Why Coffee Makes Some People Poop and Others Constipated: The Science Behind Caffeine’s Gut Effects

 “Every time I drink coffee, I have to run to the bathroom within half an hour—it’s like my body has an alarm clock.”

“I drink two cups of coffee every day, but I often feel bloated and constipated. Sometimes I even wonder if I should just give up coffee.”

On October 16, the hashtag “The first batch of coffee lovers has fallen victim” started trending online. Netizens shared their very different gut reactions to coffee—some experience diarrhea, while others get constipated.


Caffeine’s Dual Effect: It Can Both Relieve and Cause Constipation

Why can the same cup of coffee produce two completely opposite effects?
According to Cui Jun, Chief Dietitian at Beijing Electric Power Hospital, coffee’s impact on digestion is the result of multiple factors—mainly caffeine stimulation and acidic compounds.

Caffeine stimulates stomach acid secretion and speeds up intestinal movement. A single intake of 100 mg of caffeine (roughly one espresso) can increase intestinal motility by nearly 30%, shortening the time food residues stay in the intestines and reducing water absorption. When things move too quickly, diarrhea follows.

Besides caffeine, organic acids like chlorogenic acid and quinic acid lower coffee’s pH to around 4.5–5.5, directly irritating the stomach lining and boosting stomach acid production.

On the flip side, some people develop constipation after long-term coffee consumption. Cui explains that the mechanism is quite different:
“Caffeine’s diuretic effect can cause mild dehydration. If you don’t drink enough water, your stool can become dry and hard.” In this case, the intestinal contents lose moisture, making it harder for the body to pass waste. Additionally, regular heavy coffee drinkers may build a tolerance to caffeine, weakening its bowel-stimulating effects over time.

As for the online claim that “coffee grounds absorb water and cause constipation,” Cui clarifies:
“There’s no scientific basis for that. Coffee grounds do absorb water—but only before brewing. Once you drink coffee, there are no actual ‘grounds’ entering your intestines, so this claim is inaccurate.”

Gut Reactions Depend on the Individual — Smart Drinking Is Key

So why do different people react so differently to the same cup of coffee?
Cui points to individual differences as the main factor.

Your genes determine how quickly you metabolize caffeine. “Variations in the CYP1A2 gene can make caffeine metabolism differ by up to eightfold,” Cui explains. “People who metabolize caffeine more slowly experience longer and stronger effects.” Gut microbiota also play a role—people with unbalanced gut flora, due to poor diet or frequent antibiotic use, are more likely to have abnormal reactions like diarrhea or constipation after drinking coffee.

To enjoy coffee without upsetting your stomach, Cui suggests a few practical tips:

  • Avoid drinking coffee on an empty stomach. It’s best consumed about an hour after a meal to reduce irritation to the stomach lining.

  • Moderate your intake. For healthy adults, the recommended limit is 400 mg of caffeine per day—about 3–4 cups of 150 ml black coffee. Sensitive individuals should consume less.

  • Choose the right type. People with sensitive stomachs should try coffees lower in acidity. “For example, cold brew coffee has over 40% less acid than hot brew. Low-caffeine or light-roast Arabica beans are also good options.”

  • Pair it wisely. Drinking coffee with fiber-rich foods like whole-grain bread or oatmeal, and maintaining good hydration throughout the day, can ease its effects on your digestive system and counter dehydration.

Caffeine affects everyone differently, so it’s important to listen to your body.
“If you experience insomnia, anxiety, or stomach discomfort after drinking coffee, it’s a sign you’ve exceeded your personal tolerance level,” Cui advises.

So, while the internet may be divided between “coffee causes diarrhea” and “coffee causes constipation,” there’s really no need to panic. The key is understanding your own body—and finding the way of drinking coffee that works best for you.

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