Explanation of the basic structure and function of the gut and its connection with the brain helps clarify the multiple potential causes of IBS. Risks of developing IBS involve both genetics (nature) and environment (nurture). Living a healthy lifestyle is important.
Anatomy and Physiology
The gut contains a second "brain" within the lining. Both brains originate from the same tissues (neural tissue) during early-life growth. The gut brain is part of the autonomic nervous system (ANS) responsible for certain automatic functions, such as gut contractions and operations, breathing and heart rate.
There is a brain-gut connection. Both brains contain similar nerve cells and chemical messengers. They connect through the spinal cord, two vagus nerves and by way of chemical messengers that travel throughout the body (e.g., serotonin and norepinephrine). Both brains are in constant intercommunication and are both independent and interdependent. The brain affects the gut, and the gut affects the brain.
The gut is a sixth sense. We “feel” the world we live in — our environment — through such senses as touch. The gut has a surface area larger than that of a football field, which allows it to relate to and sense everything we swallow and ingest.
The gut moves contents along by peristalsis. GI contractions are complex and orderly. Peristalsis is also known as motility.
The gut is normally permeable. It allows materials to pass through, either in or out. The entire GI tract is lined with a single layer of cells (the epithelium), which is the main barrier between the environment (what is eaten and ingested) and the body. Beyond the cell lining is a deeper layer containing blood vessels containing more than half of the lymphoid tissue of the body and chemical messengers. Anything reaching this deeper layer has access to and influences the entire body. The role of the cell lining is to appropriately police what should be allowed to pass into the deeper layer (such as nutrients) or kept out of it (potential threats).
Normal function of the GI tract and overall health are dependent upon the gut microbiome. The gut is loaded with many types of bacteria and also contains fungi, viruses and protozoa. The microbiome is affected by genetics, diet, stress, infection and drugs.
Causes of IBS
The potential underlying causes of IBS are diverse and not completely understood.
Gut dysfunction includes heightened sensation, abnormal contractions (motility or peristalsis) and increased permeability (“leaky gut”) resulting in disordered immune function, inflammation and gut chemical-messenger activation.
Gut microbiome dysfunction is called dysbiosis, or small intestinal bacterial overgrowth (SIBO).
Food poisoning or bacterial gastroenteritis can cause IBS-PI/IBS-D by autoimmune small intestinal peristaltic dysfunction and subsequent SIBO. While at least half will recover over several years, IBS can be reactivated or worsened by recurrent food poisoning.
Malabsorption of bile acids can cause IBS-D symptoms.
Enzyme deficiencies or inhibition can cause malabsorption of lactose, fructose and sucrose.
Diet influences the microbiome for better or worse. Gluten (wheat, barley and rye) and certain poorly absorbed carbohydrates can cause or contribute to IBS symptoms.
Certain medications and treatments can cause IBS symptoms (e.g., iron, opioids and metformin).
Dysfunction of the brain-gut connection can result in heightened central nervous system (brain and spinal cord) sensory processing of gut and bodily sensations, pain and symptoms. This is called “central sensitization.” This process helps explain why most patients with IBS also have other functional GI disorders (FGIDs) and bodily pain disorders, such as fibromyalgia and chronic headaches. By analogy, the body and gut are a radio transmission tower responding to the environment with feelings, which are sent subconsciously to a radio located in the brain. Feelings are processed and then broadcast as pain and symptoms. When central sensitization is present, the volume control dial is turned all the way up. Stress (including early-life stressors like physical abuse), emotional distress (anxiety and depression) and negative cognition (memory and thought) can increase central sensitization and amplify pain and symptoms.
The abdominal enlargement (distention) that commonly occurs with bloating is not caused by an abnormal quantity of gas in the gut. Rather, the distention is caused by a paradoxical response to stimuli in the gut (such as eating and stool in the colon) that results in diaphragmatic contraction (descent) and relaxation of the upper abdominal wall.
While genetics (nature) can play a role in susceptibility to IBS, certain environmental (nurture) risk factors can predispose to development of, trigger, worsen or perpetuate IBS. These include adverse early-life stress (particularly sexual, physical and/or emotional abuse and maternal deprivation), psychological distress, antibiotics (systemic), certain medications and food poisoning (bacterial gastroenteritis).
Bacterial food poisoning should be avoided, as should unnecessary systemic (absorbed) antibiotics and certain medications. A healthy lifestyle — including diet, exercise, sleep and stress management — is important.