A Study on Gastrointestinal Transit Time at the Population Level

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

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

Exclusion criteria

• 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;
• Using medications which effect gastric motility (e.g., domperidone, erythromycin, metoclopramide, prucalopride, opiates, loperamide, …)
• The ingredients of blue muffins include gluten, nuts, baking powder, sugar, vanilla extract and sunflower oil. Participants with allergies to these ingredients;
• Current diagnosis of cancer;
• 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;
• Immobility;
• Neuromuscular degenerative conditions associated with muscular atrophy, such as myasthenia gravis, muscular dystrophies, fibromyalgia, or multiple Sclerosis;