{ "id": "coffee-and-digestion", "url": "https://coffearium.org/entries/coffee-and-digestion/", "license": "CC-BY-4.0", "title": "Coffee and digestion", "summary": "What studies show about coffee and the digestive tract – stomach acid, reflux, the gallbladder, the colon and gut microbes – which effects were measured in people, which are associations, and what “low-acid” claims measure.", "domain": "science", "kind": "Concept", "stage": "cup", "topics": [ "science" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-21T00:00:00.000Z", "updated": "2026-10-21T00:00:00.000Z", "added": "2026-10-21T00:00:00.000Z", "brief": [ "Regular and decaffeinated coffee both raised gastric acid output in a 1975 study; caffeine alone raised it less than either coffee.3", "A 2026 meta-analysis of 40 studies comparing drinkers and non-drinkers found gastro-oesophageal reflux disease slightly more common in coffee drinkers (OR 1.18), of unclear clinical significance.7", "In 12 healthy people, caffeinated coffee stimulated the colon about as much as a 1,000 kcal meal, 60% more than water.1", "Coffee and decaf both made the gallbladder contract in a small human study; coffee drinking is associated with fewer gallstones, but genetic studies disagree on cause.8,9,10", "No study opened for this entry showed that a “low-acid” label, measured as pH, predicts how the stomach responds.3,6,15" ], "quickFacts": [ { "label": "Gastric acid, peak output (1975)", "value": "Regular coffee 20.9, decaf 16.5, caffeine 8.4 mEq/h (cup-equivalent)" }, { "label": "Reflux (GORD), meta-analysis 2026", "value": "40 studies, 122,074 people; OR 1.18 (1.03–1.36)" }, { "label": "Colon response (1998)", "value": "Caffeinated coffee ≈ 1,000 kcal meal; 60% above water, 23% above decaf" }, { "label": "Urge to defecate after coffee", "value": "29% of 99 young volunteers (questionnaire, 1990)" }, { "label": "Gallstones, meta-analysis 2022", "value": "RR 0.86 (0.79–0.93); per cup 0.95 (observational)" }, { "label": "Bowel recovery after surgery", "value": "First defecation ≈10 h earlier (6 trials, 601 patients)" } ], "aka": [ "Coffee and the stomach", "Coffee and the gut" ], "linkTerms": [ "coffee and digestion", "gastric acid", "heartburn", "reflux", "gastro-oesophageal reflux disease", "GERD", "low-acid coffee", "stomach-friendly coffee", "gut microbiota" ], "related": [ "caffeine", "acids-in-coffee", "chlorogenic-acids", "cold-brew", "decaffeination", "bioactive-compounds", "whats-in-a-cup-of-coffee", "coffee-and-long-term-health", "how-to-read-coffee-health-news" ], "contested": [ "Gallstones, association or cause: a 2022 meta-analysis of observational studies found a lower risk with more coffee, but its genetic (Mendelian randomisation) analysis found no causal association9; a 2020 general-population study by Danish researchers found both an observational and a genetic association with lower risk.10", "Gut microbes: a 2024 review found most human studies reported more Firmicutes and Actinobacteria with coffee, but also conflicting results11; a 2024 multi-cohort study found one bacterium, *Lawsonibacter asaccharolyticus*, consistently linked to coffee drinking.12 Which changes matter for health is not established in the sources opened.", "Irritable bowel syndrome: a 2023 meta-analysis found lower odds of IBS among coffee drinkers13, while a review cites a patient survey linking caffeine with symptoms such as diarrhoea.11 Both are observational." ], "corrections": [ { "date": "2026-10-21T00:00:00.000Z", "change": "IBS meta-analysis: the 95% CI of the pooled OR is now given as 0.80–0.88 from the paper's Results, with a note that the abstract prints 0.80 to 0.84; study designs and the lack of a dose analysis are stated.", "reason": "The abstract's interval ends at the point estimate; the Results section of the full text gives 0.80 to 0.88." }, { "date": "2026-10-21T00:00:00.000Z", "change": "Removed the statement that many microbiota findings come from animal studies and