Figure 11.1. Patient blowing into the sample bag as part of the hydrogen breath test
Courtesy of Mark Washburn / Dartmouth-Hitchcock.
If you asked people who have irritable bowel syndrome what their worst symptoms are, many would say problems with excess intestinal gas and the feeling of being bloated and distended. As discussed in Chapter 6, everyone who eats or drinks anything other than water will produce some gas in the gastrointestinal (GI) tract. Researchers have found that the average adult produces approximately 700 ml of intestinal gas each day. Gas within the GI tract develops from sources other than food and drink, such as swallowed air, diffusion from the bloodstream, and a variety of chemical reactions within the GI tract. The five most common gases found within the GI tract are nitrogen, oxygen, hydrogen, carbon dioxide, and methane. Other gases are present in the GI tract, but they exist only in trace amounts. Nearly all nitrogen and oxygen within the upper GI tract comes from swallowed air. Carbon dioxide may be present from swallowed air, from carbonated beverages, or from chemical reactions that occur in the stomach and upper small intestine (acids and bases are neutralized and form carbon dioxide gas as a byproduct). Most gas production in the GI tract, however, occurs in the colon (large intestine).
Although many of us don’t like to think about it, our large intestines are full of bacteria. This extensive population of bacteria is vital to our good health and is often referred to as the gut microflora or microbiota or gut microbiome. The average individual has anywhere from five hundred to one thousand different species of bacteria living in the colon at any one time. The total number of bacteria in the colon is estimated at approximately 1013 to 1014 (ten to the thirteenth or fourteenth power)—to put this in perspective, you have more living bacterial cells in your colon (approximately 3–4 pounds worth at any one time) than you do living cells in all other organs in your body combined. These bacteria have a variety of jobs: they are critical to maintaining the health of the large intestine, they play an important role in the immune system of the gastrointestinal tract, they help metabolize (break down) medications, they produce essential fatty acids and vitamins, and they serve in digestion and nutrient absorption.
Gut microflora typically reside only in the large intestine. However, in some people who have IBS, the bacteria migrate from the colon into the small intestine (see Figure 6.1 for a review of the anatomy of the GI tract). When abnormal amounts of colonic bacteria migrate from the colon into the small intestine, a condition called small intestine bacterial overgrowth (SIBO) develops. SIBO may be caused by an infection in the gastrointestinal tract, surgery to the small or large intestine, altered motility of the intestinal tract, diverticula (abnormal sacs or pouches) in the small intestine, low levels of acid in the stomach (stomach acid helps to prevent bacterial overgrowth), various medications, changes in the immune function of the GI tract, or other unknown causes. No matter why SIBO develops, when these colonic bacteria colonize the small intestine, the physiology of the small intestine changes.
Major symptoms of SIBO include excessive gas and bloating. As the colonic bacteria in the small intestine digest and ferment food products that are much larger and more complex than normal, they produce excess gas. If you have ever made bread at home, you know that bread rises when yeast is added. Rising occurs due to gas production (carbon dioxide) when yeast is exposed to food (sugar or starch). A similar process occurs when too many bacteria are present in the upper small intestine—they ferment food products too vigorously and produce excess gas. Excess gas is not dangerous, but it can certainly be uncomfortable. In people who have long-standing SIBO, colonic bacteria in the small intestine may also lead to changes in vitamin levels (such as low vitamin B12 and vitamin D levels or high folate levels) and cause chronic diarrhea.
Studies during the past decade have shown that some, but not all, people who have IBS also have SIBO. Because nearly all patients who have IBS have symptoms of gas and bloating, how can health care providers determine if the gas and bloating are a direct result of SIBO? In other words, because symptoms of gas are nonspecific—making it extremely difficult to tell if the excess bloating is from SIBO or not—testing may be necessary. There are two main methods of accurately diagnosing SIBO. The first is upper endoscopy (see Chapter 8), in which a physician carefully passes a sterile catheter into the proximal jejunum and takes a sample of the fluid (called an aspirate) to send to the lab. Laboratory technicians count the number of bacteria present and also culture the bacteria (they attempt to grow the bacteria on a Petri dish to identify the exact type of bacteria present). Although this method of diagnosis may sound simple, there are multiple problems with this approach. One, although upper endoscopy is generally safe, it is invasive, expensive, and does involve some small risks. Two, it is very difficult to obtain a sample of small intestine fluid without contaminating it with fluid from adjacent organs, especially the oropharynx. Three, many labs do not have the expertise or technical capabilities to culture small intestine fluid. For these reasons, most health care providers now attempt to diagnose SIBO with noninvasive measures, such as a breath test.
