Plant your health: a prospective, three-arm, parallel-group, open-label, randomised, blinded-endpoint, single-centre trial comparing vegan, vegetarian, and omnivorous diets in younger to middle-aged adults with overweight or obesity
Overweight and obesity and associated cardiometabolic risk factors in adults
This study will evaluate the health effects of adopting either a vegan or vegetarian diet compared with maintaining a habitual omnivorous diet over 52 weeks. The intervention, titled Plant Your Health, is a three-arm, parallel-group, randomised controlled dietary trial with equal allocation (1:1:1) to the following arms: Arm 1: Vegan diet: Participants will adopt a fully plant-based diet excluding all animal-derived products (meat, poultry, fish, eggs, and dairy). Arm 2: Lacto-ovo vegetarian diet: Participants will adopt a lacto-ovo vegetarian diet, excluding all meat, poultry, and fish but permitting eggs and dairy products. Arm 3: Control (habitual omnivorous diet): Participants in the control arm will continue with their usual diet (i.e., ≥5 portions of meat per week). Randomisation will use a secure, computer-generated allocation sequence within the REDCap randomisation module (Research Electronic Data Capture, Vanderbilt University), hosted on the University of Leicester server. Participants will be allocated in a 1:1:1 ratio to the omnivorous, vegetarian, or vegan dietary intervention arms. The allocation sequence will be generated by an independent trial statistician using permuted blocks of variable size to maintain allocation concealment. Randomisation will be stratified by sex (male/female), age group (18–39 / 40–50 years), and ethnicity (White European / South Asian / Other). Dietary Support Programme: Core Dietary Support Programme (All study arms receive these): To ensure parity across all three study arms, all participants will receive an equivalent allocation of basic dietary support and resources over the 52-week study period. This includes access to supermarket food vouchers, up to an approximate value of £450.00 per participant. All participants will receive remote contact via telephone or video call every 4 weeks (approximately 15 minutes). These scheduled remote calls are intended to support ongoing participant engagement, ensure consistency o
1. Adults aged 18–50 years 2. Men and women 3. Body mass index (BMI) ≥25 and <40 kg/m² (≥ 23 kg/m² for individuals from ethnic minority backgrounds) 4. Self-reported stable weight over the previous 6 months (<+/- 5% of bodyweight) 5. Currently follows an omnivorous diet, defined as consumption of meat on at least 5 days per week over the previous six months, with no four-week or longer period of vegetarian or vegan eating during that time 6. Currently consumes dairy products (for example, but not limited to, milk in hot beverages, cheese, yogurt, butter, cream, or other foods containing milk) on at least five days per week over the past six months, with no four-week or longer period of abstaining from dairy products 7. Willing and able to provide written informed consent to participate in the trial 8. Able to understand written and spoken English, as interpreters will not be used 9. Willing to be randomised to any study arm (omnivore, vegetarian, or vegan) and to comply with all study procedures for the 12-month duration
1. Diagnosed with type 1 or type 2 diabetes 2. Any significant hepatic, renal, or gastrointestinal condition (e.g., inflammatory bowel disease, coeliac disease, chronic liver or kidney disease) or any other medical or lifestyle factor (e.g., severe food allergy, intolerance, or eating disorder) that, in the investigator’s opinion, would compromise safety, affect nutrient absorption, or prevent adherence to a vegan or vegetarian diet 3. Currently receiving active treatment for cancer or renal disease, has experienced a cardiovascular event within the past six months, or is living with a serious illness with a life expectancy of less than 1 year 4. Use of prescription or over-the-counter lipid-lowering agents (e.g., statins, fibrates, niacin, plant sterol/stanol products, or red yeast rice) or high-dose omega-3 fatty acid supplements (≥2 g/day eicosapentaenoic acid + docosahexaenoic acid combined). Participants taking low-dose over-the-counter omega-3 (≤1 g/day) are not excluded. 5. Pregnant, breastfeeding, or planning pregnancy during the study period 6. Use of antibiotics, probiotics, prebiotics, or other supplements/medications that may affect TMAO metabolism or gut microbiota within the past 8 weeks (e.g., L-carnitine, choline, or FMO3 inhibitors) 7. Severe psychiatric illness, substance misuse, or any other major clinical factor likely to impair consent, participation, or adherence 8. Current use of weight-loss medication (e.g., glucagon-like peptide-1 receptor agonists/ Glucose-dependent insulinotropic polypeptide agonists or similar agents) 9. Current or planned long-term systemic corticosteroid use (≥4 weeks) 10. Current participation in another interventional study that could influence outcomes or conflict with this trial. 11. Familial hypercholesterolaemia, diagnosed or clinically suspected 12. History of metabolic surgery