Transit Time Segments and Norms
Total gut transit time—from mouth to anus—normally spans 10-73 hours (mean ~35 hours). This breaks into segments: gastric emptying (1-4 hours for mixed meal), small intestinal transit (2-6 hours, oro-caecal time), and colonic transit (20-40 hours). Different segments serve different functions: gastric delays allow acid action; small intestinal speed ensures nutrient absorption; colonic time permits water reabsorption and SCFA production.
Radiopaque Marker Method (Metcalf Protocol)
The Metcalf protocol uses radiopaque markers (small plastic pellets visible on X-ray). 20 markers are swallowed on day 1; abdominal X-rays taken on days 1, 2, 3, 4, 5 show marker progression. Delayed evacuation (>5 markers remaining on day 5) indicates slow transit (constipation-type). This method is inexpensive, non-invasive, and reproducible but exposes patients to radiation and requires multiple X-rays.
Wireless Motility Capsule (SmartPill)
SmartPill is an ingestible, battery-powered capsule recording pH, pressure, and temperature as it travels the GI tract. It transmits data to an external receiver, and a computer determines transit times for each segment. Advantages: non-radioactive, high accuracy, multi-parameter data. Disadvantages: expensive (~$1,000-3,000 per procedure), may be retained in strictures or adhesions, and requires patient cooperation for consistent positioning of receiver.
Blue Dye Test (Simple Home Method)
Asnicar et al. (2021) validated a home-based blue dye test: patients ingest blue dye capsules and track their appearance in stool. Time from ingestion to first appearance estimates whole-gut transit. Advantages: no radiation, inexpensive, home-based. Disadvantages: requires patient observation, less precise than other methods. This test shows promise for epidemiological studies and monitoring but lacks clinical standardization.
Transit Time and Microbiota Correlation
Slow colonic transit (>40 hours) correlates with reduced bacterial diversity, reduced SCFA-producing bacteria (slower fermentation), and increased blooms of sulfate-reducing bacteria (generating H2S). Fast transit (<20 hours) correlates with different dysbiosis: reduced short-chain fatty acid production (insufficient fermentation time), altered bacterial community structure. Transit time is often a hidden confounder in microbiome studies: it affects composition independent of diet or antibiotics.