that human samples are often small; N-methylpyridinium is no longer called a compound formed during roasting on the strength of the cell study.", "reason": "Neither statement is in the cited sources (Saygili 2024 review; Rubach 2012 abstract). N-methylpyridinium is now described from Rubach 2014 (more in a dark roast blend)." }, { "date": "2026-10-21T00:00:00.000Z", "change": "Cold brew source now links to the 2018 paper itself; the Danish gallstone study is no longer placed in Copenhagen; surgery types, missing coffee doses and the sex result of the gallstone meta-analysis are stated as in the sources.", "reason": "The previous link opened a different paper (Fuller & Rao 2017); the cohort's location is not in the abstract; health entries must state population and dose." } ], "sources": [ { "id": "s1", "title": "Is coffee a colonic stimulant?", "authors": "Rao, S. S. C.; Welcher, K.; Zimmerman, B.; Stumbo, P.", "year": 1998, "publisher": "European Journal of Gastroenterology & Hepatology 10(2), 113–118", "url": "https://augusta.elsevierpure.com/en/publications/is-coffee-a-colonic-stimulant", "doi": "10.1097/00042737-199802000-00003", "tier": "A", "quote": "Its magnitude is similar to a meal, 60% stronger than water and 23% stronger than decaffeinated coffee.", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s2", "title": "Effect of coffee on distal colon function", "authors": "Brown, S. R.; Cann, P. A.; Read, N. W.", "year": 1990, "publisher": "Gut 31(4), 450–453", "url": "https://gut.bmj.com/content/31/4/450.abstract", "doi": "10.1136/gut.31.4.450", "tier": "A", "quote": "Twenty nine per cent (63% women) claimed that coffee induced a desire to defecate.", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s3", "title": "Gastric acid secretion and lower-esophageal-sphincter pressure in response to coffee and caffeine", "authors": "Cohen, S.; Booth, G. H.", "year": 1975, "publisher": "New England Journal of Medicine 293(18), 897–899", "url": "https://pubmed.ncbi.nlm.nih.gov/1177987/", "doi": "10.1056/NEJM197510302931803", "tier": "A", "quote": "Sphincter pressure showed minimal changes in response to caffeine, but was significantly increased by both regular and decaffeinated coffee (P less than 0.05).", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s4", "title": "Effect of regular and decaffeinated coffee on serum gastrin levels", "authors": "Acquaviva, F.; DeFrancesco, A.; Andriulli, A.; Piantino, P.; Arrigoni, A.; Massarenti, P.; Balzola, F.", "year": 1986, "publisher": "Journal of Clinical Gastroenterology 8(2), 150–153", "url": "https://www.gwern.net/doc/nootropic/caffeine/1986-acquaviva.pdf", "tier": "A", "quote": "Regular and decaffeinated coffees share a strong gastrin-releasing property.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s5", "title": "Multi-parametric approach to identify coffee components that regulate mechanisms of gastric acid secretion", "authors": "Rubach, M.; Lang, R.; Seebach, E.; Somoza, M. M.; Hofmann, T.; Somoza, V.", "year": 2012, "publisher": "Molecular Nutrition & Food Research 56(2), 325–335", "url": "https://portal.fis.leibniz-lsb.tum.de/en/publications/multi-parametric-approach-to-identify-coffee-components-that-regu-4/", "doi": "10.1002/mnfr.201100453", "tier": "A", "quote": "N-MP was shown to effectively down-regulate mechanisms of gastric acid secretion", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s6", "title": "A dark brown roast coffee blend is less effective at stimulating gastric acid secretion in healthy volunteers compared to a medium roast market blend", "authors": "Rubach, M.; Lang, R.; Bytof, G.; Stiebitz, H.; Lantz, I.; Hofmann, T.; Somoza, V.", "year": 2014, "publisher": "Molecular Nutrition & Food Research 58(6), 1370–1373", "url": "https://portal.fis.leibniz-lsb.tum.de/en/publications/a-dark-brown-roast-coffee-blend-is-less-effective-at-stimulating--4/", "doi": "10.1002/mnfr.201300890", "tier": "A", "tierNote": "Peer-reviewed, but a coffee