Caffeine is a diuretic. A diuretic is any substance that promotes the production of urine, affecting the kidneys by speeding up the urine production process. The diuretic property of caffeine causes less water to reach the colon, this means that the colon may not receive enough fluid to process stools properly. If liquid is drawn out of stools, the stools become harder to pass. For this reason drinking a coffee, or any other caffeine-rich liquid, isn’t the recommended approach to relieve constipation.
Caffeine can also trigger a bowel movement by stimulating the muscles in the digestive system to contract. For this reason many IBS (irritable bowel syndrome) sufferers who suffer from ‘incomplete evacuation’, the sensation of an incomplete bowel movement, turn to coffee to relieve their symptoms.
For those who wish to relieve their constipation-like symptoms with a cup of coffee, they can circumvent coffee’s diuretic effect by consuming water, or any other non-caffeinated fluid, in order to provide the colon with enough liquid to keep stools moist and to process solid wastes effectively.
If you rely on coffee to relieve IBS symptoms, this is a logical approach. However, it may be unnecessary for the one-cup-a-day coffee drinker because the average caffeinated coffee contains between 90-200 mg of caffeine and caffeine may only be considered a diuretic if consumed in large quantities.
According in an article on OneMedical.com,
“While we’ve been told for years that coffee (and caffeine in general) is a potent diuretic, research studies show that coffee seems to only have diuretic effects when consumed in larger amounts–more than 500 to 600 mg a day. The most recent studies on this subject show that there is, in fact, no significant difference between urine output among people who drink caffeinated drinks compared to those who drink water or other comparable non-caffeinated drinks. Therefore, the typical coffee drinker shouldn’t experience significant dehydration from a one- or two-cup habit.”