This study will investigate how adding a specific type of dietary fat to daily meals for two weeks affects the gut bacteria and their activity. Participants will be randomly assigned to consume one of four types of fat: butter, coconut oil, olive oil, or sunflower oil (60 mL each day for 2 weeks). Stool and blood samples (after an overnight fast) will be collected at 3 time points during the study (Days 1, 7, and 22). These samples will be used to measure metabolites produced by gut bacteria, the types of bacteria present in the gut, blood lipids, and inflammatory markers. The study, based on previous in vitro findings, aims to understand whether different types of fats (based on their structure and level of saturation) have different effects on gut bacteria and their activity in healthy subjects.

This is a pilot, parallel, randomised controlled dietary intervention trial investigating the effects of dietary fat composition on gut microbiota and metabolic activity in healthy adults. Participants will be randomised to one of four intervention groups (butter, coconut oil, extra virgin olive oil, or sunflower oil), representing commonly consumed dietary fats that differ in fatty acid structure.

This study builds on prior in vitro findings demonstrating that fatty acid chain length and degree of saturation influence microbial fermentation pathways and short-chain fatty acid (SCFA) production.

The primary objective is to determine whether structurally distinct dietary fats differentially influence gut microbial activity and composition in vivo. Secondary objectives include assessing the effects on cardiometabolic and inflammatory markers. It is hypothesised that dietary fats differing in chain length and degree of saturation will differentially modulate gut microbiota metabolism, with potential systemic effects on circulating cardiometabolic and inflammatory markers.

A total of 64 healthy adults aged 20-50 years will be recruited from the Glasgow area. All study visits will take place at the New Lister Building, University of Glasgow. Participants will attend three study visits over a three-week period.

The study includes a one-week run-in period (normal habitual diet) followed by a two-week dietary intervention. During the intervention, participants will replace the majority of their habitual dietary fat intake with their allocated fat while maintaining their usual diet and lifestyle.

Biological samples (blood and stool) will be collected at each study visit (Day 1, 7 and 22) to assess study outcomes. Anthropometric measurements will also be recorded. Dietary intake, gastrointestinal symptoms, and compliance will be monitored using food diaries, symptom questionnaires, and adherence logs.

The sample size is based on anticipated changes in faecal SCFA concentrations. Based on prior data, 14 participants per group provides 80% power (P=0.05) to detect a mean change of 30 μmol/g (SD: 28 μmol/g). Allowing for a 10% drop-out rate, 16 participants per group (64 total) will be recruited. As this is a pilot study, this calculation is intended to inform a reasonable sample size rather than to ensure definitive hypothesis testing.

Inclusion Criteria:

• Adults aged 20-50 years
• Body mass index (BMI) between 20 and 30 kg/m²
• Generally healthy
• Stable body weight (±2 kg) over the past 2 months
• Living in the Glasgow area
• Non-smoker

Exclusion Criteria:

• Use of antibiotics within the past 3 months
• Current use of medications affecting gut microbiota or cardiometabolic health
• Regular use of dietary supplements that may influence study outcomes (e.g. probiotics, prebiotics, vitamins, minerals, fish oil), unless willing to discontinue prior to study start
• Presence of chronic gastrointestinal, metabolic, cardiovascular, hepatobiliary, or pancreatic disease
• History of major gastrointestinal surgery
• Diagnosed diabetes or use of lipid-lowering therapy
• Known fat malabsorption disorders
• Pregnant or lactating women
• Post-menopausal women
• Following a restrictive or specific diet (e.g. vegan, ketogenic diet)
• Food allergies or intolerances relevant to study interventions (e.g. dairy, coconut)
• Recent significant weight change (±2 kg in the past month)
• High reliance on ready meals or takeaway foods (>7 main meals per week)
• Participation in another research study that may interfere with this study