company is among the author affiliations and only nine volunteers took part; cited here as the study behind dark-roast claims, not as evidence of an effect.", "quote": "Gastric acid secretion was less effectively stimulated after administration of the dark roast blend coffee compared to the market blend.", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s7", "title": "Association of Coffee Intake with Risk of Gastroesophageal Reflux Disease and Complications: A Systematic Review and Meta-Analysis", "authors": "Muftah, M.; McCarty, T. R.; Hartnett, D.; Flanagan, R.; Hiramoto, B.; Chan, W. W.", "year": 2026, "publisher": "Clinical and Translational Gastroenterology 17(4), e00996", "url": "https://scholars.houstonmethodist.org/en/publications/association-of-coffee-intake-with-risk-of-gastroesophageal-reflux/", "doi": "10.14309/ctg.0000000000000996", "tier": "A", "quote": "Coffee use was associated with a small, statistically significant increased rate of GERD, but not Barrett's.", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s8", "title": "Coffee stimulation of cholecystokinin release and gallbladder contraction in humans", "authors": "Douglas, B. R.; Jansen, J. B.; Tham, R. T.; Lamers, C. B.", "year": 1990, "publisher": "American Journal of Clinical Nutrition 52(3), 553–556", "url": "https://pubmed.ncbi.nlm.nih.gov/2393014/", "doi": "10.1093/ajcn/52.3.553", "tier": "A", "quote": "An average gallbladder contraction of 33 +/- 7% was observed after 165 mL regular coffee", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s9", "title": "Addictive behavior and incident gallstone disease: A dose–response meta-analysis and Mendelian randomization study", "authors": "Bai, Y.; Zhang, M.; Cui, H.; Shan, X.; Gu, D.; Wang, Y.; Tang, M.; Wang, X.; Jiang, X.; Zhang, B.", "year": 2022, "publisher": "Frontiers in Nutrition 9, 940689", "url": "https://www.frontiersin.org/articles/10.3389/fnut.2022.940689/full", "doi": "10.3389/fnut.2022.940689", "tier": "A", "quote": "No significant causal association of GSD risk with coffee consumption was found in this IVW.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s10", "title": "Coffee intake protects against symptomatic gallstone disease in the general population: a Mendelian randomization study", "authors": "Nordestgaard, A. T.; Stender, S.; Nordestgaard, B. G.; Tybjærg-Hansen, A.", "year": 2020, "publisher": "Journal of Internal Medicine 287(1), 42–53", "url": "https://research.regionh.dk/da/publications/coffee-intake-protects-against-symptomatic-gallstone-disease-in-t/", "doi": "10.1111/joim.12970", "tier": "A", "quote": "The estimated observational odds ratio for GSD for a one cup per day higher coffee intake was 0.97 (0.96-0.98)", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s11", "title": "Effects of Coffee on Gut Microbiota and Bowel Functions in Health and Diseases: A Literature Review", "authors": "Saygili, S.; Hegde, S.; Shi, X.-Z.", "year": 2024, "publisher": "Nutrients 16(18), 3155", "url": "https://archive.lstmed.ac.uk/25406/1/nutrients-16-03155.pdf", "doi": "10.3390/nu16183155", "tier": "A", "quote": "there are many conflicting results regarding the impacts of coffee in the gastrointestinal tract", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s12", "title": "Coffee consumption is associated with intestinal Lawsonibacter asaccharolyticus abundance and prevalence across multiple cohorts", "authors": "Manghi, P.; Bhosle, A.; Wang, K.; Marconi, R.; Selma-Royo, M.; Ricci, L.; et al.", "year": 2024, "publisher": "Nature Microbiology", "url": "https://www.broadinstitute.org/publications/broad1357361", "doi": "10.1038/s41564-024-01858-9", "tier": "A", "quote": "Plasma metabolomics on 438 samples identified several metabolites enriched among coffee consumers", "abstractOnly": true, "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s13", "title": "Examining the Association between Coffee Intake and the Risk of Developing Irritable Bowel Syndrome: A Systematic Review and Meta-Analysis", "authors": "Lee, J. Y.; Yau, C. Y.; Loh, C. Y. L.; Lim, W. S.; Teoh, S. E.; Yau, C. E.; Ong, C.; Thumboo, J.; Namasivayam, V. S. O.; Ng, Q. X.", "year": 2023, "publisher": "Nutrients 15(22), 4745", "url": "https://www.mdpi.com/2072-6643/15/22/4745", "doi": "10.3390/nu15224745", "tier": "A", "quote": "Eight studies comprising 432,022 patients were included in the final meta-analysis.