SIBO (small intestinal bacterial overgrowth) typically looks like persistent bloating, excessive gas, and abdominal pain that worsens after eating. Your belly may visibly swell over the course of the day, and your stools often change in ways that offer clues about what’s happening inside. But SIBO can also show up in less obvious places, including your skin, your energy levels, and your bloodwork.
The Core Gut Symptoms
The hallmark symptoms of SIBO are abdominal discomfort, bloating, flatulence, and chronic watery diarrhea. These happen because bacteria that normally live in the large intestine have colonized the small intestine, where they ferment carbohydrates before your body can absorb them. That fermentation produces hydrogen, carbon dioxide, and methane gas, which stretches the intestinal walls and causes distension.
Most people notice an uncomfortable fullness after eating, even after small meals. The bloating tends to build throughout the day, often starting mild in the morning and becoming more pronounced by evening. Loss of appetite, nausea, and unintentional weight loss are also common, especially in more advanced cases.
What Your Stool Looks Like
Stool changes are one of the most visible signs of SIBO, and the type of change depends on which gases the overgrown bacteria produce. Hydrogen-dominant SIBO tends to cause loose, watery diarrhea. Methane-dominant overgrowth (now called intestinal methanogen overgrowth, or IMO) is more associated with constipation, along with bloating and abdominal pain. A third subtype involving hydrogen sulfide gas also tends toward diarrhea.
In more severe cases, SIBO interferes with fat absorption. When that happens, your stools become fatty, a condition called steatorrhea. Fatty stools look noticeably different from normal bowel movements. They tend to be bulky, loose, greasy or foamy, pale or clay-colored, and unusually foul-smelling. They often float and can be difficult to flush. If you’re seeing stools like this regularly, it suggests your small intestine isn’t properly breaking down and absorbing dietary fat.
The “SIBO Belly”
Visible abdominal distension is so common with SIBO that it’s earned its own informal name. Your stomach may look noticeably flat or normal in the morning, then progressively swell as you eat throughout the day. This isn’t water retention or fat gain. It’s gas produced by bacterial fermentation physically stretching your intestines. Some people report looking several months pregnant by evening, even after eating relatively little. The distension typically improves overnight as the gas moves through or is reabsorbed.
Signs Beyond the Gut
SIBO doesn’t always stay confined to digestive symptoms. When bacterial overgrowth impairs nutrient absorption, it can cause deficiencies that show up throughout your body. The bacteria themselves consume vitamin B12 before your body can use it, which can lead to a specific type of anemia (megaloblastic anemia) that causes fatigue, weakness, numbness or tingling in the hands and feet, and difficulty concentrating. Iron-deficiency anemia is also common, bringing its own fatigue, pale skin, and brittle nails.
Fat-soluble vitamins A, D, and E can also become depleted. Low vitamin D affects bone health over time. Low vitamin A can impair vision and skin integrity. These deficiencies develop gradually, so they may not be obvious until SIBO has been present for a while.
The Skin Connection
One of the more surprising ways SIBO can show up is on your face. A prospective study found SIBO in nearly half of patients with rosacea, a rate significantly higher than in healthy controls. The link was strongest with papulopustular rosacea, the subtype characterized by red bumps and pus-filled pimples rather than just flushing. One study found that patients with papulopustular rosacea had over 12 times the risk of having SIBO compared to those with the flushing-only type. The exact mechanism isn’t fully understood, but the connection between gut bacteria and skin inflammation is an active area of investigation.
What SIBO Looks Like on a Breath Test
Since SIBO can’t be seen from the outside or confirmed by symptoms alone, most people end up taking a breath test. You drink a sugar solution (glucose or lactulose), then breathe into collection tubes at regular intervals over two to three hours. The test measures hydrogen and methane in your breath, which rises when bacteria ferment the sugar.
A positive result for hydrogen-dominant SIBO is a rise of 20 parts per million (ppm) or more above your baseline within the first 90 minutes. For methane, a level of 10 ppm or higher at any point during the test is considered positive. On a graph, a hydrogen-positive result shows a clear spike in the first 90 minutes. A lactulose breath test sometimes produces a “double peak” pattern: the first peak from bacteria in the small intestine and the second from normal fermentation in the colon.
These thresholds come from the North American Consensus guidelines and are what most gastroenterologists use to interpret results. Keep in mind that breath testing has limitations. It can miss hydrogen sulfide-dominant SIBO unless the lab specifically measures for that gas, and false negatives do occur.
How Symptoms Vary by Subtype
Not everyone with SIBO has the same experience, and the differences largely come down to which organisms have overgrown and what gases they produce.
- Hydrogen-dominant SIBO: The most common pattern. Produces diarrhea as the primary bowel symptom, along with bloating, gas, and cramping.
- Methane-dominant (IMO): Caused by methane-producing organisms called methanogens. Constipation is the defining feature, often with significant bloating and abdominal pain. The methane itself slows intestinal movement, which is why stool backs up.
- Hydrogen sulfide-dominant: Less well-studied but associated with diarrhea. May also cause sulfur-smelling gas and belching.
Knowing your subtype matters because it affects which treatment approach is most effective. This is one reason breath testing is valuable even when symptoms already strongly suggest SIBO.
Conditions That Mimic or Overlap With SIBO
SIBO symptoms overlap heavily with irritable bowel syndrome (IBS), and some researchers believe a significant portion of IBS cases involve undiagnosed SIBO. The bloating, pain, and altered bowel habits can look identical. Celiac disease, lactose intolerance, and inflammatory bowel disease can also produce similar symptoms. What distinguishes SIBO is that it responds to treatments targeting bacterial overgrowth rather than to dietary changes alone, though diet often plays a supporting role in management. If you’ve been treating IBS-like symptoms without improvement, SIBO is worth investigating with a breath test.