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What Causes Loose Stools in Adults?

author image Dr. Tina M. St. John
Dr. Tina M. St. John owns and operates a health communications and consulting firm. She is also an accomplished medical writer and editor, and was formerly a senior medical officer with the U.S. Centers for Disease Control and Prevention. St. John holds an M.D. from Emory University School of Medicine.
What Causes Loose Stools in Adults?
What Causes Loose Stools in Adults? Photo Credit: AndreyPopov/iStock/Getty Images

The colon normally absorbs the majority of water in fecal material, leaving soft, solid stool. Dietary intake, infections and digestive disorders can lead to increased water in expelled feces, or loose stools. The nutrient content of ingested foods and intestinal infections often cause transient episodes of diarrhea. Digestive disorders often underlie chronically loose stools.

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Dietary Sugars

Dietary sugar intake can increase the water content of stool, causing diarrhea. The small intestine has a limited capacity to digest certain types of sugars called sugar alcohols, which include xylitol, sorbitol, mannitol, lactitol, isomalt and maltitol, and ingesting large quantities often leads to transient diarrhea. Xylitol occurs naturally in lettuce, corn, cauliflower, raspberries and strawberries. It is used as a food additive in gums, breath mints and food bars. Sorbitol occurs naturally in peaches, pears, apples and prunes. It is used as a low-calorie sweetener in candies, gums and many "diet" foods. Mannitol is another common sugar alcohol that occurs naturally in carrots, sweet potatoes, asparagus, olives and pineapples. Processed foods commonly contain mannitol as a low-calorie sweetener.

Lactose, a naturally occurring sugar in milk, can also cause loose stools. Many adults have a low level of intestinal lactase, the enzyme that breaks down lactose into absorbable sugars. Ingestion of large quantities of milk or dairy products can lead to diarrhea among people with a lactase deficiency, also known as lactose intolerance.

Bowel Infections and Food Poisoning

Bowel infections caused by viruses, bacteria and certain parasites frequently cause loose stools. Viral gastroenteritis, an inflammation of the stomach and intestines, is a common cause of episodic diarrhea. Many viruses can cause gastroenteritis, including noroviruses, astroviruses, rotaviruses, sapoviruses and certain adenoviruses. Bacterial food poisoning caused by Salmonella species, Shigella species, Staphylococcus aureus, Clostridium perfringens, Bacillus cereus and certain strains of Escherichia coli also commonly causes episodic diarrhea. Parasites that frequently cause loose stools include Cryptosporidium, Giardia, Entamoeba histolytica and Cyclospora species.

Malabsorption Disorders

Disease processes that interfere with the breakdown of consumed foods or the absorption of nutrients by the intestine are collectively termed malabsorption disorders. These conditions typically cause loose stools. Decreased production or release of pancreatic digestive juices causes malabsorption owing to impaired breakdown of dietary fats, starches and proteins. Common causes of pancreatic malabsorption include pancreatitis, an inflammation of the pancreas, and cystic fibrosis, a genetic disorder.

Impaired production or release of bile -- which occurs with liver cirrhosis or gallstones, among other liver and gallbladder disorders -- frequently causes diarrhea related to the presence of undigested fats in the intestine. Intestinal disorders that interfere with absorption of digested nutrients include celiac disease, Crohn disease and Whipple disease. Abdominal radiation treatments for cancer can also temporarily interfere with the ability to absorb nutrients, leading to diarrhea.

Other Causes

Several other conditions and disorders can cause loose stools or diarrhea. Examples include: -- binge drinking or alcoholism -- irritable bowel syndrome -- laxative overuse -- certain weight loss medications, such as orlistat (Alli, Xenical) -- gastric bypass surgery or removal of part of the bowel -- diverticulitis -- colon or pancreatic cancer -- congestive heart failure -- overactive thyroid or underactive adrenal glands -- diabetic nerve damage

Warnings and Precautions

See your doctor if you experience unexplained loose stools, especially if the diarrhea is severe or worsening, or persists for longer than 4 or 5 days without improvement. This is particularly important if you are pregnant or have an existing health condition, such as diabetes, HIV/AIDS, cancer or inflammatory bowel disease. Seek urgent medical care if you experience warning signs or symptoms along with loose stools, including: -- severe or worsening abdominal pain -- bloody, maroon-colored or tarry stools -- fever higher than 101 F -- lightheadedness, dizziness or fainting

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