Ultimately, the cause(s) of IBS are not known—there are many theories. The advantage of these theories is that they can present a scientific framework to examine some of the possible causes of IBS.
IBS is a functional digestive disorder. Functional disorders are, by definition, those that disrupt the function or the workings of a particular system but there are no known actual abnormalities. Another way of looking at this is to say that we know something is wrong, in this case within the digestive system, but the clues as far as changes in anatomy, cells, tissues, lab values or physical examination results are simply not there. Approximately 40% of all digestive (GI) problems seen by doctors are functional disorders. IBS is the most common functional digestive disorder. Other functional disorders of the digestive system include:
- Functional bloating (also called functional distension)—this is actually a group of disorders where the predominant symptom is a sense of fullness or bloating in the abdomen.
- Functional constipation—a group of disorders with chronic (persistent or long-term) constipation or a sense of incomplete defecation or an incomplete bowel movement.
- Functional diarrhea—a group of disorders with chronic passage of loose, watery stools. There is no abdominal pain associated with functional diarrhea—this difference can allow a physician to distinguish between functional diarrhea and IBS-D, for example, though, in fact, this may represent a sub-group of IBS.
- Unspecified bowel disorders—this is a sort of “catch-all” group that is used when a person’s symptoms don’t fit well with any other disorder
- Opioid-induced constipation. One of the most common adverse effects of using opioid-based pain medications (eg. Percocet® or Oxycodone®) is constipation. Tolerance rarely develops to this effect and combinations of diet, exercise and laxative treatment is often necessary.
There are three main areas that cover the theories of the causes of IBS. There is a good deal of overlap in these three areas. The areas are:
- Changes in how the digestive system moves foods and wastes through the small and large intestine—this movement is called intestinal motility.
- Hyperalgesia or a heightened perception of pain or discomfort. This has to do with the messages sent from the digestive system to the brain and back
- A psychosomatic basis for IBS is considered possible because of the strong linkage between disorders such as depression, panic disorder, anxiety and post-traumatic stress disorder (PTSD). However, it is often to determine which disorder came first— most cases of IBS occur before the age of 35. This is also the time where most psychological disorders occur as well. Since there are genetic factors at play in both psychological disorders and IBS- that is, if one close family member has, for example, clinical depression, your risk for depression increases. In a similar way, if one close family member has IBS, your risk for IBS increases as well.
There are a number of theories that researchers can use to try and understand IBS better. As mentioned, there is a good deal of overlap in these theories—motility, for example is the result of brain-gut communication, hormonal effects and the levels of chemical messengers available. In the long run, IBS will likely be caused by a combination of the following theories. This makes holistic sense—while we like to break down the body into various systems, the fact is that all these systems form a network or interactions. One analogy that has often been made is to imagine the body as a spider web. The strands are separate but if a fly lands on one part of the spider web, that landing is immediately detected in all parts of the spider web and all the parts of the web react or interact and respond by stretching or shortening to compensate for that fly. The web is stronger for all the strands and can withstand some broken strands. However, at some stress point, that web will come apart.
The main theories for the underlying cause(s) of IBS are:
- Brain-Gut mis-communication: The gut has been called our “Second Brain” because of its rich nerve supply that is in many ways independent of the “first” brain. Our language often reflects this—phrases like “I listened to my gut” or “Seeing that made me want to vomit” and “My guts are in a knot over the tragedy” imply that sort of connection. This communication can control digestion, movement (motility) of contents of the small and large intestines, and control of smooth muscle tone (to keep the contents moving). If the brain’s nerves (the central nervous system) and the gut nerves (the mesenteric or enteric system) can’t communicate properly, this is believed to cause at least some of the symptoms of IBS. The brain and the gut communicate in a number of ways—by direct nerve transmission and via chemical messengers, including gut hormones. In addition, brain-gut communication can be affected by reproductive hormones, adrenal hormones and pituitary hormones.
- Motility problems in the gut. The contents of the small and large intestines are in constant motion. This motility is the process of peristalsis, coordinated wavelike contractions pushing food and eventually waste through the intestines. Slowed motility can lead to constipation while faster motility can result in diarrhea. If the muscles involved in peristalsis go into spasms, this can be read as discomfort or pain, depending on an individual’s perception of the sensation. Stress and other emotional or mental issues can affect this motility—remember, the brain and the gut DO communicate. About 1/3 of people with IBS have motility issues—the passage of food can be either too fast or too slow, but the peristaltic movement can also become uncoordinated and ineffective in moving the intestinal contents along through the small to the large intestines.
- Sensitivity to Pain. Doctors generally ask patients to rank pain on a scale from one to ten. This is actually pretty accurate—for that individual patient. But, we all know that one person’s “agony” or 10 on this scale is another person’s pain (perhaps a 7 or 8) is another person’s significant discomfort (maybe a 5). Pain can be perceived in different ways by different people—it is not a “wimpiness” factor—it is quite simply a difference in how pain is perceived by different people. Also, in people with IBS, there is evidence that the nerves of the gut are in fact more sensitive to foods, gas and waste passing through the intestines—and the brain perceives this as discomfort or pain. A number of disturbances have been found in individuals with IBS that may impact the sensitivity to pain or hyperalgesia. These include abnormal activation of various receptors, increased production of various pain transmitters and the increased production of various gene products that may impact the perception of pain or discomfort. Finally, the part of the brain that deals with a number of emotional and stress-related issues, the limbic system, can be active in either increasing or decreasing gut motility.
