If you’ve had the colonoscopy, the endoscopy, and multiple rounds of treatment, and you’re still bloated, exhausted, and being told it’s “just IBS,” then there’s a question worth asking: What if the real problem was never tested for?
SIBO (Small Intestinal Bacterial Overgrowth) is one of the most commonly missed gut conditions. The warning signs are real. They’re just not on most doctors’ radar.
In this article, I’ll be walking you through the 7 most commonly missed warning signs of SIBO — including some that have nothing to do with your gut at all. Please also keep in mind that this is educational information only and not direct medical advice for your specific situation.
Here is a video we made; there is also a written version underneath.
What is SIBO?
SIBO stands for Small Intestinal Bacterial Overgrowth. It occurs when bacteria that normally live in the large intestine migrate into and colonize the small intestine — a part of the gut that is supposed to be relatively bacteria-free.
When bacteria overgrow in the small intestine, they ferment the food you eat before your body has a chance to properly digest and absorb it. This produces gas, disrupts nutrient absorption, damages the gut lining, and sends inflammatory signals throughout the body, including to the brain.
There are three main types of SIBO, classified by the gas the bacteria produce:
- Hydrogen SIBO — typically associated with diarrhea
- Methane SIBO (technically produced by archaea, not bacteria) — typically associated with constipation
- Hydrogen Sulphide SIBO — associated with diarrhea and produces a characteristic sulfur or rotten egg smell
Understanding which type you have matters enormously for treatment, and standard testing often misses the third type entirely.
Summary: SIBO occurs when bacteria overgrow in the small intestine, causing fermentation, gas production, and nutrient malabsorption. There are three distinct subtypes — hydrogen, methane, and hydrogen sulphide — each with different symptom profiles. Identifying the correct subtype is essential for effective treatment.
Warning Sign #1: Severe Bloating That Looks Like You’re Pregnant
This is the most common and most dramatic sign. Clients often describe looking six months pregnant by lunchtime and not just feeling full, but visible, measurable distension.
Here is the mechanism: when bacteria overgrow in the small intestine, they ferment the food you eat (particularly carbohydrates and fiber) before your body has a chance to properly digest it. That fermentation produces gas. A lot of gas. And because it is happening in the small intestine rather than the large intestine, where fermentation is supposed to occur, the gas builds up in the wrong place.
What makes SIBO bloating distinct from ordinary bloating is the timing. SIBO bloating typically comes on fast, within an hour of eating, even small amounts of carbohydrate-containing foods. Fiber and complex carbs are often the worst triggers, which is why so many people come to us confused: they have been told to eat more vegetables and whole grains for their gut health, yet they feel significantly worse.
The evidence: A 2024 review published in Deutsches Ärzteblatt notes that bloating is one of the primary presenting symptoms of SIBO, specifically linked to bacterial fermentation in the proximal small intestine (the upper part of the gut), producing hydrogen and methane gases (1).
Summary: SIBO bloating is caused by bacterial fermentation occurring in the wrong part of the gut, producing large amounts of gas rapidly. The key distinguishing feature is timing — bloating that arrives within an hour of eating, particularly after carbohydrates or fiber, is a significant red flag. This is one reason why standard dietary advice to eat more fiber can actually worsen symptoms in undiagnosed SIBO.
Warning Sign #2: Diarrhea Or Constipation (Or Both)
Most people assume SIBO only causes diarrhea. But this is not accurate. Whether you experience diarrhea, constipation, or an alternating pattern of both depends entirely on which type of bacteria (or microorganism) is overgrowing in your small intestine.
Here is how it breaks down:
- Hydrogen-producing bacteria are associated with diarrhea-predominant symptoms
- Methane-producing organisms are associated with constipation-predominant symptoms
- Hydrogen sulphide-producing bacteria are also linked to diarrhea
A 2026 real-world study from Cedars-Sinai tested 6,000 people nationally using three-gas breath testing. Researchers found that hydrogen sulphide gas was specifically associated with diarrhea-predominant symptoms, while methane was linked to constipation. This is why a breath test — not a colonoscopy, not a standard blood panel — may give you more diagnostic clarity than years of conventional testing (2).
