of intervention, end of follow-up). Analyses include linear mixed models for group-level effects, individual-level models for specific food groups, longitudinal mediation analyses, network analysis for spillover effects, and framework analysis for qualitative data.
**Key Results:** The study was approved on October 19, 2022, and recruitment began in October 2022. Final results are expected by October 2023. No quantitative results are reported in this protocol. The authors hypothesize small-to-medium positive changes in the environmentally sustainable healthy diet index, with decreases in meat, dairy, and ultraprocessed foods and increases in plant-based foods. They expect mediation by motivational factors and heterogeneity across participants and socioeconomic groups.
**Clinical Implications:** This pilot study will provide critical feasibility and preliminary effectiveness data for a novel digital intervention targeting sustainable healthy diets as a whole. By addressing multiple sustainability dimensions (health, environment, socioeconomic fairness) and using high-resolution longitudinal assessments, it aims to inform the design of larger, more definitive trials. The inclusion of diverse socioeconomic groups will help identify potential disparities in intervention effects, guiding equitable implementation. The findings will contribute to evidence-based strategies for promoting dietary changes that benefit both human health and planetary sustainability.