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Acute diarrhea
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In one pass Diarrhea happens when the gut releases more water into its interior than it takes back.
Educational content, not medical advice — consult a clinician.
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Chapter 1
What diarrhea does to the body
Diarrhea happens when the gut releases more water into its interior than it takes back. Normally the small intestine and colon keep drawing large amounts of water, together with salt, back into the blood; when viruses, bacteria or their toxins disrupt this, the direction reverses.
Most acute diarrhea clears up on its own. The real risk is not the diarrhea itself but the water and salt lost with it, and children and older adults have the least to spare. So the most important thing at this point is to replace the water and salt you lose.
Seek care now, rather than waiting at home, if you have bloody or black stools, a persistent high fever (above 38.9°C), vomiting so much you can't keep water down, much less urine, a dry mouth, dizziness when you stand up, or diarrhea lasting more than 3 days.
Most acute diarrhea clears up on its own. The real risk is not the diarrhea itself but the water and salt lost with it, and children and older adults have the least to spare. So the most important thing at this point is to replace the water and salt you lose.
Seek care now, rather than waiting at home, if you have bloody or black stools, a persistent high fever (above 38.9°C), vomiting so much you can't keep water down, much less urine, a dry mouth, dizziness when you stand up, or diarrhea lasting more than 3 days.
Numbers · Why children and older adults need more care
Diarrhea is mostly not deadly today, and rehydration is the reason: oral and intravenous rehydration have been very successful. Yet it remains a major cause of death worldwide, concentrated at both ends of life. By an estimate cited in one review, about 577,000 children under 5 and 502,000 adults over 70 died of diarrheal disease worldwide in 2015.So for these two groups, the threshold for seeing a doctor should be lower.
This story is about acute diarrhea. Diarrhea that lasts for weeks or keeps coming back is a different matter, and a doctor needs to look for the cause (see ibs).
Chapter 2
Why the gut pours out water
The cells lining the gut do two opposite jobs at once. On one side, they move sodium inward and water follows it back into the body. On the other, a chloride channel called CFTR lets chloride out into the gut, and water and sodium follow the chloride out. Normally absorption wins.
Toxins such as cholera toxin and the enterotoxins of some E. coli strains raise signaling molecules inside the gut cells (such as cyclic AMP), which hold the CFTR door open. Chloride keeps flowing out and water follows it; at the same time, one of the main transporters that absorbs sodium is turned down. The gut switches from taking water in to pouring it out. This is called secretory diarrhea.
The key point: on the same cells, the route where sodium and glucose enter together (SGLT1) still works in most of these diarrheas. Oral rehydration salts save lives by using this route that has not been shut.
Toxins such as cholera toxin and the enterotoxins of some E. coli strains raise signaling molecules inside the gut cells (such as cyclic AMP), which hold the CFTR door open. Chloride keeps flowing out and water follows it; at the same time, one of the main transporters that absorbs sodium is turned down. The gut switches from taking water in to pouring it out. This is called secretory diarrhea.
The key point: on the same cells, the route where sodium and glucose enter together (SGLT1) still works in most of these diarrheas. Oral rehydration salts save lives by using this route that has not been shut.
Mechanism · How rehydration salts pull water back
Each time the SGLT1 door moves cargo, it carries two sodium ions together with one glucose molecule. Rehydration salts contain sodium and glucose in set proportions; the two enter the cell together, and water follows them back into the blood by osmosis. Why plain water is not enough, and what the standard formula is, are covered fully in the chapter of digestive on how the small intestine absorbs nutrients.One point here relates directly to diarrhea: toxins like cholera toxin largely leave this route alone, so even while the gut is still pouring water out, rehydration salts can still bring water back. There are exceptions: a protein made by rotavirus inhibits this transporter. For severe dehydration, the guideline advises intravenous fluids instead. That also explains why rehydration salts don't stop diarrhea: they replace water that is lost; they do not stop the gut from pouring it out.
Chapter 3
Rehydrating the right way
Reduced-osmolarity oral rehydration salts are the guideline's first choice for adults and children alike whenever there is mild to moderate dehydration. A few easily missed points are where people go wrong most often:
They don't stop diarrhea: rehydration salts correct dehydration, but they do not reduce how much comes out or shorten the illness. Many people stop after a day because they are still running to the toilet and conclude it isn't working. In fact you should keep replacing as you lose, until the diarrhea and vomiting stop.Mix with the amount of water on the label, not stronger: the concentration is part of the formula, the newer lower-concentration formula works better, and a stronger mix drifts away from what was tested.If you can't keep it down: when fluids by mouth won't stay down, the guideline advises a nasogastric tube or intravenous fluids instead, which means it is time for hospital.
