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How does caffeine (or any additional agents) act as a laxative when ingested? I'm interested in the metabolic/signaling pathway.

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As someone who has serious digestive issues,I can testify that coffee has far more strong of an effect on motility than water does. In fact, I have found that coffee works better than otc laxatives too,at least in my case, and much faster. I imagine there are a lot of ibs sufferers out there who don't realize this and wonder why they have such bad problems in the mornings –  user4384 Sep 2 '13 at 19:10
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up vote 15 down vote accepted

Coffee does have an effect on the peristaltic movement in the bowel.

Coffee increases rectosigmoid motor activity within 4 min after ingestion in some people. Its effects on the colon are found to be comparable to those of a 1000 kCal meal. Since coffee contains no calories, and its effects on the gastrointestinal tract cannot be ascribed to its volume load, acidity or osmolality, it must have pharmacological effects. Caffeine cannot solely account for these gastrointestinal effects.

Effectively, decaf and regular coffee stimulate peristaltic movement in the colon as effectively as a meal does. Caffeine is not the active agent then, but some other compound in coffee.

Source: Boekema PJ, Samsom M, et al. Coffee and gastrointestinal function: facts and fiction. A review. Scand J Gastroenterol Suppl. 1999;230:35-9. PMID 10499460.

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Thanks, I'll check the paper. Does it mention the candidate substances causing the effect instead of caffeine? –  zeller Apr 4 '12 at 12:17
I don't know because I can't access the article myself. Work done at the same time, by another group found that caffeinated coffee was 23% stronger than decaf, implying that most of the effect is not derived from caffeine. The source (PMID: 9581985; Rao, Welcher, et al. Eur J Gastroenterol Hepatol. 1998 Feb;10(2):113-8.). –  leonardo Apr 4 '12 at 19:39
It is accepted that unless 500 mg of caffeine is consumed at once, it is equivalent to drinking water for hydration purposes. From this and the results of these two papers, I speculate that water accounts for ~40% of the effect, caffeine for a further ~20%, and the remaining ~40% is an unaccounted coffee effect. –  leonardo Apr 4 '12 at 19:45
@leonardo: the effect of water would very much depend on the coffee. A cup of Italian coffee and a cup of American coffee have extremely different volumes. –  nico Apr 6 '12 at 16:34
@rwst I'm sorry but you are incorrect. That source directly concludes that coffee's effect "[...] cannot be ascribed to its volume load, acidity or osmolality, it must have pharmacological effects." While also finding that these effects are not solely the effect of caffeine. –  leonardo Nov 22 '12 at 22:56
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