The ABCs of Diarrhea Causes

By Noelle Patno, PhD

When you get “the runs,” you need to know what is actually causing the excess stool and how to control it. Diarrhea (“flow through” from the Greek), by definition, is frequent loose stools of small to moderate volume typically during waking hours, in the morning or after meals.1 Often there is a feeling of extreme urgency along with the sense of incomplete evacuation. If diarrhea is accompanied by blood or grease or is of large volume, this is cause for extra concern, and you should consult your doctor as soon as possible. Diarrhea may be due to multiple causes such as dysbiosis or alteration of the intestinal microbiota which lead to changes in the usual bacterial composition.

The ABCs and more of diarrheal causes

  1. Antibiotics. Disrupting the ecosystem of the intestinal microbiota by antibiotics often leads to frequent, loose stools (antibiotic associated diarrhea or AAD). This disruption often allows undesirable pathogenic bacteria to bloom; sometimes the pathogen Clostridium difficile may increase to levels that lead to Clostridium difficile-associated diarrhea (CDAD).1
  2. Bacterial infections/travellers’ diarrhea. These may occur with travel and consumption of contaminated water. Persistent intestinal infections by pathogenic bacteria or viruses (for example, Clostridium difficile, Aeromonas, Plesiomonas, Campylobacter, Giardia, Amebae, Cryptosporidium, and Cyclospora)1 may be associated with chronic diarrhea. Enterotoxigenic E. coli (ETEC) is a typical pathogen and may be treated by antibiotics.2 However, the emergence of antibiotic resistance has made the use of antibiotics more difficult, and their use in certain conditions is being re-evaluated.3
  3. Colitis. One of the types of inflammatory bowel disease, ulcerative colitis, is observed endoscopically as red, swollen, and inflamed tissue in the intestine, and even in mild cases, patients may experience about four loose stools a day.1 Moderate and more severe cases may include blood in the stool. Diarrhea with colitis should be managed by a healthcare practitioner.
  4. Drugs and supplements. Check medication labels that may list diarrhea as a potential side effect and talk to your doctor about whether any of your medications may cause diarrhea. Excessive intake of laxatives designed to relieve constipation or supplements, such as magnesium glycinate or magnesium citrate, may cause diarrhea.
  5. Dietary influences on electrolytes. Magnesium promotes smooth muscle relaxation, while calcium promotes its contraction.4 An imbalance of the foods containing these minerals may affect proper intestinal functioning.
  6. Dietary lubricants. Okra, flaxseed, oat, kelp, cactus, natto, toasted sesame oil, olive oil, acacia, and chia seeds encourage the “flow through” or “internal transit,”4 which may otherwise be beneficial for constipated individuals. If, for any reason, these compounds are excessively high in the diet, consult your supervising practitioner on how to balance salts and other foods to mitigate diarrhea.
  7. Malabsorption (associated with Crohn’s disease [CD], celiac, and others). Nutrients may not be absorbed well due to intrinsic or acquired problems in the transportation systems in the small intestine. Those who have CD or celiac or have had surgery on their intestines may experience problems with absorption of nutrients. Malabsorption may manifest as fatty diarrhea, which is greasy and foul smelling with unabsorbed fat, as found in chronic pancreatitis. Increased dietary intake of sorbitol (sugar alcohol used as a sugar substitute found in gums or sugar free foods and candies) and lactose may lead to diarrhea in lactose-intolerant patients. Bacterial overgrowth in the small intestine (SIBO) may also be associated with malabsorption. If you have, or think you may have, any of these conditions, consult your doctor. 1
  8. Other factors. Your doctor may examine other factors such as your your food intake, allergies and lifestyle history, family history, and any concurrent markers or symptoms (stress and depression, fevers/joint pains, inflammatory markers of inflammatory bowel disease [IBD]) that could be related to, or suggest, other causes contributing to diarrhea. Stool testing for fecal calprotectin, a zinc and calcium binding protein from immune cells, may help indicate whether the diarrhea is due to inflammatory causes.1

Chronic diarrhea is a decrease in fecal consistency lasting for four or more weeks, which requires further investigation by medical personnel. If you have any concerns related to diarrhea, you should discuss them with your healthcare practitioner.



  1. Bonis P. Approach to the adult with chronic diarrhea in resource-rich settings. Rutgeerts P and Grover S. Topic last updated June 5, 2017. Accessed July 23, 2018.
  2. Friedman G. Probiotics in the Prevention and Treatment of Gastrointestinal Disease. In: Integrative Gastroenterology. Mullin G. New York, NY: Oxford University Press, Inc; 2011.
  3. Taylor DN et al. Medications for the prevention and treatment of travellers’ diarrhea. J Travel Med. 2017;24(suppl_1):S17-S22.
  4. Koff A. Nutrition Strategies for the Treatment of IBS and the Prevention of Digestive Complaints. In: Integrative Gastroenterology. Mullin G. New York, NY: Oxford University Press, Inc; 2011.


About Noelle Patno, PhD:

Noelle Patno, PhD, is the Nutritional Scientist for Digestive Health at Metagenics. After pursuing chemical engineering at Stanford and engineering at Abbott, she sought an education in preventive nutrition from a basic science perspective by pursuing her PhD in Molecular Metabolism and Nutrition from the University of Chicago. Her current role involves researching and developing probiotics, prebiotics and other nutritional solutions and programs for promoting digestive health and overall health.



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