Bowel transit time refers to how long it takes for the food to move from the mouth to the end of the intestine (anus).
This article talks about the medical test used to determine bowel transit time.
You will be asked to swallow 2 gelatin capsules filled with a colored food dye. You take these 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 will also need to note how long it takes for the color to disappear from the stools.
In most cases, you DO NOT need to prepare for this test. However, you should follow any diet or other directions from the health care provider.
You will not feel the capsules move through your digestive system.
The test helps determine bowel function.
Your provider 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 color should first appear in the stool about 12 to 14 hours after it is taken. The last of the color will appear within 36 to 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 done these days. Instead, bowel transit is often measured with small probes called manometry. Your provider can tell you if this is needed for your condition.
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Camilleri M. Disorders of gastrointestinal motility. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 136.