The reason this connection is so frequently missed is that doctors see constipation and prescribe fiber or laxatives. They see diarrhea and look for infection or prescribe antidiarrhoeals. Neither approach addresses the underlying bacterial imbalance in the small intestine that is driving both presentations.
Summary: SIBO can cause diarrhea, constipation, or both — depending on which bacterial subtype is present. Standard treatments that target the symptom rather than the underlying overgrowth will not resolve the root cause. Three-gas breath testing is currently the most comprehensive non-invasive tool for identifying which subtype is present.
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Warning Sign #3: Brain Fog That Nobody Can Explain
This is the warning sign that surprises people the most. When we say “gut bacteria,” most people think of gut symptoms. But SIBO frequently causes cognitive symptoms that neurologists, GPs, and psychiatrists may all fail to connect to what is happening in the digestive tract.
I’m talking brain fog, difficulty concentrating, poor memory, or mental fatigue that no amount of sleep or caffeine resolves.
The mechanisms behind this are still being studied, but several appear to be at play. Bacteria in the small intestine produce toxins that enter the bloodstream through the gut lining. The gut and brain are also directly connected via the vagus nerve — and chronic gut inflammation appears to disrupt that signalling pathway in measurable ways.
A 2025 study published in the Journal of Clinical Gastroenterology examined 102 patients undergoing breath testing for suspected SIBO. While bloating was the most common symptom, 54% of patients also reported brain fog — and brain fog scores were significantly elevated in those with confirmed SIBO compared to those without it (3).
If your cognitive symptoms have never been adequately explained, and you have gut symptoms alongside them, this connection is worth raising with a specialist.
Summary: SIBO-related brain fog is driven by bacterial toxins entering the bloodstream and disrupting vagus nerve signalling between the gut and brain. Research shows that over half of confirmed SIBO patients report significant cognitive symptoms alongside their digestive complaints. Brain fog that coexists with gut symptoms should prompt investigation of a gut-brain connection rather than being treated in isolation.
Warning Sign #4: Deficiencies That Won’t Budge No Matter What You Take
This is one of the most clinically significant (and most overlooked) consequences of SIBO. Patients have often been supplementing with B12 or iron for months or years, their levels refuse to correct, their doctor is puzzled, and nobody has considered SIBO as the explanation.
When bacteria overgrow in the small intestine, they do not just produce gas. They compete directly with you for nutrients. Certain bacteria consume vitamin B12 before your body has the opportunity to absorb it. They also disrupt the uptake of iron and fat-soluble vitamins, including D, A, E, and K. The problem is not your diet or your supplement dose. Your gut is delivering nutrients to the wrong recipients first.
A 2024 study published in the International Journal of Molecular Sciences analyzed 67 patients newly diagnosed with SIBO and found that certain subtypes were associated with significantly lower serum vitamin D and lower ferritin levels — with high statistical significance (4).
So if you’re deficient in B12, iron, or vitamin D and have been unable to correct those levels through diet and supplementation, SIBO should be on the list of conditions to rule out.
Summary: SIBO bacteria compete directly for nutrients in the small intestine, consuming B12 and disrupting absorption of iron and fat-soluble vitamins before the body can access them. Persistent nutrient deficiencies that fail to correct with supplementation are a clinically significant but frequently missed indicator of SIBO. Testing for SIBO should be considered in any patient with unexplained, treatment-resistant deficiencies.
Warning Sign #5: Your Food Intolerances Are Multiplying
A pattern we see consistently in clinical practice: a patient starts with one or two food intolerances — typically dairy or gluten. Over time, the list grows. Onions. Apples. Beans. Legumes. Eventually, they are reacting to foods they used to eat without any issue, and their diet has shrunk to a narrow set of supposedly “safe” options.
This escalating pattern of food intolerances is a red flag for SIBO.
The mechanism involves the gut lining itself. When bacteria produce excessive gas and toxins over time, they can damage the tightly packed cells that form the intestinal wall. When that wall is compromised, partially digested food particles cross into the bloodstream more easily, triggering immune reactions. Each new immune reaction can sensitize the body to additional foods, expanding the list of intolerances progressively.
This is one of the reasons why elimination diets alone frequently fail to provide lasting relief in SIBO: the more foods are removed without addressing the underlying bacterial overgrowth, the more the gut lining continues to deteriorate, and the more intolerances develop.
