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What Causes Loose Stools in Adults?

author image Dr. Tina M. St. John
Tina M. St. John runs a health communications and consulting firm. She is also an author 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?
There are many causes of loose stools, from dietary factors such as lactose intolerance, to bacterial infections to certain diseases. Photo Credit Chris Warren/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 content of ingested foods and intestinal infections often cause transient episodes of diarrhea. Digestive disorders often underlie chronically loose stools.

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 components. 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 cirrhosis, which is a liver disease, or gallstones—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.

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