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s14", "title": "Effect of postoperative coffee consumption on gastrointestinal function after abdominal surgery: A systematic review and meta-analysis of randomized controlled trials", "authors": "Eamudomkarn, N.; Kietpeerakool, C.; Kaewrudee, S.; Jampathong, N.; Ngamjarus, C.; Lumbiganon, P.", "year": 2018, "publisher": "Scientific Reports 8, 17349", "url": "https://link.springer.com/10.1038/s41598-018-35752-2", "doi": "10.1038/s41598-018-35752-2", "tier": "A", "quote": "Six trials involving 601 participants were included.", "accessed": "2026-10-21T00:00:00.000Z" }, { "id": "s15", "title": "Acidity and Antioxidant Activity of Cold Brew Coffee", "authors": "Rao, N. Z.; Fuller, M.", "year": 2018, "publisher": "Scientific Reports 8, 16030", "url": "https://www.nature.com/articles/s41598-018-34392-w", "doi": "10.1038/s41598-018-34392-w", "tier": "A", "quote": "Varying the temperature of the extraction water did not result in distinguishable pH values between hot and cold brew coffees.", "accessed": "2026-10-21T00:00:00.000Z" } ], "markdown": "## What “digestion” covers here\n\nCoffee meets the whole digestive tract on its way through the body. Studies have looked at the oesophagus, the stomach, the gallbladder, the colon and the bacteria living in the gut. This entry sorts what was measured in people from what is only an association or a laboratory mechanism.\n\nThe difference matters. An observational study compares people who drink coffee with people who do not; it shows an association, not a cause.7,9 A trial gives coffee or a comparison drink and measures the response.1,14 Cell studies show how a compound can act, not what a cup does in a person.5 How caffeine itself is absorbed and broken down is covered in *Caffeine*; the liver belongs to *Coffee and long-term health*.\n\n
\n
\n
    \n
  1. Oesophagus: reflux. Mixed. Reflux disease was slightly more common in coffee drinkers across 40 studies; the authors call the clinical significance unclear.7
    2. \n
  2. Stomach: acid and gastrin. Measured in humans. Regular and decaffeinated coffee both raised acid output and gastrin, the hormone that drives acid secretion.3,4
    3. \n
  3. Stomach cells: which compounds. Mechanism only. In cultured human stomach cells, caffeine raised and N-methylpyridinium lowered signs of acid secretion.5
    4. \n
  4. Gallbladder: contraction. Measured in humans. Coffee and decaf both made the gallbladder contract in six healthy coffee drinkers.8
    5. \n
  5. Gallbladder: gallstones. Mixed. Fewer gallstones among coffee drinkers in cohort studies; genetic studies disagree on whether coffee is the cause.9,10
    6. \n
  6. Colon: motility. Measured in humans. Caffeinated coffee stimulated the colon about as much as a meal; some people respond within minutes, others not at all.1,2
    7. \n
  7. Colon: gut microbes. Mixed. Coffee drinking is linked with differences in gut bacteria; results between studies conflict.11,12
    8. \n
  8. Bowel after surgery. Measured in humans. In six randomised trials, coffee after abdominal surgery shortened the time to first bowel movement.14
    9. \n
\n
Where coffee has been studied along the digestive tract, and how strong the evidence is at each site, from the sources cited. The mark beside each number gives the class of evidence: filled, measured in humans; half-filled, mixed; open, mechanism only. The liver (dashed) is not covered here. Point at a number or a line of the legend to highlight the site. Schematic, not to scale.