Breath testing is based on the principle that bacteria (whether in the small intestine or in the colon) produce hydrogen and methane gas in response to nonabsorbable carbohydrates; hydrogen gas produced within the intestinal tract can then diffuse across the wall of the intestinal tract and into the bloodstream, where it travels to the lungs and is exhaled. During a breath test, a patient ingests a carbohydrate “meal” (typically either a premeasured dose of lactulose or glucose) after an overnight fast, and then samples of his or her exhaled breaths are analyzed at routine intervals (usually every 15 minutes for 3 to 4 hours). In a healthy individual, a breath test would show a sharp rise in breath hydrogen (and/ or methane) after the carbohydrate “meal” passes through the ileocecal valve into the colon (see Figure 11.1). In a patient who has IBS and bacterial overgrowth, the “ideal” positive breath test would show an early peak (within 60 to 90 minutes) of hydrogen or methane, due to the test meal being metabolized by small intestine bacteria, and then a second peak as the carbohydrate reaches the colon. If the test is positive, meaning that there is evidence of excess hydrogen or methane production, then the patient is diagnosed with SIBO and treatment can be initiated (see Chapters 17 and 19).
Different laboratories use different carbohydrate test “meals.” The most commonly ordered breath test is the lactulose breath test. Lactulose is a nonabsorbable sugar that passes through the stomach and small intestine without being broken down. When it passes into the colon, colonic bacteria digest it and produce gas (hydrogen) as a byproduct. Advantages of the lactulose breath test include ease of use, well-defined standards, and ability to compare data from one lab to another (because of its widespread use). Other labs use glucose as the test meal; some believe that this is a slightly better test, although it is used less commonly. At present, there are no large comparisons of the lactulose and glucose breath tests, so whichever test is offered to you by your health care provider is a reasonable option.
As discussed earlier in this chapter, because bloating is a nonspecific symptom that may be caused by many different pathophysiological (abnormal physiological) processes, it cannot reliably be used to diagnose SIBO. For that reason, some members of the medical community recommend testing all people who have IBS with symptoms of bloating for bacterial overgrowth and then treating them only if the test is positive. Treatment usually involves antibiotics, which are expensive and are associated with some risk, so the practice of testing for bacterial overgrowth helps ensure that antibiotics are given only to those patients who have SIBO. However, testing all people who have IBS with symptoms of bloating for bacterial overgrowth is a difficult strategy to implement, as that would mean testing 15 percent of the U.S. adult population. In the current economic and health care climate, such a strategy is impractical and financially untenable. Other practitioners recommend empiric therapy, which means that if they think the symptoms of gas and bloating are due to SIBO, treatment is initiated because of those symptoms, without any specialized testing. The following case study illustrates many of the issues that patients encounter when faced with persistent symptoms of gas and bloating and the question of SIBO.
Figure 11.1. Patient blowing into the sample bag as part of the hydrogen breath test
Courtesy of Mark Washburn / Dartmouth-Hitchcock.
Susan is a 27-year-old woman referred for further evaluation due to persistent symptoms of lower abdominal discomfort, bloating, gassiness, and occasional loose, nonbloody stools accompanied by a sensation of urgency. These symptoms appeared approximately five years ago, after Susan developed a presumed viral gastroenteritis while traveling in Mexico. She initially had severe watery diarrhea, which slowly improved with time. Her major symptom now is feeling gassy and bloated. She states that sometimes she looks “five months pregnant” due to the bloating and that some days it is so severe that it is uncomfortable to wear skirts or tight pants. Susan is single; she works full-time as a copyright editor, does not use tobacco products, and has two to three glasses of wine per weekend. No one in her family has a history of celiac disease or inflammatory bowel disease. Her weight has remained stable over the past five years, and she is not allergic to any medications. She takes a daily oral contraceptive and occasional acetaminophen for headaches. Susan’s past medical history is unremarkable—her only surgery was an appendectomy as a child.