In mild illness with almost no dehydration, what matters most is that you can drink at all, and rehydration salts are not essential; a randomized trial in children supports this.
They don't stop diarrhea: rehydration salts correct dehydration, but they do not reduce how much comes out or shorten the illness. Many people stop after a day because they are still running to the toilet and conclude it isn't working. In fact you should keep replacing as you lose, until the diarrhea and vomiting stop.Mix with the amount of water on the label, not stronger: the concentration is part of the formula, the newer lower-concentration formula works better, and a stronger mix drifts away from what was tested.If you can't keep it down: when fluids by mouth won't stay down, the guideline advises a nasogastric tube or intravenous fluids instead, which means it is time for hospital.
In mild illness with almost no dehydration, what matters most is that you can drink at all, and rehydration salts are not essential; a randomized trial in children supports this.
Evidence · Diluted juice can help mild cases in kids
A pediatric emergency department in Toronto, Canada, ran a randomized trial with 647 children aged 6 months to 5 years who had gastroenteritis but almost no dehydration. One group drank apple juice diluted half and half with water, then whatever they liked; the other drank an electrolyte solution. Treatment failure within 7 days (needing an IV, admission, another emergency visit, lingering symptoms or significant fluid loss) was 16.7% in the juice group and 25.0% in the electrolyte group; IV fluids were needed by 2.5% and 9.0%.The abstract does not conclude why the juice group did better; one possibility is that children were more willing to drink it. The authors set their own limits: it applies only to children with mild illness and almost no dehydration, in the medical setting of high-income countries.
So the reading is this: if a child is alert, passing normal amounts of urine and has just had a few bouts, getting them to drink, and keep drinking, matters most. Once they become listless, pass little urine, cannot drink or vomit whatever they take, switch to rehydration salts and see a doctor promptly.
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Evidence · Why the reduced-osmolarity formula
The World Health Organization later switched oral rehydration salts to a reduced-osmolarity formula: a little less sodium and glucose than the old formula, and a lower overall concentration (for the exact figures, see the chapter of digestive on small-intestine absorption). One basis for the change was a Cochrane systematic review: in children admitted to hospital with diarrhea, the reduced-osmolarity formula meant fewer unplanned IV infusions (8 trials, 0.59), less stool output and less vomiting, with no extra risk of low blood sodium detected.That is also why you mix it with the amount of water on the label: the concentration is part of the formula, and a stronger mix is no longer the thing that was tested. When you buy rehydration salts at a pharmacy, check that it is the reduced-osmolarity formula.
Chapter 4
Eating, and how to use medicines
Diarrhea does not call for starving yourself. The Infectious Diseases Society of America guideline advises returning to your usual diet during or right after rehydration, and breastfed children should keep breastfeeding throughout. You don't have to live on rice porridge or a handful of foods.
Some rules for medicines:
Antidiarrheal drugs (such as loperamide): they can be used by adults with a normal immune system who have watery diarrhea. With a fever or blood in the stool, do not use them at any age, and do not use them under 18. They are no substitute for fluids.Antibiotics: for most watery diarrhea without recent international travel, the guideline advises against taking antibiotics on your own.Probiotics: people with a normal immune system may use them to ease symptoms and shorten the illness, but the recommendation is weak; if your immunity is weakened, ask a doctor first.
If you recently took antibiotics, or the diarrhea started in hospital, tell your doctor; a test for a bacterium called C. difficile may be needed.
Some rules for medicines:
Antidiarrheal drugs (such as loperamide): they can be used by adults with a normal immune system who have watery diarrhea. With a fever or blood in the stool, do not use them at any age, and do not use them under 18. They are no substitute for fluids.Antibiotics: for most watery diarrhea without recent international travel, the guideline advises against taking antibiotics on your own.Probiotics: people with a normal immune system may use them to ease symptoms and shorten the illness, but the recommendation is weak; if your immunity is weakened, ask a doctor first.
If you recently took antibiotics, or the diarrhea started in hospital, tell your doctor; a test for a bacterium called C. difficile may be needed.