\n
\n\n## The stomach: acid and gastrin\n\nCoffee makes the stomach produce more acid. In a 1975 study in healthy people, regular coffee and decaffeinated coffee gave similar peak acid outputs.3 Pure caffeine, at the amount found in a cup, gave less than half that of regular coffee.3 So caffeine is not the main driver.\n\nGastrin is a hormone that tells the stomach to release acid. In a 1986 study, both regular and decaffeinated coffee raised gastrin in healthy volunteers.4 The authors concluded that decaffeination removes part of this effect, not all of it.4\n\nWhich compounds are responsible is less clear. In a 2012 study on cultured human stomach cells, caffeine increased signs of acid secretion. N-methylpyridinium, a compound found in larger amounts in a dark roast blend than in a medium roast blend6, reduced them.5 That is a laboratory mechanism, not a measurement in people.\n\n## Heartburn and reflux\n\nHeartburn is a burning feeling caused when stomach contents flow back into the oesophagus. Frequent reflux is called gastro-oesophageal reflux disease (GORD; GERD in US spelling).7\n\nA 2026 meta-analysis pooled 40 studies with 122,074 people. Reflux disease was reported in 34.9% of coffee drinkers and 30.7% of non-drinkers.7 The authors call this a small association of unclear clinical significance. They found no link with Barrett’s oesophagus, a change in the oesophageal lining.7 The studies differed a lot, and association is not causation.7 The abstract gives no amounts, such as cups a day.\n\nThe authors conclude that advising everyone with reflux to avoid coffee needs further evaluation.7 Individual triggers are a matter for a doctor.\n\n## The colon and the urge to go\n\nMany people feel an urge to open their bowels after coffee. In a 1990 questionnaire of 99 healthy volunteers aged 17–27, 29% said coffee caused it.2 In 14 healthy subjects, pressure sensors then measured the lower colon. Activity rose within four minutes of coffee in those who reported the effect, and not in the others.2 Decaf did the same in the responders; hot water did nothing.2\n\nA 1998 study measured colonic activity in 12 healthy people. Caffeinated coffee stimulated it about as much as a 1,000 kcal meal.1 The effect was 60% stronger than water and 23% stronger than decaf.1\n\nThe effect has been tested after abdominal surgery, when the bowel can be slow to restart.11,14 In a 2018 meta-analysis of six randomised trials after caesarean, colorectal or gynaecological cancer surgery, coffee shortened the time to the first bowel movement by about 10 h.14\n\n## The gallbladder and gallstones\n\nThe gallbladder stores bile and squeezes it into the gut after a meal. In a 1990 study of six healthy coffee drinkers, 165 mL of coffee made it contract by about a third.8 Decaf did much the same.8\n\nGallstones are less common among coffee drinkers in observational studies.9,10 Whether coffee prevents them is disputed. Two genetic studies, which test cause more directly, reached opposite conclusions (see Contested).9,10\n\n## Gut microbes and the bowel\n\nThe bacteria living in the gut are called the gut microbiota. A 2024 review found that most human studies reported shifts in some bacterial groups with coffee. It also stresses many conflicting results.11\n\nThe clearest signal so far comes from a 2024 study of 22,867 people in the US and the UK. One bacterium, *Lawsonibacter asaccharolyticus*, was linked to coffee drinking, and the link was highly reproducible across populations.12 What that means for health is not known from the sources opened.\n\nFor irritable bowel syndrome (IBS), a common disorder of bowel habit and pain, evidence is thin and observational. See Deep dive.11,13\n\n## “Low-acid” and “stomach-friendly” coffee\n\nSome coffees are sold as low-acid or gentle on the stomach. Two different things are mixed up in such claims. One is the acidity of the drink, measured as pH or titratable acidity. The other is how much acid the stomach itself makes.\n\nThe studies above measured the second. Decaf raised the stomach’s acid output almost as much as regular coffee.3 Compounds such as caffeine and N-methylpyridinium acted on stomach cells in the laboratory.5 No study opened for this entry tested whether a coffee with a higher pH causes less heartburn.\n\nCold brew is often called low-acid. One study found hot and cold brews of the same light roasts had similar pH, but the hot brews held more titratable acid.15 Other studies disagree on pH; see *Cold brew* and *Acids in coffee*.\n\n\n\n## Gastric secretion in detail\n\nThe 1975 study compared dose responses of caffeine, regular coffee and decaffeinated coffee in normal subjects.3 Peak acid output was 20.9 ± 3.6 mEq/h for regular coffee, 16.5 ± 2.6 mEq/h for decaf and 8.4 ± 1.3 mEq/h for caffeine, compared on a cup-equivalent basis.3 The abstract does not give the number of subjects.\n\n
\n
\n
Peak gastric acid output after regular coffee, decaffeinated coffee and an equivalent amount of caffeine in normal subjects, mean ± variation as given in the abstract.3 One study from 1975.
