This study is a randomized controlled trial nested in the "Le French Gut" e-cohort aiming to test the efficacy of 4 reminder strategies (emails and/or SMS with standard and/or institutional wording) in increasing participation of inactive volunteers.

This trial is led in collaboration between the "Le French Gut" project and SKEZI, a DeepTech start-up which markets the data collection solution (SKEZIA online platform) for the "Le French Gut" project and carries out the InnoPRO program, aimed at generating new knowledge in the field of e-cohorts.

The "Le French Gut" project (NCT05758961) is an ongoing online research cohort aiming to collect faecal samples and associated nutritional and clinical data to better understand the observed heterogeneity between gut microbiome of healthy subjects, its connections with the diet and lifestyle, but also changes in gut microbiota associated with diseases (diabetes, obesity, allergies, cancer, Parkinson, Alzheimer…). Le French Gut is initiated by MetaGenoPolis-INRAE, supported by INRAE with Prof. R. Benamouzig, (AP-HP) as principal investigator of the project.

When participants first register to participate in the cohort, they must complete a short "About You" ("A propos de vous") questionnaire before they can receive any study kits and be considered as active participants in the research. In October 2024, almost half of the ~43,000 individuals who have created an account never finish this first questionnaire needed to validate their participation in the cohort.

Purpose of the study This study will test which type of reminder works best for encouraging those inactive volunteers to return to the platform and complete the missing questionnaire.

How the study will be done

All eligible volunteers (i.e. people who registered on the online platform and provided a valid e mail address or mobile number but have not yet finished completing the "A propos de vous" questionnaire) will be randomly assigned to one of four groups:

1. Standard e mail - a regular, semi-formal email. Same email sent three times, 72h apart.
2. SMS - a short text message sent to the participant's phone. Same SMS sent three times, 72h apart.
3. Institutional e mail - a plain, formal email signed by the "Le French Gut" study coordinator. Same email sent three times, 72h apart.
4. Combined sequence - a standard e mail, followed 72 hours later by the SMS, and 72 hours after that by the institutional e mail.

All messages contain the same content (a link that takes the participant directly to the questionnaire). The study will last about one month after the last reminder is sent.

Questions the study will answer

• Which reminder method results in the highest completion rate of the "A propos de vous" questionnaire (i.e. proportion of people completing the questionnaire) within 30 days?
• How quickly do participants respond after each type of reminder?
• Does the time since a person first registered (recent vs. long time registrant) influence which reminder works best?

Hypothesis Investigators hypothesized that the combined sequence (standard e mail → SMS → institutional e mail) will lead to a higher proportion of participants completing the questionnaire than any single channel reminder alone.

Why this matters Finding the most effective, low cost reminder strategy will help the French Gut team-and other online cohorts-keep more participants engaged, thereby improving the quantity and quality of the data that can be used for future health research. The results will be shared in a peer reviewed scientific article and could guide the design of future e cohort studies worldwide.

Inclusion Criteria:

• Being registered on the "Le French Gut" platform but not active participant (e.i. eligibility questionnaire not completed, "Le French Gut" consent not validated, "A propos de vous" questionnaire not completed)
• Having provided a valid email address and/or mobile-phone number

Exclusion Criteria:

• Completed "A propos de vous" questionnaire before the first reminder
• Being non eligible to the "Le French Gut" cohort (eligibility to "Le Fench Gut" cohort: being over 18 years-old and living in France)

Participants were unaware that the trial was being conducted, and the researchers and data analysts were blind to participants' assigned groups.