The brain-gut connection

A better understanding of the brain-gut connection may offer hope to the millions of people who suffer from recurring gastrointestinal symptoms. Most people can relate to gut feelings during accident. Recall a time you lost sight of your daughter in a crowded playground for a few moments; remember those few agonizing minutes before a major presentation at work; think of a time when you felt the sick punch of disappointment in the pit of your stomach. If all of these emotions occur in our brains – fear, anxiety, disappointment, and stress – why, then, do we feel them in our guts? This very question has led scientists to analyze how the mid-gut connection may cause or exacerbate a variety of gastrointestinal (GI) symptoms and disorders, from any number of known functional disorders such as irritable bowel syndrome to physical disorders such as Crohn’s disease and ulcerative colitis. Until now, when a patient suffered GI symptoms with no identifiable physical cause (described as functional disorders), doctors often blamed the brain, prescribed antidepressants or threw their hands up and offered no solution. Many of these patients had to learn to live with symptoms such as chronic and severe diarrhea, constipation and stomach pain.

Years ago, for instance, ulcers were thought to be caused by stress, partly because patients admitted to stress in their lives and partly because the gut was thought to be under the control of the brain and spinal cord. Later, ulcers were found to be caused by bacteria called H. pylori, not by signals of stress transmitted from the brain, but stress is still thought to play a role in several other GI disorders.

The second brain. The enteric nervous system (ENS), which lines the esophagus, stomach and small and large intestines, can be likened to a second brain, according to AGA member. One doctor calls the gut a second brain because of its functional ability and because it contains as many nerve cells as the spinal cord (over 100 million). Furthermore, the gut can function independently of the brain. The gut is the only part of the body where reflex behavior does not require participation of the brain and spinal cord. This means that, were all the nerves cut that connect the gut to the brain and spinal cord, your gut would still digest, your bowels would still move and your intestines would still function.

Brain-gut communication. Though the gut can function without the help of the brain, the two brains are connected by the vagus nerve, which carries messages form the gut to the brain and vice versa. Contrary to the common notion that many GI disorders originate in the brain, 90 percent of the nerve fibers in the vagus nerve carry signals that are ascending from gut of brain, not the other way around. This 90 percent of information traveling out of the gut and into the brain is sensory, communicating sensations that we recognize, such as hunger, fullness, nausea and discomfort. The 10 percent of the information that travels from brain to gut is motor information – messages dictated by chemicals that tell the stomach how much acid to produce, when to churn and when to rest. Scientists understand most of the information that travels form the brain to the gut, but the nature of most of the information that travels from the gut to the brain is still unknown. What we do know is that, when signals from the gut reach consciousness, they are not the kind of information anyone likes to receive. The gut is not an organ from which you wish to receive frequent progress reports.

Stressors that muddle the brain-gut connection. In any given month, an astounding 40 percent of people receive frequent progress reports from the gut by way of moderate to severe bloating, abdominal pain and diarrhea, according to a 2000 study published. Though the messages the gut sends to the brain remain a mystery, AGA member, authors of Irritable Bowel Syndrome and the Mind Body-Spirit Connection, suggest that the way our bodies respond to stressors or triggers may put a kink in our gut-brain communication that can aggravate or initiate GI symptoms or disorders.

The "good stress" response. In their book, they explain that when we experience stress, our internal balance (homeostasis) can be thrown off, and our bodies respond by trying to defend our stability in order to survive. This "good stress" response is called allostasis. Our good stress response suppresses functions that are not necessary for immediate survival, like digestion. Usually, allostasis lasts only for the duration of a stressful event, but sometimes it may be activated without any conscious awareness of a stress trigger. People with functional GI disorders, for example, know that sometimes stress triggers symptoms, sometimes food does and sometimes nothing really seems to have triggered the symptoms, but the symptoms are still present. This may be because our gut has a memory. Our gut records events and survival responses and then responds – either appropriately or inappropriately – to certain familiar conditions.

Remember eating too much of something and throwing up, or eating something gone sour and spending the night in the bathroom with diarrhea? Chances are your gut hasn’t forgotten either, and rarely are you able to look at that food without feeling a bit nauseated. Your brain may rationalize that this time the food is fresh and you won’t overindulge, but your gut isn’t falling for it, and symptoms persist.

The "bad stress" response. When GI symptoms won’t go away, our good stress response may be overloaded. In their book, they suggest that when you experience a stressor (disease, emotion, pain) with severity or chronicity, a "good stress" response may turn into a "bad stress" response that can either aggravate existing disease conditions or predispose you to the development of new ones. For example, a person usually develops an ulcer because of H. pylori, but the ulcer may be aggravated in times of stress. The stress hormones normally released by the central nervous system to defend the body can create harmful physiological changes when produced repeatedly or in excess. The result is allostatic overload, which is described as the wear and tear resulting from chronic overactivity or underactivity of the stress response systems. Experienced allostatic overload in college, when recurrent and severe stomach pains sent her repeatedly to the doctors’ office. Multiple tests showed nothing physically wrong, but for two years working over 30 hours per week and taking 21 credits per semester, and the stress may have caught up with her. To this day, her gut seems to have recorded that response to stress. Whenever she is anxious or worried, one can count on another painful bout of symptoms. Chronic stress during college years resulted in a painful physiological change that one’s been unable to reverse.

Causes of functional gut symptoms. We have identified four main causes of functional GI disorders: gut reactions to stressors/triggers, altered gut motility and spasm, increased gut sensitivity and psychological distress.

• Gut reactions to stressors/triggers. When you have a gut infection or when you eat high-fat foods, your gut may respond with symptoms like abdominal cramping and diarrhea. People with functional disorders such as irritable bowel syndrome (IBS), where symptoms are present with no diagnosed physical cause, are more sensitive to triggers like food and infections.

• Altered gut motility and spasm. Disturbances in the brain-gut connection also can cause gut motility problems and peristalsis, which may result in painful spasms, constipation or diarrhea.

• Increased gut sensitivity. People with IBS have enhanced sensation and perception of what is happening in their digestive tract. IBS sufferers and those with other functional GI disorders have a lower threshold for internal pain because even normal sensation of activity in the gut (like the movement and distention of the gut by food and gas) may be uncomfortable or painful.

• Psychological distress. When a person experiences allostatic overload, they also experience symptom vigilance, which means they are more likely to pay attention to symptoms and be concerned by them. Research shows that allostatis overload may generate anxiety and depression in addition or in conjunction with functional symptoms and syndromes.

Research offers potential hope. Until the communication from gut to brain is more fully understood, we will continue to battle with our bad stress responses. However, he has increasing hope for patients. Recently, for example, he and AGA members, discovered that patients with IBD and IBS have reduced SERT expression in the lining of their gut.

SERT is a serotonin transporters, and without SERT, the actions of serotonin in the bowel cannot be terminated properly. As a result, the effect of serotonin becomes too strong and causes diarrhea until patients can’t stand the diarrhea any more and simply lose their ability to respond to serotonin; then they become constipated. Further studies are needed, but this finding indicates that an abnormal bowel, not the brain, may be the culprit in IBS symptoms. As we gain a better understanding of the gut-brain connection through research, patients can look forward to better medical relief from their GI symptoms.