The goal of this study is to investigate the baseline of gastrointestinal transit time (GITT) and explore its relationship with other health indicators in 932 healthy adult volunteers.

The main questions it aims to answer are:

What are the population-level baseline values for GITT using the blue dye method? How is GITT associated with key physiological parameters including blood metabolic markers, life style, and stool characteristics?

Participants will:

Consume a standardized blue dye muffin after overnight fasting at the research center; Offer a finger prock blood samples for metabolic profiling; Collect stool samples for moisture measurement; Complete detailed questionnaires on diet, lifestyle, and bowel habits; Record the time of first blue stool appearance to determine GITT.

This explorative study is designed as a cross-sectional investigation aimed at establishing population-level reference values for gastrointestinal transit time (GITT). Adult participants from the general population will undergo GITT measurement using the blue-dye method. This muffin contains blue colorant that cannot be absorbed; after ingestion, it passes through the entire gastrointestinal tract and is eventually excreted in feces. Following a two-day lifestyle standardization period (including dietary and activity guidance) to minimize potential confounding effects on gastrointestinal motility, participants will attend a clinical visit where they will ingest a blue muffin. GITT will be determined by measuring both the time interval between ingestion of the blue muffin and the first appearance of blue-colored stool, and the duration from its first appearance to its complete disappearance.

The blue muffin method has been well-established in previous research for GITT assessment. Building upon the methodology developed by Berry et al., we will introduce stricter pre-test condition controls (includes dietary advice and lifestyle recommendations) and optimized the blue muffin formulation.These improvements allow for reliable measurement with a single muffin administration, which has been evaluated through KU Leuven Health Pass in a large sample size(S-70168).

To investigate potential determinants and correlates of GITT, we will collect biochemical parameters, fecal moisture, and resting metabolic rate. Additionally, participants will complete 24-h diet record and health questionnaires assessing bowel movement, physical activity habits, stress levels, sleep quality and hydration condition for subsequent correlation analyses.

Inclusion Criteria:

1. 18-65 years old
2. 18.5 kg/m² ≤ BMI ≤ 25 kg/m² or BMI ≥ 30 kg/m² (people with normal weight or obesity)
3. Provision of informed consent to participate.

Exclusion Criteria:

1. Gastrointestinal conditions which may affect nutrient absorption or gastric emptying, including coeliac disease, Crohn's disease, previous resection of the small intestine other than bariatric surgery, gastroparesis;
2. Using medications which effect gastric motility (e.g., domperidone, erythromycin, metoclopramide, prucalopride, opiates, loperamide, …)
3. The ingredients of blue muffins include gluten, nuts, baking powder, sugar, vanilla extract and sunflower oil. Participants with allergies to these ingredients;
4. Current diagnosis of cancer;
5. Advanced organ failure, including chronic kidney disease Stage 5, liver cirrhosis Child-Pugh B or C, intestinal failure, heart failure stage D, or chronic obstructive pulmonary disease stage 4;
6. Immobility;
7. Neuromuscular degenerative conditions associated with muscular atrophy, such as myasthenia gravis, muscular dystrophies, fibromyalgia, or multiple Sclerosis;

Healthy volunteers from the Flanders region who volunteered to participate in the research