Dietary Education Intervention for Subthreshold Depression (MIND-Trial)

This randomized controlled trial evaluates the efficacy of an 8-week dietary education intervention in adults with subthreshold depression. Participants will be randomly assigned in a 1:1 ratio to either a dietary education intervention or a befriending control condition. The dietary intervention consists of online dietary education and counseling designed to encourage a Mediterranean dietary pattern. The control group will receive online befriending sessions of similar frequency and duration. The study will evaluate changes in depressive symptoms and investigate changes in oral and gut microbiota, dietary measures, psychiatric symptoms, metabolic measures, and neurobiological markers.

Inclusion Criteria: * Male or female, aged 21 to 60 years. * Meets the operational diagnostic criteria for subthreshold depression. * Has not been following a Mediterranean-style diet, based on self-report and a - Mediterranean Diet Adherence Screener (MEDAS) score ≤9. * Able to converse in English. * Able to use a computer or mobile device with internet access. * Capable of providing informed consent. Exclusion Criteria: * Current use of antidepressants, antipsychotics, or mood stabilizers. * Self-reported lifetime diagnosis of bipolar disorder, a psychotic disorder, an eating disorder, or a current substance use disorder. * Major life trauma, such as recent bereavement or job loss, within the preceding 3 months. * Current participation in an intervention targeting diet. * Severe food allergies, intolerances, or aversions. * Unwilling to accept a Mediterranean-style diet. * Current daily or regular use of over-the-counter or prescription probiotic/prebiotic supplements. * Systemic oral or intravenous antibiotic use within 30 days before the study. * Self-reported chronic gastrointestinal disorder, including inflammatory bowel disease, irritable bowel syndrome, or celiac disease. * Self-reported history of significant gastrointestinal surgery, such as gastric bypass or bowel resection. * Self-reported systemic autoimmune disease or active chronic infectious disease. * Currently pregnant or lactating, or planning pregnancy during the study.