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If you have ever experienced “butterflies in your stomach” or felt like your guts were “tied up in knots” while under stress, you have experienced the gut-brain connection.  According to the Cleveland Clinic, stress can cause diarrhea, it can also cause other symptoms of GI distress like bloating, belly pain, nausea, and constipation. 
If you have GI symptoms under stress or when experiencing anxiety, they can make stress worse. Imagine a sudden attack of diarrhea under stress, and then add the stress and anxiety of not being able to get to a bathroom. Learning how to deal with stress can help you avoid this vicious cycle. [1,2]
The Gut-Brain Connection
Harvard Medical School explains that when GI symptoms are caused by stress or anxiety and not by an infection or disease, it is called a functional GI disorder. Just because it is not caused by infection or disease does not mean it’s not real. 
The part of your brain and nervous system that controls functions like breathing and digestion is called your autonomic nervous system. It is divided into sympathetic and parasympathetic systems. Stress triggers your fight or flight reaction and causes your sympathetic nervous system to slow down digestion. To avoid the damage of long-term stress, your parasympathetic nervous system kicks in to return things to normal. 
Both your sympathetic and parasympathetic nervous systems are connected to your enteric (or gut) nervous system, sometimes called your “second brain.” Your gut’s nervous system uses the same brain chemicals and chemical messengers – called hormones – as your brain. 
In fact, your gut has the largest area of nerves outside your brain. When stress causes your brain to produce stress chemicals and hormones they go into your gut and cause GI symptoms, including diarrhea.  When your GI system is under stress, GI stress chemicals and hormones go to your brain triggering more stress and inhibiting your autonomic nervous system from turning off the stress switch. 
The Gut Bacteria Connection
There are millions upon millions of bacteria living in your gut. A healthy GI system relies on good bacteria outnumbering or balancing bad bacteria. There is a connection between gut bacteria and mental health, so stress or anxiety can upset the balance: [1,2,5]
- Stress may reduce good gut bacteria and favor bad bacteria.
- Bad bacteria cause GI symptoms.
- Bad bacteria reduce the production of the mood-lifting chemical serotonin. That means less serotonin going to your brain and a depressed or anxious mood.
Possible GI Symptoms Caused by Stress
According to the Anxiety and Depression Association of America, stress-related GI symptoms can include: 
- Upset stomach
- Diarrhea or constipation
- Loss of appetite or hunger cravings
So yes, stress can cause diarrhea, an upset stomach, and other uncomfortable GI issues. One thing that stress does not cause is a stress ulcer. According to the American College of Gastroenterology, emotional stress causing an ulcer is a myth. Ulcers are caused by bacteria, called H. pylori. Although stress does not cause stomach ulcers, it may irritate an existing ulcer and make ulcer symptoms worse. 
How to Avoid Stress-Related GI Symptoms
Stress is a normal part of life. Nobody can avoid stress all the time, but you can learn better ways to cope with it: 
- Don’t try to ignore stress with diversions like eating, drinking alcohol, gambling, or shopping. These diversions end up making stress worse over the long term. 
- Identify your stress triggers and avoid them when you can. 
- Learn techniques that help you deal with stress like deep breathing, guided relaxation, or meditation.
- Exercise regularly. Exercise increases gut and brain chemicals called endorphins that improve your mood. 
- If you are really struggling with stress and stress-related GI symptoms, talk to your health care provider. Talk therapy – called psychotherapy – can often help. 
Harvard Medical School says three types of psychotherapy may be helpful for stress, anxiety and the stomach pain that follows if you’re affected by the gut-brain connection.
- Cognitive-behavioral therapy helps you replace negative thoughts and behaviors with positive ones.
- Relaxation therapy teaches you ways to reduce stress like guided relaxation exercises.
- Gut-directed hypnosis combines deep relaxation with calming thought suggestions.
- Anxiety and Depression Association of America, How to Calm an Anxious Stomach: The Brain-Gut Connection | Anxiety and Depression Association of America, ADAA
- Cleveland Clinic, Is Your Stomach Churning? You May Have ‘Gut Stress’ – Health Essentials from Cleveland Clinic
- Harvard Health, Stress and the sensitive gut – Harvard Health
- American Psychological Association, Stress effects on the body (apa.org)
- American College of gastroenterology, https://gi.org/topics/peptic-ulcer-disease/