Bowel transit time refers to how long it takes for the food to move from the mouth to the anus.
This article discusses the medical test used to determine bowel transit time.
You will be asked to swallow two gelatin capsules filled with carmine red or another food dye. You take the special capsules with a meal.
Afterwards, you observe your bowel movements and write down how long it takes for the colored dye to first appear. You'll also need to note how long it takes for the color to disappear from the stools.
No preparation is usually needed. However, you should follow any diet or other directions from the health care provider.
You will not feel the capsules move through your gastrointestinal system.
The test helps determine bowel function.
Your doctor may ask you to record transit times as you introduce fiber into your diet. Your diet affects the bowel transit time. For example, if you eat a lot of foods rich in fiber (whole grains, vegetables, and fruits), you will have a more rapid transit time and a heavier, bulkier stool.
The bowel transit time varies even in the same person. The first of the color should appear in the stool about 12 - 14 hours after it is taken. The last of the color will appear within 36 - 48 hours.
If the transit time is 72 hours or more, you may have slowed bowel function.
There are no risks.
The bowel transit time dye test is rarely performed these days.
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Camilleri M. Disorders of gastrointestinal motility. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 138.