This study will systematically investigate the effects of a diet with decreased energy density, reduced glycaemic index, and significantly increased dietary fibre, on post-prandial glycaemic response, satiety, gastrointestinal tolerability and gut microbiota composition and function in individuals with excess body weight (Body Mass Index (BMI) 25-35kg/m2).

Hypothesis: The investigators hypothesise that a diet enriched in fibre will be beneficial to post-prandial glycaemic response, well tolerated and satiating, as compared to the standard Western-style diet.

Participants: Healthy men and pre-menopausal, non-pregnant and non-lactating women, 18-45 years of age, with a Body Mass Index (BMI) between 25-34.9 kg/m2 will be recruited (n=20).

Study Design: This exploratory study aims to investigate the effects of a fibre-enriched (FR) diet in adults with excess body weight in comparison to a control diet. This randomized, single-blinded, placebo-controlled intervention/trial with a crossover design compares the effects of consumption of fibre-enriched food staples delivered within a full meal plan (fibre-enriched diet plan) with a matched control meal plan (control diet plan).

Participants stratified by age and BMI, will be randomly assigned to receive either the control diet or fibre-enriched (FR) diet for 8 days. After these 8 days, a washout period (13 day minimum) will commence, and participants will then crossover to the opposite diet. All foods and prepared meals within the menu plan will be provided to participants for both the control and FR diets. Participants are required to strictly adhere to their assigned meal plan for the 8-day period.

Given the higher dietary fibre intake of the FR-diet, the study design incorporates a dose escalation of the fibre content to allow participants on the FR diet to adjust to the increase in dietary fibre intake (4-day reduced dose, wherein study foods contain only a portion of the required dose of fibre, and 4-day full dose, high fibre intake).

The effect of the FR diet on satiety and overall food intake will be assessed. Participants on the FR diet and the control diet will be provided with non-fibre enriched snacks which they may consume at designated times each day, if they wish to and in ab libitum. These snacks will be an optional component of the menu plan. All food intakes will be monitored, and all leftovers will be returned to the research team. Additionally, dietary assessments will be completed at regular intervals.

Study Treatments:

• The FR diet consists of a full meal plan containing engineered common food products with decreased energy density and glycaemic index, in which a large proportion of refined carbohydrates are replaced with dietary fibre.
• The control diet consists of a full meal plan with non-fibre enriched food products.

Study Outcome measures will include post-prandial glycaemic response, satiety, gastrointestinal tolerability and gut microbiota composition and function in individuals with excess body weight (BMI 25-35kg/m2).

Inclusion Criteria:

• Be willing and able to give written informed consent.
• Be between 18 and 45 years of age
• Have a Body Mass Index (BMI) of 25-34.9kg/m2 (Overweight or Obese Class I)
• 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, prebiotics, probiotics, or fibre-rich supplements within four weeks prior to the baseline visit, and until the end of the study.
• Be willing to avoid physical exercise for the duration of the study (physical exercise defined as any physical activity that is planned to achieve a fitness goal)
• Be willing to consume the investigational products daily for the duration of the study.

Exclusion Criteria:

• Pregnant, lactating, menopausal or post-menopausal women, or women who are planning to become pregnant over the study period.
• Have had antibiotic treatment within three months prior to 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, antacid, proton pump inhibitors, anti-hypertensive medications, corticosteroids, laxatives, enemas, antibiotics, anti-coagulants, 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 onto the study if the dose has been stable for at least two months prior to 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 prior to 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, irritable bowel syndrome, 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 prior to 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.
• Participants may not be 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 prior to this study.
• Individuals with pacemakers or implantable cardioverter defibrillators.
• Individuals that regularly undertake rigorous exercise, defined by International Physical Activity Questionnaire with a score within category 3, Health Enhancing Physical Activity (HEPA) Active.
• Individuals who smoke or vape regularly (i.e., daily or habitual use).

An equal number of male and female participants will be enrolled