The aim of this randomised controlled trial is to investigate the effects of plant foods on gastrointestinal and cardiometabolic health in healthy adults aged 30-70 years old. The main question it will answer is "what is the effect of plant foods on alpha-diversity of the gut microbiota?".

Researchers will compare two diets containing plant foods to see if they have different effects on gastrointestinal and cardiometabolic health.

Participants will be asked to replace all their meals, snacks and drinks with those provided by the study team for four weeks (6 days/week). Participants will also be asked to attend the research facility on three occasions to:

• Provide a blood sample
• Provide a urine sample
• Provide a stool sample
• Have their body composition assessed
• Have an ultrasound of their artery (arm)
• Have their blood pressure measured
• Answer questionnaires related to dietary intake, mental health, physical activity, quality of life, diet acceptability and adherence

Inclusion Criteria:

• Adults aged 30-70 years of age
• Capacity to give informed consent in English
• Consume equal or less than the average UK plant food intake (number and quantity)
• Consume less than 30 g/d of fibre
• BMI 18.50-29.99 kg/m2
• Willing to complete the trial by adhering to the diet intervention, complete questionnaires, provide stool, urine and blood samples, record weight and other anthropometric values and record food intake throughout the study
• Willing and able to adhere to the protocol for the diet intervention for the whole duration of the trial (e.g., appropriate space in freezer to store meals, access to oven/microwave, no planned travelling)
• Willing to discontinue use of prebiotics and probiotics 4 weeks prior and during the intervention period
• Willing to consume animal products daily
• Medication use is stable in women receiving hormone replacement therapy (stable use for the past 3 months)

Exclusion Criteria:

• Medical history of any of the following: diabetes, major active or historic psychiatric conditions (e.g., schizophrenia), current eating disorder, alcohol abuse, active treatment for cancer in the last year, severe neurological, endocrine, renal, cardiac or pulmonary disease (or any other chronic medical condition), severe oesophagitis, gastritis or duodenitis, active diverticulitis or intestinal/colonic strictures, Coeliac disease, Crohn's disease or Ulcerative colitis, stem cell or organ transplant, gut resection surgery (excluding appendicectomy and cholecystectomy), bleeding disorder, anaphylaxis or any other major or chronic condition known to impact study outcome measures.
• A current diagnosis of IBS, as defined by the Rome IV criteria.
• Taking medications known to affect outcomes of interest, including:
• Prebiotics and probiotics 4 weeks prior to enrolment.
• Antibiotics within 4 weeks of enrolment.
• Medications known to affect blood pressure, appetite, gut motility, glucose control, or lipid levels within 4 weeks of enrolment.
• Taking any dietary supplements, apart from vitamin D or calcium supplements, 4 weeks prior to enrolment.
• Allergy or intolerance to any study food ingredients.
• Unable/dislike/unwilling to consume study foods or adhere to study protocol (e.g., not willing to drink a maximum 9 units of alcohol a week or consume animal products).
• Currently reported pregnant or lactating.
• Unwilling or unable to consume non-halal or non-kosher products.
• Estimated energy requirements of <1,500 kcal/day or >3,000 kcal/day.
• Unexplained or unintentional weight loss in the past six months.