Safety · When antidiarrheal drugs must not be used
Antidiarrheal drugs slow the gut down. For ordinary watery diarrhea, that can save an adult a few trips to the toilet, but in diarrhea with fever or inflammation the guideline is concerned it may lead to serious problems such as toxic megacolon (acute widening of the colon).So the Infectious Diseases Society of America guideline draws three lines:
Do not use them under 18 (strong recommendation). In the studies of children the guideline reviewed, 0.54% of children given loperamide died, all of them under 3 years old.Do not use them at any age with a fever or suspected inflammatory diarrhea (such as blood in the stool) (strong recommendation).Consider them only once fluids are adequate; they do not replace rehydration (weak recommendation).
Adults with a normal immune system who have only watery diarrhea, without fever or blood, can use them as the label directs (weak recommendation). If you are unsure which case you are, hold off and rehydrate first.
Clinical · When tests and antibiotics are needed
Most acute diarrhea clears up on its own, and identifying the germ often makes little difference to you personally, so testing is not needed every time. The guideline advises testing when:There is a fever or blood in the stool: check for bacteria that antibiotics can treat, such as Salmonella, Shigella and Campylobacter.You recently took antibiotics, or the diarrhea started in hospital: consider a test for C. difficile.
On antibiotics, the guideline advises against empirical use for most acute watery diarrhea without recent international travel (strong recommendation); the exceptions are people with weakened immunity and young infants who look seriously ill. For watery diarrhea lasting 14 days or more, antibiotics should not be given blindly either; the cause should be investigated.
Whether to use them, and which one, is for a doctor to decide.
Chapter 5
When to see a doctor
Most diarrhea can be handled at home with fluids, but in these situations seek care now rather than waiting at home:
Bloody or black stoolsA persistent high fever, above 38.9°CVomiting so much you can't keep water downMuch less urine, a dry mouth, dizziness when you stand upDiarrhea lasting more than 3 days
For children, act sooner: if a child is listless or drowsy, passes much less urine, cannot drink or vomits whatever they take, or has blood in the stool, see a doctor promptly. Older adults, pregnant women, and people with weakened immunity or a chronic illness should see a doctor early once symptoms appear.
Any illness after eating suspect soaked Wood Ear Mushroom mushrooms, wet rice noodles or homemade fermented corn flour, or blurred vision, drooping eyelids, slurred speech, trouble swallowing, weak limbs or difficulty breathing, is not ordinary diarrhea: go to the emergency department now (see Foodborne Illness).
Bloody or black stoolsA persistent high fever, above 38.9°CVomiting so much you can't keep water downMuch less urine, a dry mouth, dizziness when you stand upDiarrhea lasting more than 3 days
For children, act sooner: if a child is listless or drowsy, passes much less urine, cannot drink or vomits whatever they take, or has blood in the stool, see a doctor promptly. Older adults, pregnant women, and people with weakened immunity or a chronic illness should see a doctor early once symptoms appear.
Any illness after eating suspect soaked Wood Ear Mushroom mushrooms, wet rice noodles or homemade fermented corn flour, or blurred vision, drooping eyelids, slurred speech, trouble swallowing, weak limbs or difficulty breathing, is not ordinary diarrhea: go to the emergency department now (see Foodborne Illness).
Red flag · When diarrhea drags on
Acute diarrhea usually lasts a few days. If it has gone on for more than two weeks, it no longer belongs in this story: persistent diarrhea needs a doctor to find the cause, not more antidiarrheal drugs or antibiotics on your own. The guideline is explicit that watery diarrhea lasting 14 days or more should not be treated with antibiotics empirically.Diarrhea that keeps coming back, tracks stress and shows no infection may be related to irritable bowel syndrome (see ibs); diarrhea after milk may be a lactose problem (see the page on milk and gas in Gas and bloating).
Blood in the stool, belly pain that wakes you at night, or unexplained weight loss or anemia need a gastroenterologist soon, however long the diarrhea has lasted.
