SIBO, or small intestinal bacterial overgrowth, causes bloating, excessive gas, abdominal pain, and changes in bowel habits. These core symptoms affect roughly two-thirds of people with the condition. But the full picture often extends beyond the gut, with fatigue, nutrient deficiencies, and weight loss showing up in more advanced cases.
SIBO develops when bacteria that normally live in the large intestine multiply in the small intestine, where they don’t belong in high numbers. These bacteria ferment carbohydrates before your body can absorb them, producing hydrogen, methane, and carbon dioxide. That gas production is what drives the hallmark bloating and distension that most people with SIBO describe.
The Core Digestive Symptoms
The most frequently reported symptoms are abdominal distension, gas and flatulence, a persistent feeling of fullness, and diffuse cramping. Diarrhea is the most common change in bowel habits, though some people experience constipation or an alternating pattern between the two. The bloating tends to worsen after meals, particularly meals high in carbohydrates, because those are the foods the misplaced bacteria ferment most readily.
In more severe or long-standing cases, the diarrhea can become fatty, a sign that your body isn’t absorbing dietary fat properly. This type of stool is pale, greasy, and tends to float. When fat absorption breaks down to this degree, weight loss and malnutrition often follow.
How Gas Type Shapes Your Symptoms
Not everyone with SIBO has the same bowel pattern, and the type of gas the overgrown bacteria produce explains why. Bacteria that primarily generate hydrogen are associated with diarrhea. Organisms that produce methane, particularly a species called Methanobrevibacter smithii, are linked to constipation. Methane directly slows intestinal movement. In animal studies, methane gas reduced intestinal transit by 59% compared to regular air.
This distinction matters because methane-dominant overgrowth is now increasingly recognized as its own entity, called intestinal methanogen overgrowth (IMO). If your main complaint is constipation rather than diarrhea, methane-producing organisms are the more likely culprit. Methane-dominant patterns are significantly more common in people with constipation-predominant irritable bowel syndrome.
Fatigue and Nutrient Deficiencies
Beyond digestive complaints, chronic fatigue and difficulty concentrating are commonly reported. These symptoms are partly explained by the nutritional toll SIBO takes on the body. Overgrown bacteria in the upper small intestine break down bile acids, which your body needs to absorb dietary fat. When bile acids are disrupted, the fat-soluble vitamins A, D, and E aren’t absorbed efficiently.
Vitamin D deficiency is especially consequential. It can lead to low calcium levels, and untreated SIBO is a recognized contributor to osteoporosis over time. Iron deficiency is another common finding, particularly in people with methane-dominant overgrowth, where more than half of patients in one study had ferritin levels low enough to indicate depleted iron stores. Vitamin B12 deficiency and low protein levels round out the nutritional picture in advanced cases, contributing to anemia and muscle wasting.
These deficiencies can develop silently. Someone might attribute their fatigue or brain fog to stress or poor sleep without realizing their gut is failing to extract basic nutrients from food.
The Overlap With IBS
SIBO symptoms overlap so heavily with irritable bowel syndrome that the two are frequently confused. A large meta-analysis pooling data from 50 studies and nearly 10,000 participants found that 38% of people diagnosed with IBS tested positive for SIBO. People with IBS were almost five times more likely to have SIBO than healthy controls. Women, older adults, and those with diarrhea-predominant IBS had the highest rates.
This doesn’t mean IBS and SIBO are the same condition. But if you’ve been told you have IBS and your symptoms haven’t responded to typical management, bacterial overgrowth is worth investigating.
What Causes the Overgrowth
Your small intestine has a built-in cleaning mechanism called the migrating motor complex. During fasting, roughly every 90 to 120 minutes, a wave of muscular contractions sweeps residual material from the small intestine into the colon. When this sweep is disrupted, bacteria linger and multiply where they shouldn’t.
Several conditions impair this process or otherwise set the stage for overgrowth:
- Motility disorders like gastroparesis, chronic intestinal pseudo-obstruction, or diabetes-related nerve damage slow the gut’s ability to clear bacteria.
- Structural changes including prior abdominal surgery, small intestine diverticula, strictures from Crohn’s disease or radiation, and removal of the ileocecal valve (the one-way gate between small and large intestine).
- Low stomach acid from long-term acid-suppressing medications or other causes removes a key barrier that normally kills bacteria before they reach the small intestine.
- Immune suppression and organ dysfunction, including cirrhosis, kidney failure, and chronic pancreatitis.
- Repeated antibiotic use, which disrupts the normal microbial balance.
How SIBO Is Diagnosed
The most accessible test is a breath test. You drink a sugar solution (glucose or lactulose), then breathe into collection tubes at timed intervals. Bacteria in the small intestine ferment the sugar and produce gases that cross into your bloodstream and reach your lungs. A rise in hydrogen of 20 parts per million or more within 90 minutes is considered a positive result. Methane levels at or above 10 parts per million at any point during the test indicate methane-producing overgrowth.
Breath testing is practical and noninvasive, but it isn’t perfect. A recent meta-analysis found the glucose breath test has a sensitivity of about 55% and specificity of 83%, meaning it catches just over half of true cases but is reasonably good at ruling out false positives. The lactulose version is slightly less accurate in both measures. A negative breath test doesn’t guarantee you’re free of SIBO, especially if your symptoms strongly suggest it.
The most definitive test involves collecting fluid directly from the small intestine during an endoscopy and culturing it for bacteria. A count of 100,000 or more bacterial colony-forming units per milliliter confirms the diagnosis. This approach is more invasive and less commonly used in routine practice.
When Symptoms Don’t Respond to Treatment
SIBO symptoms overlap almost completely with another condition called small intestinal fungal overgrowth, or SIFO. In SIFO, yeast and other fungi, rather than bacteria, overgrow in the small intestine. The bloating, gas, diarrhea, and abdominal pain can look identical. The key difference is that fungi don’t produce hydrogen or methane, so a breath test won’t detect SIFO. If you’ve tested positive for SIBO and completed antibiotic treatment without improvement, fungal overgrowth becomes a strong possibility. SIFO is more common in people who’ve taken repeated courses of antibiotics, are immunosuppressed, or have significant disruption to their gut microbiome. Diagnosis requires culturing fluid from the small intestine directly.