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\n\nThe same study measured pressure in the lower oesophageal sphincter, the muscle ring that keeps stomach contents down. Caffeine changed it little; regular and decaffeinated coffee both raised it.3 The authors warned that advice based on caffeine’s known effects may bear little relation to what coffee actually does.3\n\nIn the 1986 gastrin study, 10 healthy volunteers first drank 50 mL of coffee solution, equal to one home-made cup.4 In the comparison of 100 mL servings, integrated gastrin output was 2.3 times the control after regular coffee and 1.7 times after decaf.4\n\nThe 2012 cell study used the human gastric cell line HGT-1. It tested six coffee components alone and in mixtures: chlorogenic acid, caffeine, pyrogallol, catechol, N-alkanoyl-5-hydroxytryptamides (C5HTs) and N-methylpyridinium.5 N-methylpyridinium reduced markers of acid secretion, including expression of the proton pump.5 Chlorogenic acids and their fate in roasting are covered in *Chlorogenic acids*; the wider set of coffee compounds in *Bioactive compounds*.\n\n## Dark roast claims\n\nDark roasts are sometimes marketed as easier on the stomach. The main human study behind this is small. In 2014, nine healthy volunteers drank 200 mL of a dark roast blend or a medium roast market blend.6 The dark roast held more N-methylpyridinium (87 against 29 mg/L) and less C5HT, chlorogenic acids and trigonelline; caffeine was similar.6 Intragastric pH showed less acid secretion after the dark roast.6\n\nA coffee company is among the study’s author affiliations, and the authors themselves call for further studies.6 No independent replication was found for this entry. The claim is therefore not established.\n\n## Reflux evidence in detail\n\nThe 2026 meta-analysis included 40 studies: 85,400 coffee drinkers and 36,674 non-drinkers.7 The pooled odds ratio for reflux disease was 1.18 (95% CI 1.03–1.36), with high heterogeneity between studies (I² = 89.38%).7 For Barrett’s oesophagus it was 1.13 (0.79–1.61), not significant.7 The studies compared drinkers with non-drinkers, so they show an association, not a cause.7\n\nThe 1975 finding that coffee raised sphincter pressure points the other way from a simple “coffee relaxes the sphincter” story.3 No source opened for this entry settles the mechanism.\n\n## Colonic motility in detail\n\nThe 1998 study used ambulatory colonic manometry: a six-sensor probe placed up to the middle of the transverse colon in 12 healthy subjects.1 Over 10 h, each subject received 240 mL of three drinks at 45 °C in random order – black Colombian coffee with 150 mg caffeine, decaffeinated coffee and water – and a 1,000 kcal meal.1 The response was greater in the transverse and descending colon than in the rectosigmoid colon.1 The abstract reports that decaf, like caffeinated coffee and the meal, induced more colonic activity than water, but also that the effects of decaf were not significantly different from those of water or caffeinated coffee.1\n\nIn the 1990 study, motility rose within 4 min after regular and decaffeinated coffee in eight responders and lasted at least 30 min.2 Six non-responders showed no rise.2 Both studies are small, in healthy young or adult volunteers.\n\nThe surgery meta-analysis pooled six randomised trials with 601 participants.14 Coffee shortened time to first defecation by 9.98 h (95% CI 2.99–16.97) and time to first flatus by 7.14 h.14 Hospital stay fell by 0.74 days.14 All the trials had a high risk of performance bias, because patients could not be blinded.14 The meta-analysis does not report how much coffee was given.