Overview

This study is a single-center micro-randomized trial, aiming to examine the effects of two theory-driven reminder strategies on adherence to repeated, image-based meal logging in adults age 45 and older. Three times daily, participants are automatically randomized to receive one of three reminder messages prior to each habitual mealtime. The main research questions the trial aims to answer are:

• Do loss-framed reminders (emphasizing loss of a daily financial reward for not logging) increase meal logging adherence in the two hours following a reminder, compared to a neutral reminder?
• Do logging consistency reminders (providing feedback on recent logging streaks) increase meal logging adherence in the two hours following a reminder, compared to a neutral reminder?
• Do the effects of loss-framed and logging consistency reminders differ from each other?

Primary aim: To estimate the proximal effect of each reminder type on whether a participant logs a meal within two hours of receiving a reminder.

Secondary aim: To examine whether within-person physiological patterns (e.g., heart rate variation) moderate response to reminders, with the goal of identifying physiological markers of receptivity to behavioral prompts.

The trial aims to enroll 200 adults aged 45 and older. Participants attend a baseline visit for onboarding, consent, and device setup, followed by a 28 day remote intervention period during which they should photograph their meals three times daily (breakfast, lunch, dinner) and wear a smartwatch and smart ring continuously. Prior to each mealtime, they receive an automatically randomized reminder message. Participants earn a daily financial reward contingent on completing meal logging for that day.

Note: This trial was registered retrospectively as enrollment began prior to registration. However, registration was completed before data collection was finished and before any data analysis was conducted.

Eligibility

Inclusion Criteria:

• Citizens or residents of Switzerland who are German-speaking (main place of living or employment in Switzerland).
• Age ≥45 years.
• BMI ≥25 kg/m² (overweight or obese).
• Regular access to a smartphone (iOS or Android) with a data plan.
• Able to use smartphone applications.
• Able to walk independently.

Exclusion Criteria:

• History of stroke, heart disease, renal failure, cancer, or diabetes (type 1 or type 2).
• Past vascular bypass surgery or angioplasty.
• Current or planned use of glucose-lowering medications during the upcoming 4 weeks (e.g., GLP-1 receptor agonists, metformin).
• Pregnant or breastfeeding.
• Relevant skin conditions at wearable placement sites (e.g., upper arm).
• BMI \<25 kg/m² confirmed at enrollment.
• FPG ≥7.0 mmol/L and/or HbA1c ≥6.5% (newly identified type 2 diabetes); Individuals who meet type-2 diabetes criteria at enrollment, based on Swiss/ADA diagnostic criteria will be informed of blood test results and referred to a GP. These individuals will be deemed ineligible and excluded from the study.