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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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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
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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
Yes, this is true. For hydration purposes, coffee is as effective as tap water. The effect on gut motility is strongest with caffeinated coffee, then decaf, then water, with both coffee types equal to a 1000 kcal meal. I've speculated how much of the effect they are able to attribute to the individual components (caffeine, coffee, water) based on their experimental results. What you suggest nico is a comparison now of two kinds of coffee, of which they would expect the differences seen from an espresso vs cup of coffee. – leonardo Apr 6 '12 at 17:07
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