\n\n## Gallstones in detail\n\nThe 2022 meta-analysis pooled seven longitudinal observational studies from America and Europe with 333,773 participants.9 Coffee drinking was associated with a lower risk of gallstone disease (RR 0.86, 0.79–0.93), and each extra cup a day with 0.95 (0.94–0.96).9 The association held for caffeinated coffee (0.84) but not decaf (1.00). It was significant in women but not in men, although the two did not differ significantly.9 The same paper’s Mendelian randomisation, which uses genes linked to coffee intake as a natural experiment, found no causal association.9\n\nA 2020 study by Danish researchers followed 104,493 people from the general population for a mean of 8 years.10 Those drinking more than six cups a day had a 23% lower risk of symptomatic gallstone disease (HR 0.77, 0.61–0.94).10 Its genetic analysis in 114,220 people gave an odds ratio of 0.89 (0.83–0.95) per extra cup a day.10 The authors read this as a causal effect; the 2022 analysis did not confirm it.9,10\n\n## Microbiota and IBS in detail\n\nThe 2024 multi-cohort study combined detailed diet data from 22,867 participants with public data from 54,198 people in 211 cohorts.12 The coffee–microbiome link was highly reproducible across populations.12 Plasma from 438 samples showed quinic acid and related compounds linked to both coffee and *L. asaccharolyticus*, and coffee stimulated the bacterium’s growth in vitro.12 These are associations and laboratory growth tests, not a trial.\n\nThe 2024 review found that most human faecal studies reported more Firmicutes and Actinobacteria, and several more *Bifidobacterium*, with coffee; one study found fewer Firmicutes.11\n\nFor IBS, a 2023 meta-analysis of eight studies with 432,022 participants found lower odds of IBS among coffee drinkers (pooled OR 0.84, 95% CI 0.80–0.88; the abstract prints the interval as 0.80 to 0.84).13 Most of the studies were cross-sectional, six from Asia, one from the UK and one from Africa; they compared any coffee intake with none, without a dose–response analysis.13 The authors call for prospective cohort studies that track new cases in healthy people.13 The 2024 review notes a patient survey linking caffeine with symptoms such as diarrhoea, and concludes that controlled studies are needed.11\n\n## Reference values\n\nValues as reported in the sources. Study types differ; read each row with its population and design.\n\n| Parameter | Value reported | Study type and population | Source |\n|---|---|---|---|\n| Peak gastric acid output | Regular 20.9 ± 3.6; decaf 16.5 ± 2.6; caffeine 8.4 ± 1.3 mEq/h (cup-equivalent) | Dose–response study, normal subjects | Cohen & Booth3 |\n| Gastrin, integrated output vs control | 2.3 × (regular); 1.7 × (decaf), 100 mL | Healthy volunteers | Acquaviva et al.4 |\n| N-methylpyridinium, dark vs market blend | 87 vs 29 mg/L | Brew analysis; trial in 9 volunteers (industry affiliation) | Rubach et al.6 |\n| Reflux disease, drinkers vs non-drinkers | 34.9% vs 30.7%; OR 1.18 (1.03–1.36) | Meta-analysis, 40 studies, 122,074 people (drinkers vs non-drinkers) | Muftah et al.7 |\n| Barrett’s oesophagus | OR 1.13 (0.79–1.61), not significant | Same meta-analysis | Muftah et al.7 |\n| Colonic motor response | Coffee ≈ 1,000 kcal meal; 60% > water; 23% > decaf | Manometry, 12 healthy subjects; 240 mL, 150 mg caffeine | Rao et al.1 |\n| Urge to defecate after coffee | 29% of 99 | Questionnaire, volunteers aged 17–27 | Brown et al.2 |\n| Onset and duration, rectosigmoid response | Within 4 min; at least 30 min | Manometry, 8 responders of 14 | Brown et al.2 |\n| Time to first defecation after surgery | 9.98 h shorter (2.99–16.97) | Meta-analysis, 6 randomised trials, 601 patients | Eamudomkarn et al.14 |\n| Gallbladder contraction | 33 ± 7% after 165 mL regular coffee; 29 ± 10% after decaf | 6 healthy coffee drinkers | Douglas et al.8 |\n| Gallstone disease, coffee