July 18, 2024

When Digestion Goes Wrong—Why Diarrhea Happens and How to Treat It

Author: Kate Findley
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By Roberta H. Anding, M.S., Baylor College of Medicine and Texas Children’s Hospital
Edited by Kate Findley, The Great Courses Daily

Diarrhea is one of the most common gastrointestinal (GI) disorders. Professor Anding describes the mechanism of this disorder as it relates to the digestive system, which will give you a new perspective on why it takes place and what you can do to lessen the impact.

Woman holding her stomach in pain
One of the most common GI disorders is diarrhea, which involves the digestive system and the small and large intestines. Photo by Fizkes / Shutterstock

Why Diarrhea Impacts Digestion

In order to understand how diarrhea occurs and how to better cope with it, you first need to gain an understanding of the small and large intestines. The small intestine is the workhorse of digestion, and when unraveled, is about 23 feet long. 

The surface area is increased by villi—small, finger-like projections that stick out of the small intestine lining. Any disorder of the small intestine often results in malabsorption (poor absorption) because you lost some of that absorptive surface area. 

On the tips of the villi are enzymes. Some digest sugar, while lactase digests milk sugar (lactose) into its most basic form so it can be absorbed. 

If you’ve had diarrhea, you shave off the tips of your villi. Temporarily, you may not be able to digest milk at all. The villi will regenerate over time—typically three to five days—but in the meantime, you may have to stay away from milk sugar.

Within the deepest recesses of the villi are Peyer’s patches, which are immune-secreting lymphoid tissue. Most people do not realize this, but a good percentage of your immune system lies in your small intestine.

Because your normal food supply contains a large amount of bacteria, whatever escapes the hydrochloric acid in your stomach can now be taken care of by immune-secreting lymphoid tissue within the small intestine. Due to this connection to the immune system, diseases of the small intestine actually increase the likelihood that you end up with some other form of infection.

Causes: Food Poisoning, Antibiotics, Etc.

The large intestine, which is connected to the small intestine by a series of one-way valves, finishes the work of digestion in about 12 hours. Its primary role is to control fluid balance; so if you’re dehydrated, your large intestine pulls water out of waste products to increase the amount of water that you have in your blood. 

Often, people end up with diarrhea within the large intestine. Diarrhea can be caused by GI irritants such as caffeine, alcohol, and fatty foods. 

The loss of fluid can be excessive, and if the diarrhea is caused by food poisoning, it can be more than you can make up voluntarily. That’s why people with significant diarrhea often have to go to the emergency room and get “rescue fluids,” or intravenous fluid therapy.

Diarrhea can have causes other than food poisoning. Any concentrated solution that is dumped into the small intestine faster than it can be processed can cause what is known as “dumping syndrome.” 

Literally, food is dumped from the stomach to the small intestine in a super-concentrated form. When this happens, the body’s job is to pull in water to dilute it. All along the GI tract, and particularly in the large intestine, fluid is added in an attempt to keep that concentrated solution in a manageable form for the gut. 

Individuals who’ve had gastric bypass—a weight-loss surgery where your stomach is rearranged so you feel full faster—often face challenges with GI issues. If they eat more food than they should, the concentrated solution in the stomach is dumped into the small intestine. Water is pulled in, and they can have significant diarrhea. 

Inflammatory bowel disease can cause diarrhea, as well. Additionally, antibiotic use can cause diarrhea for two reasons. For one, it can irritate the villi of the small intestine, which leads to malabsorption.

Secondly, antibiotics kill the healthy bacteria in the large intestine that help to process food and digest fiber. If you are on antibiotics, you should eat kimchi or yogurt, which are great sources of probiotics—live bacteria and yeast containing beneficial bacteria.


If you have significant diarrhea, your first line of defense is fluid. Adequate amounts of fluid should have some electrolytes, which could come from an oral electrolyte replacement fluid or a sports drink.

When the diarrhea is from malabsorption, you can eat light carbohydrates to replace lost nutrients. The carbohydrates should come from easily digested sources such as white rice, potatoes, or plain pasta. You may also want to eliminate substances and foods that are not absorbed well—for example, certain fibrous foods such as wheat if they’re irritating you.

The challenge lies in the fact that there’s an individual basis for the tolerance of fat, fiber, and lactose. If, as part of your GI disorder, you’re malabsorbing fat and losing fat in your stool, a specialized product called medium-chain triglycerides can help. 

Medium-chain triglycerides are absorbed and utilized in your body a bit differently than regular fats. A common example is coconut oil. They can be a valuable adjunct in keeping your calories high enough if you’ve got a chronic gut disorder that’s causing diarrhea.

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University. In addition, she is a registered dietitian with the American Dietetic Association and a dietitian for the Houston Texans NFL franchise. 

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