: How does anxiety and stress affect the bowels? It appears to be accepted that anxiety and stress can cause constipation and Irritable Bowel Syndrome, but how does your mental state actually affect the functioning of the bowel?

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One’s mental state has a major effect on bowel function. In fact, there is a high correlation between IBS and stress.

Given the high levels of anxiety and stress commonly seen in patients with Irritable Bowel Syndrome, evidence suggests that the syndrome may be linked to a disruption of the stress system. The stress response in the body involves the sympathetic nervous system which has been shown to operate abnormally in IBS patients.

In an article by Johns Hopkins School of Medicine, scientists are inferring that ‘the little brain’, or enteric nervous system (ENS) is the link between what's going on in the gut and it's effect on the brain, or the relationship between digestion and mood.

Anxiety and depression have been thought to contribute to gastro conditions like irritable bowel syndrome (IBS).

Jay Pastricha, MD, Director of the Johns Hopkins Center for Neurogastroenterology states that the main role of the ENS is:

"...controlling digestion, from swallowing to the release of enzymes that break down food, to the control of blood flow that helps with nutrient absorption to elimination."

As described in Human Physiology: An Integrated Approach, when your sympathetic nervous system is activated by a stressful event, for example, it initiates a “fight or flight” response -- a mechanism that primes the body for action, particularly in situations that threaten survival. Activation of the sympathetic nervous system causes vasoconstriction of most blood vessels, including those in the digestive tract. Because the blood vessels in the gastrointestinal organs constrict, the digestive tract will inhibit peristalsis (digestion) and result in a variety of GI symptoms.

The GI tract is generally sensitive to emotion in most people (even those who do not have IBS) as anger, anxiety, sadness, and elation (among others) can all trigger symptoms in the gut. For example, you've likely heard or even spoken the words "going with your gut" when making a decision or feeling like you have "butterflies in your stomach" when feeling nervous. In people who suffer from IBS however, the ENS may trigger big emotional shifts which can lead to functional bowel problems such as constipation, diarrhea, bloating, pain, and stomach upset.

People with IBS can experience either constipation, diarrhea, or both depending on the individual and how their emotionally triggered symptoms present. According to Mayo Clinic:

For the purpose of treatment, IBS can be divided into three types based on your symptoms: constipation-predominant (IBS-C), diarrhea-predominant (IBS-D), or mixed.

Research also suggests that digestive system activity may affect cognition (thinking skills and memory). As explained by a Harvard Health article:

The brain has a direct effect on the stomach. A troubled intestine can send signals to the brain, just as a troubled brain can send signals to the gut. Therefore, a person’s stomach or intestinal distress can be the cause or the product of anxiety, stress, or depression.

In an effort to provide further evidence in response to "How does your mental state actually affect the functioning of the bowel?":

Psychological factors influence the actual physiology of the gut, as well as presenting symptoms. As it relates to IBS, stress can affect movement and contractions of the GI tract or can increase inflammation. Additionally, people with IBS may perceive pain more acutely than other people do because their brains do not properly regulate pain signals from the GI tract. When faced with a stress-causing event, the existing pain can feel worse. As stated in the article, 'The Brain-Gut Connection':

Irritation in the gastrointestinal system may send signals to the CNS that trigger mood changes, which is why a higher than normal percentage of people with IBS and other bowel disorders develop depression and anxiety.