High-glycaemic foods contribute to elevated risk of obesity, type 2 diabetes, and cardiometabolic disease. Replacing digestible carbohydrates with dietary fibres is known to reduce postprandial glycaemic excursions, enhance satiety, and support beneficial microbial fermentation. However, limited evidence exists on how interactions between different isolated fibres within a processed food matrix may modulate these responses, particularly when such interactions could recreate structural features of intrinsic plant fibre networks that naturally restrict starch accessibility and alter fermentation dynamics.

This randomized, single-blinded, placebo-controlled crossover trial will investigate how isolated dietary fibres, alone and in combination, influence metabolic and microbial responses when incorporated into a commonly consumed cereal-based food (scone). Overweight but otherwise healthy adults (BMI 25-<30 kg/m²) will consume seven fibre-enriched scone formulations across two consecutive mornings per intervention phase. Outcomes include postprandial glycaemic response measured via continuous glucose monitoring (primary outcome), perceived satiety and energy intake, gastrointestinal symptoms, fermentation dynamics via breath hydrogen and methane, and gut microbiota composition assessed through 16S rRNA sequencing. This study will generate novel insights into potential synergistic interactions between isolated fibres within a food matrix and their consequences for glycaemic control, satiety, microbial fermentation, and community. Findings will inform next-generation food design strategies aimed at replicating complex intrinsic fibre structures to enhance the health impact of processed foods.

The study will be conducted over a 28-day period, during which each participant will complete a structured sequence of controlled dietary interventions while continuous metabolic and gastrointestinal data are collected. Participants will rotate through seven intervention phases, each lasting two consecutive days. In each phase, they will consume one of the fibre-enriched scone formulations provided by the research team, accompanied by standardized, calorie-controlled meals. Snacks will be provided but may be consumed ad libitum, ensuring consistent macronutrient intake across intervention days while preserving ecological validity.

To maintain data quality and support participant adherence, each intervention phase is followed by a washout day during which participants return to their habitual diet. These washout periods minimize carryover effects from previous fibre exposures and allow both glycaemic responses and microbial activity to stabilize before the next intervention. Weekends are intentionally left free of study foods and structured routines, a design choice that improves flexibility, reduces participant fatigue, and helps maintain high compliance without compromising continuous data collection.

Upon enrolment, participants will receive two continuous glucose monitoring (CGM) sensors, each worn for 14 days, ensuring uninterrupted interstitial glucose measurement throughout the study. This approach allows precise characterization of postprandial glycaemic responses to each test food under free-living conditions while minimizing participant burden.

To assess fermentation dynamics, participants will provide breath samples using a portable breath-analysis device capable of quantifying hydrogen and methane. Measurements will be collected immediately before and after each intervention meal, enabling assessment of both the magnitude and timing of microbial fermentation in response to the different fibre combinations.

Gastrointestinal sensations and tolerance will be evaluated using validated questionnaires that capture bloating, discomfort, stool frequency, and form (including the Bristol Stool Scale), and other relevant symptoms. Satiety and hunger perception will be assessed using standardized visual analogue scales administered before and after each intervention meal, providing detailed insight into subjective digestive and appetite-related responses.

Eligible participants are healthy men and women aged 18-45 years with a body mass index between 25 and <30 kg/m². All participants must be weight-stable for at least three months prior to enrolment and free from metabolic, gastrointestinal, or chronic inflammatory diseases. Exclusion criteria include diagnosed diabetes or pre-diabetes, use of glucose-modulating or lipid-lowering medications, recent antibiotic therapy, adherence to restrictive diets (such as ketogenic, vegan, or medically prescribed regimens), significant food allergies relevant to study products, smoking, pregnancy or breastfeeding, and any condition that may interfere with digestion, absorption, or gut microbiota composition.

Overall, the combination of controlled feeding, continuous glucose monitoring, breath-based fermentation measurements, validated gastrointestinal and satiety assessments, and microbiome profiling will generate a comprehensive dataset describing how isolated dietary fibres shape metabolic, digestive, and microbial responses when incorporated into a familiar cereal-based food matrix.

Inclusion Criteria:

• Be willing and able to give written informed consent.
• Be between 18 and 45 years of age.
• Have a BMI of =25<30kg/m2(overweight).
• Have a waist circumference of >94cm for a male, >80cm for a female (increased risk of metabolic syndrome).
• Have had a stable body weight (<5% change over the past three months).
• Be in general good health as determined by the investigator through interview and vital signs (blood pressure, pulse, temperature). Systolic blood pressure less than 160mm Hg and diastolic blood pressure less than 100 mm Hg (defined as Hypertension stage 2).
• Be willing to avoid consuming dietary supplements (at the discretion of the investigator), prebiotics, probiotics, or fibre-rich supplements within four weeks before the baseline visit, and until the end of the study.
• Be willing to avoid vigorous physical activities on the interventional days (defined as any physical activity that is planned to achieve a fitness goal).
• Be willing to consume the investigational food products and menu plan daily for the duration of the study.

Exclusion Criteria:

• Pregnant, lactating, or post-menopausal women, or women who are planning to become pregnant over the study period.
• Have had antibiotic treatment within three months before baseline.
• Are taking a medication that the investigator believes would interfere with the objectives of the study, pose a safety risk, or confound the interpretation of study results; to include anti-inflammatory drugs, H2 blockers, antacids, proton pump inhibitors, anti-hypertensive medications, corticosteroids, laxatives, enemas, antibiotics, anti-coagulants, and immunosuppressant medication. Participants should have a wash-out period of at least two weeks for each of these medications except for antibiotics, which should not have been taken in the previous three months. Participants taking proton pump inhibitors and medications for chronic conditions (e.g., anti-hypertensive medication) will be allowed into the study if the dose has been stable for at least two months before the study baseline visit.
• Have a history or indication of drug and/or alcohol abuse at the time of enrolment.
• Have a habitual alcohol consumption of >2 alcoholic beverages/day (>28g ethanol daily).
• Follow a vegetarian or vegan diet.
• Have a typical fibre intake of >30g per day.
• Have experienced major dietary changes within three months before the study baseline.
• Plan major lifestyle changes (diet, physical activity, or travel) during the study period.
• Have a clinically diagnosed eating disorder.
• Have a food allergy or intolerance that would preclude study product intake (for example, eggs, gluten, nuts, milk, or any other food allergy or intolerance).
• Have an active gastrointestinal disorder or previous gastrointestinal surgery.
• Have a significant active and medically-diagnosed acute or chronic co-existing illness including: metabolic, psychiatric, cardiovascular, endocrinological, immunological condition, gastrointestinal disease or any other condition which contraindicates, in the investigator's judgement, entry to the study (such as, diarrhoea, Crohn's disease, ulcerative colitis, IBS, diverticulosis, stomach or duodenal ulcers, hepatitis A/B/C, HIV, cancer, diabetes etc) or a significant history of such diseases.
• Are severely immunocompromised (e.g., HIV positive, transplant patient, on anti-rejection medications, on a steroid for >30 days, or chemotherapy or radiotherapy within the last 12 months).
• Have a malignant disease or concomitant end-stage organ disease.
• Have symptomatic respiratory or cardiac illness.
• Experience alarm features such as sudden weight loss, rectal bleeding, a recent change in bowel habits, or significant abdominal pain within three months before baseline.
• Individuals who, in the opinion of the investigator, are poor attendees or unlikely for any reason to be able to comply with the study protocol.
• Are receiving treatment involving experimental drugs.
• If the participant has been in a recent experimental trial, these must have been completed not less than 30 days before this study.
• Individuals who regularly undertake rigorous exercise.
• Individuals who smoke or vape.