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Gut Transit Time
You know how often you go. You have no idea how long food takes to get through you, and the two are different numbers. Someone who goes every morning can still run a four-day transit and never know it MΓΌller-Lissner et al. 2005. A free home test settles it: eat whole sweetcorn, watch for the kernels, count the hours. Most adults land between 24 and 48. Land well outside that and you've found a lever worth pulling before anything gets diagnosed.
Screen Β· As-needed Evidence Moderate Chapter Gut

The number is a real signal. Fed a blue-dyed muffin, healthy adults had a median transit of 28.7 hours, with the healthy middle running 17 to 42. People outside that band had worse blood-sugar and blood-fat responses to meals, different gut bacteria, and more visceral fat, independent of age, sex, and weight 1.

The colon is where the slowness lives. The longer stool sits, the more your bacteria switch from making short-chain fats the body uses to protein-breakdown products it doesn't. Slow transit tracks with faster turnover of the colon lining, a pattern linked to colorectal cancer risk 2.

Sweetcorn works because it survives the trip. The cellulose skin on a kernel is something your body can't break down, so a whole one lands in the bowl looking like the corn you ate. The dye used in modern research relies on the same trick: something visible that doesn't get destroyed on the way through 3.

Run it on an ordinary day, not a feast and not a fast.

Reading it, for healthy adults: under 12 hours is very fast and worth a doctor if it holds; 12 to 24 is the fast end of normal; 24 to 48 is typical; 48 to 72 is the slow end; over 72 is slow transit, and the levers are fibre, fluid, and walking. One caveat: women run 30 to 50 percent slower than men at every age, so a 36-hour result sits near the median for a woman and toward the slow end for a man 4.

Act on a slow number and the change is quick. Add insoluble fibre, more water, and more walking, and within a week most people feel it: softer stools, less evening bloat, less effort in the bathroom. Retest at two weeks and a 78-hour transit often comes down to 40-something 5. For a fast number the move is opposite β€” soluble, viscous fibre like psyllium, and a clinician if it persists 5. Over years, the slower-fermenting profile you avoid is the one observational studies tie to the colon-disease cluster 2.

The fine print β€” when to skip it, and what people get wrong

"More fibre always speeds things up." Insoluble fibre (wheat bran) shortens transit; soluble fibre (psyllium) normalises it, speeding slow guts and slowing fast ones 5. "Toxins build up in the colon." Dropped a century ago; long transit changes what bacteria make, and nothing leaks through the gut wall 6.

This is a screen, not a diagnosis. Rectal bleeding, unexplained weight loss, new bowel changes after 50, unexplained iron-deficiency anaemia, or a family history of colorectal cancer means see a clinician first 7. The test is also unreliable during a stomach bug, after antibiotics, on opioids, or just after surgery.

References
  1. 1Asnicar F, Leeming ER, Dimidi E et al. (2021). Blue poo: impact of gut transit time on the gut microbiome using a novel marker. Gut. link
  2. 2Roager HM, Hansen LB, Bahl MI et al. (2016). Colonic transit time is related to bacterial metabolism and mucosal turnover in the human gut. Nature Microbiology. link
  3. 3Cummings JH, Wiggins HS (1976). Transit through the gut measured by analysis of a single stool. Gut. link
  4. 4Degen LP, Phillips SF (1996). Variability of gastrointestinal transit in healthy women and men. Gut. link
  5. 5McRorie JW, McKeown NM (2017). Understanding the physics of functional fibers in the gastrointestinal tract: an evidence-based approach to resolving enduring misconceptions about insoluble and soluble fiber. Journal of the Academy of Nutrition and Dietetics. link
  6. 6MΓΌller-Lissner SA, Kamm MA, Scarpignato C, Wald A (2005). Myths and misconceptions about chronic constipation. American Journal of Gastroenterology. link
  7. 7Drossman DA, Hasler WL (2016). Rome IV β€” Functional GI disorders: disorders of gut-brain interaction. Gastroenterology. link
Related in the handbook (8)
Works well with
  • β€” If your transit is off, fiber is the first lever β€” and the type matters more than the amount.
  • β€” Slow transit usually means too little fibre or fluid β€” water is half of what keeps things moving at a healthy pace.
Related
  • β€” The Bristol scale reads stool form; transit time reads the speed β€” together they screen your gut.
  • β€” If your transit is slow, this is the constipation playbook β€” daily PEG fixes most of it.
  • β€” Curious how fast things actually move? The corn test puts a number on your transit time before you go chasing a cause.
  • β€” The cleaning wave that spacing your meals restarts is the same gut motility that moves food along β€” let it run and transit tends to normalize.
  • β€” Psyllium is a simple way to nudge a transit time that's running too fast or too slow.
  • β€” Slow gut motility is what lets colon bacteria creep upstream β€” sluggish transit is a real risk factor for overgrowth.
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