Optimal gut transit time is 12 to 48 hours. Many adults on low-fibre, ultra-processed food diets have transit times of 72 hours or more. Prolonged gut transit time increases the exposure time between colonic contents, including potential carcinogens, and the colonic epithelium. It is associated with increased colorectal cancer risk, altered microbiome composition favouring proteolytic species over fermentative species, and higher concentration of secondary bile acids which are carcinogenic at high concentrations.
Fibre and Transit Time
Insoluble fibre from whole grains, vegetables, and fruit skins adds bulk to the stool and stimulates peristaltic movement. The stool bulk mechanically activates the muscular contractions of the colon that propel contents forward. In randomised trials, increasing dietary fibre from 15 to 30 grams per day consistently reduces transit time by 20 to 40 hours in adults with previously slow transit. This requires changing the food pattern, not just adding a psyllium supplement to an otherwise unchanged diet.
Water Content of Whole Foods
Whole vegetables and fruits are 70 to 95 percent water by weight. This water contributes to stool hydration and softening, which is essential for comfortable and rapid transit. Ultra-processed foods are typically 5 to 20 percent water by weight. Populations who eat predominantly whole plant foods consistently have significantly shorter transit times and better stool consistency than populations relying on low-water-content processed foods, even when total fluid intake appears similar.