The aim of this crossover intervention study is to evaluate the volatile organic compounds (VOCs) present in participants at baseline and how these levels change after consuming either a low- or high-dietary fiber diet for three days. This study will involve adults, 18-65 years old, without any gastrointestinal diseases.

The primary objectives are to characterize baseline VOC profiles, assess changes in these profiles after a short-term dietary fiber intervention, determine if specific VOC patterns correlate with dietary fiber intake, and identify the time points at which VOC levels peak. This information will help establish the optimal timing for breath sampling and potentially provide insight into how VOC patterns relate to dietary fiber consumption and degradation.

The expected outcome is that a high-dietary fiber diet will produce a VOC profile enriched in metabolites associated with fiber fermentation compared to a low-dietary fiber diet, and that these VOCs will be consistent with those identified in vitro during fecal fermentation. Each participant will serve as an internal control, enabling within-subject comparisons of VOC levels at baseline and following the dietary fiber intervention.

Participants will receive a whole-food product that is rich in dietary fiber and will consume either a low-fiber diet (10 g/day) or a high-fiber diet (40 g/day) for three days. On the first day of each intervention, participants will also ingest a pH capsule to monitor gastrointestinal pH continuously throughout the three days. After a three-week washout period, participants will cross over to the other intervention arm and undergo another round of gastrointestinal pH monitoring.

In addition to the dietary intervention, participants will complete food and health questionnaires to assess habitual fiber intake and dietary habits. Fecal samples will be collected for in vitro fermentation to functionally profile the gut microbiota by assessing fecal VOCs, short-chain fatty acids (SCFA), and the microbial enzymatic activity and its pH-dependent modulation.

Inclusion Criteria:

1. Signed informed consent obtained prior to any study-related procedures.
2. Willingness to refrain from regular consumption of prebiotic or probiotic products, as well as medications known to affect gastrointestinal function, for at least four weeks prior to the study visits.

Exclusion Criteria:

1. History of complicated gastrointestinal surgery.
2. Presence of a gastrointestinal diagnosis or any condition that, in the opinion of the principal investigator, may influence the study outcomes.
3. Current diagnosis of a psychiatric disorder.
4. Systemic use of antibiotics or steroid medications within the past 3 months.
5. Frequent use of NSAIDs (non-steroidal anti-inflammatory drugs) within the 2 months preceding the study visit.
6. Regular consumption of prebiotic or probiotic products within the past 4 weeks.
7. Alcohol or substance abuse.
8. Current smoking or use of nicotine pouches.
9. Frequent use of laxatives, antidiarrheal agents, or anticholinergic medications within the 12 weeks preceding the study visit.
10. Pregnancy or breastfeeding.
11. Respiratory disease or infection.