drinkers | RR 0.86 (0.79–0.93); 0.95 per cup a day | Meta-analysis, 7 observational studies, 333,773 people | Bai et al.9 |\n| Gallstone disease, > 6 cups a day | HR 0.77 (0.61–0.94) | Cohort, 104,493 people from the general population | Nordestgaard et al.10 |\n| Gallstone disease, genetic estimate | OR 0.89 (0.83–0.95) per cup a day; null in a second study | Mendelian randomisation | Nordestgaard et al.10; Bai et al.9 |\n| IBS, coffee drinkers | Pooled OR 0.84 (0.80–0.88) | Meta-analysis, 8 mostly cross-sectional studies, 432,022 people; any intake vs none | Lee et al.13 |\n| Microbiome study size | 22,867 participants; 54,198 in public cohorts | Multi-cohort, US and UK | Manghi et al.12 |\n\n## Open questions\n\n- **Reflux mechanism.** Coffee raised sphincter pressure in one 1975 study3, yet reflux is slightly more common in drinkers.7 No source opened here explains the difference.\n- **Low-acid labels.** No study opened for this entry linked the pH or titratable acidity of a coffee to heartburn or gastric acid output in people.\n- **Dark roast.** The human evidence is one nine-person study with an industry affiliation.6\n- **Gallstones.** Observational studies agree on a lower risk; the genetic studies disagree on cause.9,10\n- **Microbes and health.** Coffee drinkers carry different gut bacteria12, but what that does for health is not established in the sources opened.11\n\n## Key studies\n\n
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Cohen & Booth, 1975 – Coffee, decaf and caffeine in the stomach. Compared dose responses of caffeine, regular coffee and decaffeinated coffee for gastric acid output and sphincter pressure. Both coffees stimulated more acid than caffeine alone, and both raised sphincter pressure while caffeine barely did.

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New England Journal of Medicine 293(18), 897–899 · doi:10.1056/NEJM197510302931803

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Rao et al., 1998 – Is coffee a colonic stimulant? Measured colonic pressure over a day in 12 healthy people given coffee, decaf, water and a large meal. Caffeinated coffee stimulated the colon about as much as the meal and clearly more than water.

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European Journal of Gastroenterology & Hepatology 10(2), 113–118 · doi:10.1097/00042737-199802000-00003

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Muftah et al., 2026 – Coffee and reflux disease. Pooled 40 studies with more than 122,000 people. Reflux disease was slightly more common in coffee drinkers, Barrett’s oesophagus was not, and the authors question routine advice to avoid coffee.

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Clinical and Translational Gastroenterology 17(4), e00996 · doi:10.14309/ctg.0000000000000996

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Bai et al., 2022 – Gallstones, association and cause. Combined a dose–response meta-analysis of cohort studies with a Mendelian randomisation analysis. Coffee drinking was associated with fewer gallstones, but the genetic analysis found no causal link.

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Frontiers in Nutrition 9, 940689 · doi:10.3389/fnut.2022.940689

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Manghi et al., 2024 – Coffee and one gut bacterium. Analysed gut metagenomes and diet in tens of thousands of people across many cohorts. Coffee drinking was consistently linked with *Lawsonibacter asaccharolyticus*, which coffee also helped grow in the laboratory.

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Nature Microbiology · doi:10.1038/s41564-024-01858-9

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\n\n*General information, not medical advice.*" }