Diarrhea is triggered by anything that disrupts the normal balance of fluid absorption in your gut. The list of culprits is long, ranging from a stomach bug to your morning coffee to the sugar-free gum in your pocket. Understanding the most common causes helps you figure out what’s behind your symptoms and whether you need to change something or just wait it out.
How Your Gut Produces Loose Stools
Your small intestine normally absorbs most of the fluid that passes through it. Diarrhea happens when that process breaks down, and it does so in a few distinct ways. Osmotic diarrhea occurs when something in your gut pulls extra water into the intestine. This is what happens with sugar alcohols, lactose intolerance, and certain laxatives. The key giveaway: this type stops when you stop eating the offending food.
Secretory diarrhea is the opposite scenario. Your intestinal lining actively pumps fluid outward instead of absorbing it, usually because a bacterial toxin or hormone has hijacked the signaling process. This type keeps going even if you stop eating entirely. Infections from bacteria like E. coli and the cholera bacterium work this way.
Inflammatory diarrhea involves actual damage to the intestinal wall, producing blood, mucus, or pus that adds to stool volume while also impairing absorption. Conditions like Crohn’s disease, ulcerative colitis, and infections from Salmonella or Campylobacter fall into this category. Finally, problems with gut motility, where food moves through too quickly or too slowly, can cause diarrhea by preventing proper absorption or encouraging bacterial overgrowth.
Infections: The Most Common Acute Cause
Norovirus is the leading cause of sudden-onset diarrhea in adults. It spreads easily through contaminated food, surfaces, or close contact, and symptoms typically hit hard within 12 to 48 hours. Rotavirus is another major player, particularly in young children. Both tend to resolve on their own within a few days.
Bacterial infections from Salmonella, E. coli, Campylobacter, and Shigella usually come from undercooked meat, contaminated produce, or unsafe water. These tend to cause more intense symptoms, sometimes including bloody stool and fever. Parasites like Giardia and Cryptosporidium are less common but can cause diarrhea that lingers for weeks, particularly after drinking untreated water while camping or traveling abroad.
Foods and Drinks That Trigger It
Sugar alcohols are one of the sneakiest dietary triggers. Found in sugar-free gum, mints, protein bars, and diet foods, these sweeteners (sorbitol, xylitol, mannitol, lactitol) are poorly absorbed in the small intestine. They pull water into the gut through osmosis, causing bloating and loose stools. Sorbitol can trigger osmotic diarrhea at doses as low as 20 grams, which is easy to hit if you’re chewing through a pack of sugar-free gum or eating multiple “keto” snacks in a day.
Caffeine stimulates contractions in the colon, speeding up transit time. For some people, a single strong cup of coffee is enough to send them to the bathroom. Alcohol irritates the gut lining and disrupts fluid absorption, which is why a night of heavy drinking often ends with loose stools the next morning. Spicy foods containing capsaicin can also accelerate gut motility, particularly if your body isn’t accustomed to them.
Greasy or high-fat meals are another frequent offender. Fat takes longer to digest, and large amounts can overwhelm your bile acid production, leading to undigested fat reaching the colon and triggering watery stool.
Food Intolerances You Might Not Realize You Have
Lactose intolerance is extremely common, affecting the majority of adults worldwide. When you lack enough of the enzyme that breaks down milk sugar, undigested lactose draws water into the intestine and gets fermented by gut bacteria, producing gas, bloating, cramps, and diarrhea. In studies of patients with chronic digestive symptoms, 78% showed evidence of lactose malabsorption.
Fructose malabsorption is less well known but surprisingly widespread. When healthy adults consumed 50 grams of pure fructose (roughly the amount in a large soda plus an apple), 80% showed signs of malabsorption and about half experienced bloating or diarrhea. Even at 25 grams, around 40% of people tested positive. Fructose that isn’t absorbed in the small intestine travels to the colon, where bacteria ferment it and produce gas, while the unabsorbed sugar pulls water into the gut.
High-fructose corn syrup causes fewer problems than pure fructose at equivalent doses, likely because the glucose it contains helps with fructose absorption. Still, about 1 in 3 people with irritable bowel syndrome (IBS) experience flare-ups when consuming large amounts of fructose from any source. If you notice that fruit juice, honey, agave syrup, or foods sweetened with high-fructose corn syrup consistently upset your stomach, fructose malabsorption is worth investigating.