On physical examination, she does not appear distended and is not tympanic (meaning that when the abdomen is tapped [also called percussed] by the examining health care provider, it sounds like a drum). Previous medical providers performed many different tests on Susan, including stool samples to look for a bacterial or parasitic infection, several sets of blood work with a CBC (complete blood count), tests for celiac disease (serum TTG antibody and serum IgA), a colonoscopy with random biopsies, an x-ray study of the small intestine, and an upper endoscopy with small bowel biopsies. All of these tests were completely normal. Because Susan read somewhere that bloating could be a sign of ovarian cancer, she also had a pelvic ultrasound (which showed normal results).
Susan tried many different methods of appeasing her symptoms. Although loperamide improved her diarrhea, it did not help her bloating. She avoided all dairy products for two weeks without improvement. She took all fructose-containing products out of her diet for one month, also without benefit. She lowered her fiber intake to less than 10 grams per day, which helped her diarrhea a little bit but did not improve her bloating. She even went on a gluten-free diet for two months, but this did not seem to help (and it was expensive and hard to maintain). She tried two different over-the-counter probiotics, each for approximately six weeks, without any benefit.
After discussing Susan’s symptoms with her, I diagnosed her as having postinfectious IBS from the GI tract infection she developed during her trip to Mexico (see Chapter 3). Although the original infection had long since resolved, it may have injured the nerves to the GI tract (the enteric nervous system) and may also have changed her colonic micro-flora or caused her to develop SIBO. We had a long discussion about her symptoms and the medication and dietary restrictions she had tried thus far (and how they had failed to improve her symptoms). We also discussed the fact that symptoms of gas and bloating are nonspecific, meaning that they can develop for a multitude of reasons. Finally, we discussed empiric treatment (antibiotics) versus objective breath testing. Susan said that she really wanted to understand why she had these symptoms, especially because all of her tests had been normal to date. In addition, as a teenager and young adult she had had some problems with antibiotics (GI upset, diarrhea, yeast infections) and wanted to avoid these problems if possible. For these reasons, we decided to have Susan undergo a breath hydrogen test. The test showed clear evidence of SIBO—probably a result of her prior gastrointestinal infection. I prescribed a course of nonabsorbable antibiotics (rifaximin). Susan called 2 weeks after finishing the 10-day course of antibiotics to say that her bloating and loose, urgent stools had resolved and that her lower abdominal pain had improved. She hadn’t suffered any side effects from the antibiotic and was very pleased with the results.
I generally recommend performing a breath test for SIBO only after patients have failed a thorough trial of dietary changes. These dietary changes typically include sequential weeks of eliminating dairy, fructose, fiber, and gluten; going on a strict elimination diet for at least 10 days (the classic elimination diet includes only water, broth, white rice, boiled chicken, and egg whites); or using the low-FODMAP diet for at least 1 month (see Chapter 15).
Although some providers recommend empiric treatment with antibiotics, which eliminates unnecessary breath tests, a positive response to the antibiotic does not provide any insight into the underlying cause of the symptoms. Patients who respond to antibiotics may really have had SIBO, or their results might have been due to a change in colonic bacteria (or were simply a placebo response). For many people who have IBS and excessive gas and bloating, it does not matter why their symptoms improved—they just want to feel better. Other treatments for gas and bloating are discussed in Chapter 19.
• Small intestine bacterial overgrowth (SIBO) occurs when abnormal amounts of colonic bacteria migrate from the colon into the small intestine.
• SIBO can cause significant problems with gas, bloating, distention, and diarrhea.
• SIBO is present in some, but not all, people who have IBS.
• SIBO cannot be diagnosed by x-rays, upper endoscopies, CT scans, blood work, or colonoscopies. It is most commonly diagnosed using a breath hydrogen test.
• Treatment of SIBO typically involves a nonabsorbable antibiotic.