Small Intestinal Bacterial Overgrowth, or SIBO, is a condition characterized by an abnormal increase in the number of bacteria within the small intestine. This upper part of the digestive tract normally maintains a relatively low bacterial count compared to the large intestine. The overgrowth of bacteria, often those typically found in the colon, can lead to fermentation of ingested carbohydrates and the production of gases. The central question of how many people are affected by SIBO is complex, lacking a single, universally accepted answer.
Understanding SIBO and Its Diagnostic Complexity
SIBO arises when the natural protective mechanisms of the gut fail, allowing bacteria to multiply to an excessive degree in the small bowel. These bacteria consume nutrients and release byproducts that cause a range of uncomfortable gastrointestinal symptoms. Common complaints include bloating, excessive gas, abdominal pain, and diarrhea, which can mimic symptoms of many other digestive disorders.
The challenge in determining SIBO prevalence stems from diagnostic difficulty. The gold standard is a technically invasive procedure called a small bowel aspirate and culture, which directly measures the bacterial count. Due to its invasive nature and expense, this method is rarely used in routine clinical practice.
Instead, diagnosis primarily relies on non-invasive breath tests, which measure hydrogen and methane gases produced by the bacteria after a patient consumes a sugar solution. However, the reliability of these breath tests can be limited by factors like the movement speed of food through the gut and the specific sugar used, which contributes to the wide variation in reported case numbers.
Prevalence Estimates in the General Population
Estimating the true prevalence of SIBO in the general, healthy population is difficult due to the scarcity of studies on truly well individuals. Reliable figures for the average person without underlying health issues tend to fall on the lower end of the spectrum. Available data suggests the prevalence in healthy adults without digestive symptoms is likely between 0% and 15%.
Some studies focusing on younger, healthy control groups have reported SIBO rates as low as 5.9% or even 0% when using the more specific Glucose Breath Test (GBT). Conversely, other studies that have used the Lactulose Breath Test (LBT) on apparently healthy individuals have reported higher figures, sometimes closer to 20%. This difference highlights a major diagnostic uncertainty even within the healthy population, as the LBT is known to be less specific.
Many SIBO cases in the general population remain undiagnosed because affected individuals are either asymptomatic or experience mild symptoms. Since SIBO symptoms overlap heavily with general digestive distress, a person must seek medical attention and be specifically tested to be counted in prevalence statistics. Estimates for the general population often range from 2.5% to 22%, though the lower figures are considered more accurate for those without predisposing factors.
SIBO Rates Among High-Risk Patient Groups
Prevalence figures increase dramatically when moving from the general population to individuals with specific underlying health conditions. These conditions often compromise the gut’s natural defenses against bacterial overgrowth, such as impaired motility or structural abnormalities. For patients diagnosed with Irritable Bowel Syndrome (IBS), the overlap with SIBO is substantial, with meta-analyses suggesting a prevalence of approximately 35.5% based on breath testing.
The type of IBS influences the rate, with SIBO being more common in the diarrhea-predominant subtype (up to 37%). Patients with Inflammatory Bowel Disease (IBD) also demonstrate elevated SIBO rates, with a pooled prevalence across all IBD patients of about 31.0%. Those with Crohn’s Disease specifically have a prevalence of around 32.2%, often due to structural issues like strictures or prior abdominal surgery.
Other conditions that alter the gut environment also show high SIBO rates. For example, the condition is found in nearly 29% of patients with Diabetes Mellitus, likely due to nerve damage that impairs the small intestine’s movement. Patients with Celiac Disease, another condition affecting the small intestine, have a pooled SIBO prevalence of 18.3%, which is about five times higher than in healthy controls.
Why Prevalence Figures Vary Widely
The wide-ranging prevalence figures are rooted in methodological variations and a lack of diagnostic standardization. A primary source of inconsistency is the choice of substrate used in the breath test: glucose or lactulose. The Glucose Breath Test (GBT) is rapidly absorbed in the upper small intestine, giving it higher specificity but potentially leading to false-negative results if the overgrowth is in the lower small intestine.
Conversely, the Lactulose Breath Test (LBT) travels the entire length of the small intestine, but its use is associated with a higher rate of false-positive results. This occurs because lactulose can reach the large intestine too quickly, causing normal colonic bacteria to produce a gas spike incorrectly attributed to SIBO. Studies using the LBT often report a much higher prevalence than those using the GBT or the gold-standard culture method.
Furthermore, the diagnostic cut-off threshold for a positive test lacks universal agreement. The current North American consensus defines a positive result as a rise of 20 parts per million of hydrogen above the baseline within 90 minutes. However, some research suggests a lower cut-off might be more sensitive, meaning that changing this number alone can significantly alter the reported prevalence of SIBO.