- For many years, physicians blamed certain foods, stress, or personality (eg. a Type A personality) for peptic (stomach) ulcers. Then, a bacterium—Helicobacter pylori or H. pylori—was found to infect the stomachs of some people with ulcers. Similarly, some researchers believe that some unidentified bacteria or combination of bacteria are the root cause of IBS. About 1 in 7 people with IBS have what is technically known as postinfectious IBS which is a form of IBS which follows an infection of the digestive system.
- The gut’s population of bacteria is huge—more bacteria live in the gut than the number of your own cells in your own body. These bacteria perform a number of important functions—including the immune system, nutrient absorption and vitamin synthesis. Research into the gut microbiome is really just in its infancy—there is so much that we do not know that it is fair to say that there is a long way to go in understanding exactly what these bacteria are doing for us. The proper functioning of the gut clearly depends on the presence of these bacteria—but exactly what types of bacteria (eg. Lactobacillus, Acidophilus, Bifidobacterium etc) and in what proportions will vary depending on where you live, what you eat and your overall health.
- Small Intestinal (or Bowel) Bacterial Overgrowth (SIBO). When you hear people talking about the gut microbiome, the gut flora or the need to take probiotics, they are primarily talking about the bacteria found in the large intestine. These bacteria as a group are critically important in a number of functions including helping to maintain a functional immune system and the production of a number of vitamins. Normally, there should be very few bacteria in the small intestine where most of the absorption of nutrients from food takes place. Small intestinal bacterial overgrowth (SIBO) is the increased number of bacteria or a change in the types of bacteria in the small intestine. These bacteria can produce extra gas and possibly toxins which can cause the symptoms of IBS. These bacteria can also interfere with the absorption of nutrients, particularly fats and fat-soluble vitamins such as Vitamins K, D, E and A.
- Altered Levels of Chemical Messengers (Neurotransmitters and Hormones). There are a number of chemical messengers which appear to have altered levels in IBS. Serotonin appears to be one of the most important neurotransmitter that impacts IBS. Interestingly, serotonin is active in the gut AND in the brain—in the brain, disturbances in serotonin are related to depression and anxiety as well as sleep disorders. There is growing evidence in alterations in serotonin function that are different depending on the form of IBS. Reproductive hormones appear to affect IBS as well—changing levels of reproductive hormones during the menstrual cycle can affect the symptoms of IBS, often worsening during the monthly period. Women with IBS often have more difficult periods as well, even though the blood level of estrogen and progesterone are not abnormal.
- Family history or genetics. While there haven’t been any specific genes that have been associated with IBS. IBS tends to be more common in some families than others. This parallels the family association of various psychological diseases such as depression, anxiety, panic disorder and PTSD.
- Food Sensitivities. Many people with IBS report sensitivity or intolerance to certain foods, including gluten, casein, carbohydrates, spices, fatty foods, coffee or alcohol. It is not certain if these sensitivities are involved in the causes of IBS or appear as a result of the effects of IBS. As an aside—these are not generally food allergies, though the term is often used. Technically, allergies involve a specific set of antibodies (IgEs) and a specific set of cell reactions. Food sensitivities on the other hand are much more common, use a different set of antibodies (IgG, IgM and IgA) and another specific set of cell reactions. A food sensitivity could in theory damage either the small or large intestine (or both) in some currently unknown way or ways and create a set of circumstances that, given a susceptibility to IBS, can result in IBS.
- Poor absorption of foods. There is some evidence indicating that simple and/or complex carbohydrates may not be well absorbed in people with IBS, though again, it is not certain which came first. In addition, there are some people with IBS also may have problems with their gall bladder and the production of bile acids. These bile acids are required for the absorption of dietary fat. This may be more important in some forms of IBS such as IBS-D and in functional diarrhea (see above).
- Mental health issues are also often found commonly in people with IBS. These mental health issues can include depression, anxiety, panic disorder and post- traumatic stress disorder (PTSD) but are not limited to these disorders. Once again, the cause and effect relationship is not clear. As mentioned, serotonin is involved as a neurotransmitter in the gut and in the brain—and altered serotonin levels are seen in both IBS and in various psychological disorders. Emotional factors can become more important in dealing with IBS, especially in social situations or when dealing with IBS for a prolonged period of time. This is a research area still in its infancy, but it is reasonable to think that these relationships may be clarified by future research.
IBS research is ongoing and growing with the recognition that IBS is not only a real disorder but affects a significant population all around the world. While it may be confusing to think of all the theories of possible causes of IBS, from a scientific and medical point of view it is actually good news in the sense that it provides a wide array of possibilities to examine.