It's a commonly known fact that stress, and especially fear, can potentially cause sudden and unwanted bowel movements. Stress is often linked with diarrhea and is a strong risk factor in IBS (irritable bowel syndrome).
But what's less known is that stress can also prevent bowel movements. In some cases stress can actually lead to constipation. But how precisely can stress cause constipation and diarrhea?
The reason that stress can cause diarrhea is that it produces more contraction in the muscles including the colon. This can then be enough to cause a bowel movement and thus the evacuation of the bowels.
But when asking how can stress cause constipation, we need to look at a different mechanism.
Here, ongoing chronic stress can suppress the gastrocolic reflex. This is an unconscious action controlled by the gastrointestinal tract that would normally trigger bowel movements. Because stress causes blood and oxygen to be redirected to the muscles and the brain, this means that secondary functions like this one are suppressed.
In other cases, being in stressful situations can cause us to consciously suppress our bowel movements. If you're stressed at work for instance then you might 'hold it' so that you can focus on work. If you do this for too long or too regularly, this can result in the stool hardening up. This can also be an issue with the aforementioned suppression of the gastrocolic reflex and create something of a vicious cycle.
This still doesn't answer the question of how can stress cause constipation in some people and diarrhea in others. Why do some of us experience one effect and others have the exact opposite experience?
What's important to remember here though is that everyone is different and that every case of stress is also different. This means that we will all react in different ways when stressed and even react in different ways from one instance to the next.
So while some of us will find ourselves consciously and unconsciously preventing bowel movements, others will unintentionally flex their colon in order to encourage more bowel movements.
Ultimately, neither of these outcomes are really desirable so the main take-home message is to avoid stress and to be aware of the serious and profound effects it can have on the body.