Small Intestinal Bacterial Overgrowth (SIBO) is a condition where the small intestine experiences an abnormal increase in its bacterial population. The small intestine is naturally designed to have a relatively low number of bacteria, with the majority residing in the large intestine for digestion. When this balance is disrupted, bacteria migrate upward or proliferate excessively in the small bowel, leading to digestive issues. SIBO is a major cause of excessive gas and associated uncomfortable bloating, resulting in the production of significant volumes of gas.
The Mechanism of Gas Production in SIBO
The excessive gas characteristic of SIBO results from the bacteria in the small intestine fermenting carbohydrates prematurely. These microorganisms consume undigested dietary carbohydrates, such as fiber and certain sugars, that should normally be absorbed or passed to the large intestine. This bacterial fermentation releases gases as metabolic byproducts.
The specific types of gas produced influence a person’s symptoms. Hydrogen gas is a common byproduct of fermentation and is often associated with diarrhea or loose stools. Archaea consume hydrogen gas and convert it into methane gas.
Methane gas slows down gut motility, often leading to chronic constipation. A third type of gas, hydrogen sulfide, can also be produced by certain bacteria consuming hydrogen. Hydrogen sulfide is sometimes linked to foul-smelling gas and diarrhea.
Other Common Indicators of SIBO
The pressure from gas buildup frequently causes abdominal discomfort and cramping. This discomfort can range from mild aching to intense pain, often occurring shortly after eating.
Changes in bowel habits are a frequent sign, presenting as either chronic diarrhea or chronic constipation, depending on the dominant gas type. The presence of excess bacteria interferes with the small intestine’s primary role of nutrient absorption. This can lead to malabsorption, where the body struggles to take in nutrients effectively.
Malabsorption may lead to unexplained weight loss or specific nutrient deficiencies. The bacteria can compete with the body for Vitamin B12, which is necessary for nerve function and blood cell production, potentially leading to a deficiency. Poor absorption of fats and fat-soluble vitamins (A, D, E, K) can also occur.
Confirming SIBO Through Testing
The primary non-invasive method used to confirm SIBO is the carbohydrate breath test. This test measures the levels of hydrogen and methane gas exhaled in the breath over a timed period. The patient drinks a standardized sugar substrate solution, such as lactulose or glucose.
If bacterial overgrowth is present, the bacteria will ferment this substrate early, causing a measurable rise in hydrogen or methane gas concentration in the breath. The timing of this gas spike is crucial; an early rise indicates fermentation is happening in the small intestine. A positive test result identifies whether the overgrowth is predominantly hydrogen-producing or methane-producing.
Glucose is rapidly absorbed in the upper small intestine, making it specific for detecting overgrowth in that section. Lactulose is not absorbed by the body and travels the entire length of the small intestine, offering a broader assessment. The results guide the physician in creating a targeted treatment plan based on the specific gas profile identified.
Treatment Approaches
Treating SIBO focuses on two main pillars: eradicating the bacterial overgrowth and managing symptoms through dietary modifications. The most common approach involves a course of targeted antibiotics. Rifaximin is frequently prescribed because it acts primarily within the gut and is minimally absorbed into the bloodstream.
For dominant methane production, rifaximin is often combined with a second antibiotic, such as neomycin, to increase effectiveness against methane-producing archaea. The goal of this therapy is to significantly reduce the bacterial population.
Dietary management is frequently employed due to SIBO’s tendency to recur. Temporary changes, such as a low-FODMAP diet, limit the food supply for remaining bacteria. In severe cases, an elemental diet (a liquid diet of pre-digested nutrients) may be used to starve the bacteria. Preventing recurrence requires addressing the underlying cause, such as impaired gut motility.