Summary: Progressively expanding food intolerances, starting with one or two triggers and growing over time, are a clinically recognized pattern in SIBO patients. The mechanism involves damage to the intestinal lining, which allows food particles to enter the bloodstream and trigger escalating immune reactions. Addressing the underlying bacterial overgrowth is essential; elimination diets alone will not reverse this process.
Warning Sign #6: Excessive Belching and Upper GI Symptoms
Frequent belching, particularly in the 30 to 90 minutes following a meal, is commonly dismissed as acid reflux or attributed to eating too quickly. In patients with SIBO, it can signal something quite different: bacterial fermentation occurring too high up in the digestive tract.
The small intestine is supposed to be relatively bacteria-free. When bacteria are present in excess, they begin fermenting food in the upper gastrointestinal tract rather than waiting for it to reach the large intestine. Gas produced in this location travels both downward and upward. What the patient experiences as belching is often gas produced too early and in the wrong section of the gut.
A 2024 review confirms that nausea and upper abdominal discomfort are recognized presenting symptoms of SIBO alongside bloating — which is consistent with fermentation occurring in the proximal small intestine (5).
Importantly, many patients with these symptoms are prescribed PPIs (proton pump inhibitors) for presumed acid reflux. This can worsen SIBO, because stomach acid is one of the body’s natural defences against bacterial migration into the small intestine. Reducing it can inadvertently allow the overgrowth to continue or progress.
Summary: Excessive belching and upper GI discomfort can indicate that bacterial fermentation is occurring in the upper small intestine rather than the large intestine where it belongs. Standard treatment with PPIs for presumed reflux can worsen SIBO by reducing stomach acid — one of the key defences against bacterial migration. This is an important reason why an accurate diagnosis matters before beginning treatment.
Warning Sign #7: Unexplained Weight Changes
Unexplained changes in body weight (in either direction) can be a sign of SIBO, and the direction depends on which subtype is present.
Some patients with SIBO lose weight despite eating normally. This occurs because malabsorption means the body is not effectively extracting nutrition from food. Calories, macronutrients, and micronutrients are being intercepted by bacteria before they can be properly absorbed.
Others (particularly those with methane-dominant SIBO) gain weight unexpectedly. The proposed mechanism involves slowed gut transit caused by methane gas, which may increase the amount of calories extracted from food as it moves more slowly through the digestive tract.
The 2024 SIBO subtypes study found distinct metabolic patterns across different SIBO types, supporting the idea that the condition affects body composition and nutritional status differently depending on which microorganism is overgrowing (6).
If you have experienced unexplained weight changes alongside any of the other warning signs in this article, SIBO is worth including in the conversation with your doctor.
Bonus Sign: The Skin Connection Most Doctors Overlook
This one deserves its own dedicated article — but it is too important to leave out entirely.
There is a growing body of research connecting SIBO to inflammatory skin conditions, particularly rosacea, as well as eczema and chronic hives. The proposed mechanism is systemic: bacteria in the small intestine produce inflammatory compounds that enter the bloodstream and trigger immune activity throughout the body, including in the skin.
If you have been managing a chronic skin condition with topical treatments for years and nobody has ever suggested investigating your gut, this connection is worth looking into.
Summary: Emerging research links SIBO to inflammatory skin conditions including rosacea, eczema, and chronic hives through systemic inflammatory pathways. Bacterial toxins produced in the small intestine can enter the bloodstream and drive immune activity that manifests in the skin. Patients with treatment-resistant skin conditions alongside gut symptoms should consider SIBO as a potential contributing factor.
What Should Your Next Steps Be?
If you’re feeling overwhelmed navigating treatment, diet, recovery, and prevention on your own, professional guidance can make all the difference. At Diet vs. Disease, we specialize in SIBO and have helped thousands of patients rebuild a resilient gut through an individualized, holistic approach.
To learn more about our integrated approach, I invite you to apply for a nutrition assessment call with us. We’ll help you make sense of what’s really happening and map out the next steps to get you feeling confident again — not just temporarily avoiding symptoms, but addressing the root cause and fixing your gut issues for good.
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