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References · 8
- Thiagarajah, J. R., Donowitz, M., & Verkman, A. S. (2015). Secretory diarrhoea: mechanisms and emerging therapies. Nature Reviews Gastroenterology & Hepatology, 12(8), 446-457. Fluid absorption is driven by active Na+ transport (NHE3, the sodium-glucose cotransporter SGLT1/SLC5A1, Cl-/HCO3- exchangers); secretion by Cl- exit through apical CFTR channels, mostly in crypts. Enterotoxins such as cholera toxin and E. coli heat-stable toxin raise intracellular cyclic nucleotides, activating CFTR Cl- secretion; NHE3 is inhibited. ORS is the mainstay outside hospital and effectively treats dehydration, but does not change fluid losses, diarrhoeal output or duration of illness. Estimated 2015 deaths from diarrhoeal disease: 577,000 children under 5 and 502,000 adults over 70 (author manuscript PMC4786374; PMID 26122478). 10.1038/nrgastro.2015.111
- Shane, A. L., Mody, R. K., Crump, J. A., Tarr, P. I., Steiner, T. S., Kotloff, K., Langley, J. M., Wanke, C., Warren, C. A., Cheng, A. C., Cantey, J., & Pickering, L. K. (2017). 2017 Infectious Diseases Society of America clinical practice guidelines for the diagnosis and management of infectious diarrhea. Clinical Infectious Diseases, 65(12), e45-e80. Rec 3: people with fever or bloody diarrhea should be evaluated for treatable pathogens (strong, low). Rec 18: consider C. difficile testing after antimicrobial use or healthcare-associated diarrhea. Rec 36: in most people with acute watery diarrhea and no recent international travel, empiric antimicrobial therapy is not recommended (strong, low). Rec 39: reduced osmolarity ORS is first-line for mild to moderate dehydration in infants, children and adults with acute diarrhea from any cause (strong, moderate). Recs 44-45: continue breastfeeding; resume an age-appropriate usual diet during or immediately after rehydration (strong, low). Rec 46: antimotility and antiemetic agents only once adequately hydrated, never a substitute for fluids (weak, low). Rec 47: no antimotility drugs such as loperamide under 18 (strong, moderate); loperamide may be given to immunocompetent adults with acute watery diarrhea (weak, moderate) but avoided at any age with suspected inflammatory diarrhea or diarrhea with fever (strong, low); evidence text: 'Deaths have been reported in 0.54% of children given loperamide, and all of these events occurred in children <3 years old.' Rec 49: probiotics may be offered to reduce severity and duration in immunocompetent people (weak, moderate). Rec 50: oral zinc shortens diarrhea in children 6 months to 5 years in settings with zinc deficiency or malnutrition (strong, moderate) (full text PMC5850553; PMID 29053792). 10.1093/cid/cix669
- Centers for Disease Control and Prevention. (2024). Food poisoning symptoms - five signs of severe food poisoning. See a doctor for bloody diarrhea, fever above 38.9 C, persistent vomiting, dehydration, or diarrhea lasting more than 3 days. www.cdc.gov/food-safety/signs-symptoms/index.html
- Wright, E. M., Loo, D. D. F., & Hirayama, B. A. (2011). Biology of human sodium glucose transporters. Physiological Reviews, 91(2), 733-794. 10.1152/physrev.00055.2009
- Hahn, S., Kim, S., & Garner, P. (2002). Reduced osmolarity oral rehydration solution for treating dehydration caused by acute diarrhoea in children. Cochrane Database of Systematic Reviews, (1), CD002847. In children admitted to hospital with diarrhoea, reduced osmolarity ORS versus the older WHO standard ORS: fewer unscheduled intravenous infusions (8 trials, OR 0.59, 95% CI 0.45-0.79), lower stool output and less vomiting, with no detected extra risk of hyponatraemia (abstract, PMID 11869639). 10.1002/14651858.CD002847
- Centers for Disease Control and Prevention. (2024). Symptoms of botulism. Difficulty swallowing, muscle weakness, double vision, drooping eyelids, slurred speech, difficulty breathing. Improperly home-canned, preserved, or fermented foods are the most common source. www.cdc.gov/botulism/signs-symptoms/index.html
- Zhang, H., Guo, Y., Chen, L., Liu, Z., Liang, J., Shi, M., et al. (2023). Epidemiology of foodborne bongkrekic acid poisoning outbreaks in China, 2010 to 2020. PLOS ONE, 18(1), e0279957. 19 outbreaks, 146 illnesses, 43 deaths; CFR 29.5%; 79.0% occurred at home; Auricularia auricula 3/5 deaths (60%). 10.1371/journal.pone.0279957
- World Health Organization. (2006). Oral rehydration salts: production of the new ORS. WHO Press. iris.who.int/handle/10665/69227