Fibres found in food are mainly carbohydrates that are not broken down or absorbed during human digestion but instead pass to the colon to be fermented by microbes. Gases produced during bacterial fermentation (hydrogen, methane, and carbon dioxide), can cause unpleasant gastrointestinal symptoms in some individuals, such as bloating, flatulence, and stomach pain. Gas production varies between individuals and is influenced by the chemical structure of dietary fibres, the gut microbes' ability to ferment fibre, intestinal pH, and the transit time of intestinal contents. This randomized clinical trial in healthy adults will investigate how individual variations in the microbiome influence the level of gastrointestinal (GI) symptoms induced by different fibre types during short-term exposure.

The objective of this dietary intervention is to examine the effects of different dietary fibre types on gastrointestinal symptoms in apparently healthy adults and to investigate how these effects relate to the composition and functional capacity of the gut microbiota.

The study is designed as a randomized, crossover intervention trial with a total duration of approximately 6 weeks. A 5-day run-in phase is carried out to stabilize baseline dietary patterns and ensure adherence to study procedures. During the intervention, each participant consumes 3 different fibre-rich products, with each product administered for 4 consecutive days (referred to as a "fibre period"). Each fibre period is separated by a 10-day washout period.

Participants (n=28) are apparently healthy adult males and females aged 18-65 years who experience gastrointestinal/ digestive discomfort during or after eating. Individuals without such symptoms are unlikely to exhibit measurable responses, which would limit the ability to address the study's primary research questions.

During the 4-day fibre period, the daily fibre dose provided by the study product is 20 g for women and 25 g for men (corresponding to 2.5 g of fibre per MJ of energy intake, assuming daily energy intake of 8 MJ for women and 10 MJ for men). During the trial, participants consume three structurally different types of dietary fibre: cellulose (poorly fermentable fibre; microcrystalline cellulose), beta-glucan rich fibre (oat bran-based fibre product), and arabinoxylan rich fibre (rye bran-based fibre product). Participants are randomly assigned into one of the six predefined intervention sequences, which represent all possible orders of exposure to the 3 fibre products. Sequence allocation will be generated prior to enrolment, and participants will receive each fibre-rich test product in a randomized order. Participants are instructed to keep the rest of their diet as habitual as possible during the study. Regular use of dietary supplements must be discontinued for the study duration

Participants report their daily GI symptoms as well as perceived stress and mood through a symptom questionnaire using a mobile app. A total of nine GI symptoms are reported (i.e., stomach/ abdominal pain, stomach cramps, bloating, flatulence, borborygmus, nausea, heartburn, discomfort in the upper abdomen/ unpleasant sensation of fullness following eating, a sudden need to defecate), and their severity of symptoms is assessed on a 0-100 VAS scale. Participants report daily the number of bowel movements through the online form, as well as their daily amount of physical activity. In addition, physical activity is assessed using an accelerometer. Dietary intake is assessed by food records (3-day food records during the run-in period and 2-day food records during each fibre period).

Stool samples are used for the analysis of the gut microbiota and metabolome. Additionally, the the effects of different fibre types on metabolic profiles in blood, urine, and stool samples (including SCFAs), as well as on gases in exhaled breath will be analysed.

Inclusion Criteria:

• healthy male and female
• aged 18-65 years
• experience gastrointestinal discomfort during or after eating
• willing to commit to the 6-week study period and follow the study schedule.

Exclusion Criteria:

• Body mass index ≤ 18.5 or ≥ 35 kg/m².
• Inflammatory bowel diseases, irritable bowel syndrome (IBS), celiac disease, endocrine or lipid metabolism disorders, liver or kidney diseases, cancer within the past 5 years, eating disorders.
• Medication for gastrointestinal symptoms, bowel function, diabetes, or hypercholesterolemia.
• Regular or recent use of antibiotics (within the last 3 months).
• Food allergies to the ingredients of the study products.
• Heavy, long-duration exercise several times a week.
• Smoking, use of snus or nicotine pouches.
• Planned or ongoing pregnancy or breastfeeding.
• In addition, the site-specific principal investigator may exclude a person due to any illness, medication, or factor they deem relevant for the participant's health and safety or for the conduct of the study.

Data sharing in agreement with the EU General Data Protection Regulation