PlanetDiet: Sustainable diets and cardiometabolic health, study protocol for a six-month parallel-group randomized controlled trial in middle-aged adults at cardiovascular risk

Background: Cardiovascular disease (CVD) remains the leading cause of death worldwide. Diet plays a key role in CVD prevention while linking human health with environmental sustainability. The EAT-Lancet Commission proposed the Planetary Health Diet (PHD), a predominantly plant-based dietary pattern emphasizing high-quality foods and reduced meat consumption. Despite growing interest in the PHD, no randomized controlled trial (RCT) has evaluated its effects using the PHD index on established and novel cardiometabolic biomarkers. Methods: The PlanetDiet study is a six-month, three-arm parallel RCT investigating the effects of sustainable diets on traditional and novel cardiometabolic risk factors in 180 Danish adults aged 45?70 years at high cardiovascular risk. Participants are randomized to a PHD, an ovo-lacto-vegetarian diet, or their habitual diet (control). The primary outcome is changes in a composite cardiometabolic health score, with secondary outcomes including changes in metabolomic and proteomic profiles. Clinical assessments include the analysis of urine, blood, and faecal samples; continuous glucose monitoring; anthropometry and body composition measurements; dietary records; and self-reported questionnaires assessing physical activity, quality of life and dietary acceptance. To support adherence, participants receive four individualized dietary counselling sessions, two group sessions, seasonal food boxes tailored to their assigned diet, and access to a personalized digital application with recipes, individualized timelines, and visit scheduling. Conclusions: PlanetDiet will provide evidence on the effects of sustainable dietary patterns on cardiometabolic health. It will also investigate molecular pathways linking diet to cardiometabolic disease. Ultimately, the study may inform the development of effective, personalized dietary interventions that promote both human and planetary health. Trial registration: Sustainable Diets and Cardiometabolic Health (PlanetDiet): NCT07189676 [ClinicalTrials.gov] [registered before inclusion start; 2025-09-15]; Unique identifier: H-25030722.