Medications, Especially Antibiotics
About 1 in 5 people who take antibiotics develop diarrhea as a side effect. Antibiotics don’t just kill the bacteria causing your infection. They also wipe out beneficial gut bacteria, disrupting the microbial balance that keeps digestion running smoothly. Broad-spectrum antibiotics are the most common offenders. Symptoms can start during the course of treatment or even weeks after you finish.
In rare cases, antibiotic-associated diarrhea is caused by an overgrowth of C. difficile, a bacterium that thrives when competitors have been killed off. This form is more severe, producing watery diarrhea many times per day along with fever and abdominal pain.
Other medications that commonly cause loose stools include magnesium-containing antacids, certain blood pressure drugs, chemotherapy agents, and nonsteroidal anti-inflammatory drugs like ibuprofen when used regularly.
Stress and Exercise
Your gut and brain communicate constantly through the vagus nerve and shared chemical signals. Anxiety, acute stress, and panic can directly speed up gut motility, causing cramping and urgent diarrhea. This is why you might experience “nervous stomach” before a presentation, exam, or flight.
Runners are particularly prone to exercise-induced diarrhea, sometimes called “runner’s trots.” The causes are a combination of reduced blood flow to the gut during intense activity, mechanical jostling of the intestines, and nutritional factors like consuming energy gels or sugary sports drinks during a run. Long-distance runners report this most frequently, and it tends to improve when you avoid high-fiber and high-fat foods in the hours before exercise.
Chronic Conditions Behind Ongoing Symptoms
If loose stools last more than a few weeks, a chronic condition may be responsible. IBS with diarrhea (IBS-D) is one of the most common. It’s considered a functional disorder, meaning the gut looks structurally normal but doesn’t behave normally. Visceral hypersensitivity, where your gut nerves overreact to normal stimulation, plays a central role. Symptoms include abdominal pain tied to bowel movements, bloating, and alternating periods of diarrhea.
Inflammatory bowel disease (IBD), which includes Crohn’s disease and ulcerative colitis, involves actual inflammation and damage to the intestinal lining. Unlike IBS, IBD produces measurable inflammation that can be detected through stool tests and colonoscopy. A stool marker called calprotectin helps distinguish the two: levels below 40 micrograms per gram generally rule out significant inflammation, while levels above 100 suggest IBD. The overlap between IBS and IBD symptoms can make diagnosis tricky, since symptoms like cramping and urgent diarrhea appear in both.
Celiac disease, hyperthyroidism, and chronic infections like Giardia are other conditions that cause persistent diarrhea and are worth considering if symptoms don’t resolve.
Managing a Bout of Diarrhea
Most episodes resolve within two to three days without treatment. The biggest risk is dehydration. Signs to watch for include dark urine, dry mouth, dizziness, and a rapid heart rate. Water alone isn’t ideal for rehydration because you’re losing electrolytes too. Oral rehydration solutions, broth, or drinks with sodium and potassium help replace what’s being lost.
Over-the-counter options work differently. Loperamide (the active ingredient in Imodari) slows gut motility, giving your intestines more time to absorb water. In comparative studies, it provided faster and more complete relief than bismuth subsalicylate (found in Pepto-Bismol), which works by reducing inflammation and binding toxins in the gut. Loperamide is the better choice when you need quick control of symptoms, but it should be avoided if you have bloody diarrhea or a high fever, since slowing the gut during a serious bacterial infection can make things worse.
The old advice to eat only bananas, rice, applesauce, and toast (the BRAT diet) is no longer recommended by major health organizations. While those foods are gentle, the diet provides about 300 fewer calories per day than a normal toddler’s diet and is deficient in protein, fat, fiber, and several key nutrients. Clinical trials show that eating a normal, age-appropriate diet as soon as you’re rehydrated leads to shorter illness duration, lower stool output, and better nutritional recovery compared to restricting your diet. You don’t need to force yourself to eat heavy meals, but there’s no benefit to limiting yourself to bland starches.