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What is Irritable Bowel Syndrome?

Irritable bowel syndrome (IBS) is the most common functional gastrointestinal syndrome in the U.S. It is estimated that up to 45 million people, or 10-15% of the American population, suffer from IBS. About 2 in 3 IBS sufferers are female, while about 1 in 3 IBS sufferers are male. IBS affects people of all ages, including children. There are three primary types of IBS: IBS with constipation predominance (IBS-C), IBS with diarrhea predominance (IBS-D), and mixed IBS (IBS-M), which presents as an alternating pattern of constipation and diarrhea.  Some patients don’t fit into these categories easily, and may be referred to as un-subtyped IBS, or IBS-U. Historically, before the development of IBSchek®, a proprietary blood test to diagnose IBS, it would take patients approximately 4-6 years to receive a diagnosis of IBS, usually after undergoing a variety of costly and invasive diagnostic procedures.

The symptoms of IBS include:

• Diarrhea

• Constipation

• Constipation alternating with diarrhea

• Gas

• Abdominal bloating

• Belly pains or cramps, usually in the lower half of the belly, that get worse after meals and feel better after a bowel movement

• Change in bowel movement frequency (more than three bowel movements per day (diarrhea or loose stool) or less than three bowel movements per week (constipation))

• Change in appearance of stool: hard, pellets, not well formed, undigested food, liquid, mucus

• Harder or looser stools than normal (pellets or flat ribbon stools)

• Digestive issues beginning after an episode of gastroenteritis more commonly known as food poisoning

IBS is often, but not always, associated with another condition called small intestinal bacterial overgrowth (SIBO). Small intestinal bacterial overgrowth, often referred to as SIBO, has been widely studied as an underlying cause of the irritable bowel syndrome (IBS) phenotype. This overview of SIBO as it relates to IBS may help you decide if this is something about which you should speak to your doctor.

What is Small Intestinal Bacterial Overgrowth (SIBO)?

SIBO is the accumulation of excessive amount of gut bacteria in the small intestine (at least 100,000 bacteria per ml of fluid). While bacteria naturally exist throughout the digestive tract, with highest concentrations of bacteria in the colon, a healthy individual should have relatively low levels of bacteria present in the small intestine. Any condition which impairs the normal transit or motion of the small intestine can increase the likelihood of getting SIBO, including lack of adequate stomach acid, damage to the intestine by toxins, or a decrease in the speed at which the small intestine transfers material to the colon. In the U.S., some research studies have demonstrated that up to 80% of the IBS population, or 36 million individuals, suffer from SIBO, and some researchers even hypothesize a connection between SIBO and common skin disorders like acne and rosacea.

How is SIBO diagnosed?

Due to the difficulty and invasive nature of performing a local study of the small intestine to identify the presence of bacteria (using small bowel aspiration or a culture), providers often prefer using a test commonly known as the hydrogen and methane breath test. The hydrogen and methane breath test for SIBO is a non-invasive diagnostic tool to identify SIBO, and can be administered in the comfort of a patient’s own home. Patients are given a solution to drink, which is a mixture of water and a carbohydrate substrate. After drinking the substrate solution, the patient will collect a series of breath samples by simply breathing into a test tube using a straw. In a healthy individual, one would not expect to see any hydrogen or methane in the breath samples for approximately 90 minutes – 2 hours, the approximate time it would take for the substrate to travel from the small intestine to the colon where, in a healthy system, the substrate would be fed upon by bacteria, thus releasing the hydrogen and methane gas.

During the hydrogen and methane breath test for SIBO, the presence of elevated levels of hydrogen or methane gas, or both, identified within 90 minutes of ingesting the substrate solution provides evidence of bacteria in the upper region of the digestive tract, i.e. at the level of the small intestine. This excessive build up bacteria is often diagnosed as SIBO. Our hydrogen and methane breath test for SIBO can be administered in-office or in the comfort of the patient’s own home.

The SIBO Theory of IBS

There is often a striking similarity between the symptoms of IBS and small intestinal bacterial overgrowth (SIBO). In addition, although IBS patients often point to specific foods as triggering symptoms, the research and data supporting this proposition is still in its early stages.

Evidence that SIBO may be an underlying cause for IBS comes from two main findings. The first is the observation that many patients with IBS are found to have an abnormal hydrogen and methane breath test, a test used to diagnose SIBO. The second is data showing that many patients see a significant decrease in IBS symptoms following a trial of specific antibiotics, the primary treatment for SIBO.

The SIBO theory of IBS seeks to explain why bacteria end up in the wrong place. The small intestine has a natural "cleansing wave" -- movement of muscles in the lining that serves to empty the small intestine at regular intervals. Research suggests that impairment to this muscle movement may result in the bacteria remaining and flourishing in the small intestine. One theory is that an episode of food poisoning, scientifically referred to as “acute gastroenteritis”, may damage the muscles and cells responsible for this cleansing action, which helps explain the phenomenon of post-infectious IBS. It is also thought that stress may slow down the action of these muscles, thus explaining the relationship between stress and IBS.

The SIBO theory also attempts to account for the fact that IBS can manifest itself as either diarrhea or constipation. Research studies have confirmed that different types of bacteria, and the gasses they produce (for example, hydrogen and methane) have different effects on gut movement and patient symptoms. Some studies have found that patients who demonstrate a higher amount of methane are more likely to experience IBS with predominant constipation (IBS-C), while diarrhea-predominant (IBS-D) patients demonstrate a higher level of hydrogen in a hydrogen and methane breath test for SIBO.

It is also theorized that SIBO may be the underlying cause of lactose malabsorption, fructose malabsorption, sucrose malabsorption, and other sugar malabsorption.

The Relationship between IBS and SIBO

The relationship between SIBO and IBS remains a subject of rigorous clinical research and scientific discussion. The general consensus appears to be that SIBO may be the underlying mechanism for a particular sub-set of IBS patients. Continued scientific research, which is being undertaken at CDI, will hopefully drive new learnings and insights regarding the relationship between SIBO and IBS.

Should you talk to your doctor about taking a hydrogen and methane breath test for SIBO? Given that SIBO is an extremely common condition for IBS patients, and the fact that antibiotic treatments may bring symptom relief, SIBO may certainly be worth further investigation, particularly if bloating is a predominant part of your symptom picture.


American College of Gastroenterology IBS Task Force "An Evidence-Based Position Statement on the Management of Irritable Bowel Syndrome" American Journal of Gastroenterology2009:S1-S35.

Lin, H. "Small Intestinal Bacterial Overgrowth: A Framework for Understanding Irritable Bowel Syndrome" The Journal of the American Medical Association 2004 292:852-858.

Pimentel, M. "A New IBS Solution" Health Point Press 2006.