Could It Be SIBO? Symptoms Worth Paying Attention To
Key Takeaways
SIBO can cause a range of digestive symptoms, many of which also occur with other digestive conditions.
SIBO and IBS can look very similar, so symptoms alone usually cannot tell them apart.
Your medical history can provide important clues, as other medical conditions that affect the small intestine can make bacterial overgrowth more likely.
SIBO can sometimes affect nutrient absorption, which can contribute to weight loss, nutritional deficiencies, or fatigue.
There is no single symptom that diagnoses SIBO. A healthcare provider can help determine whether testing or further evaluation makes sense.
How long have you been brushing off the bloating that seems to follow every meal, or trying to explain away frequent trips to the bathroom? These symptoms can have many causes, but one possibility that may be worth considering is small intestinal bacterial overgrowth, or SIBO.
The challenge is that SIBO does not have one telltale symptom or set of symptoms that always appears first. Knowing what to watch for, what other conditions can look like SIBO, and whether you have factors that make bacterial overgrowth more likely can help you decide when to talk with your healthcare provider.
Common Digestive Symptoms of SIBO
For many people diagnosed with SIBO, the first clue that something was not right was a change in how their digestive system feels or behaves. SIBO is commonly associated with bloating, gas, abdominal discomfort, belching, and changes in bowel habits, although symptoms vary considerably from person to person.
Bloating stands out among people being evaluated for microbial overgrowth. In a 2026 study of more than 3,000 people referred for SIBO testing based on symptoms, 90% complained of bloating. Of those, 53% reported severe bloating, far more than reported severe diarrhea, constipation, or abdominal pain. Yet only about half of the patients were ultimately diagnosed with microbial overgrowth, showing why even a common symptom like bloating cannot tell us whether someone has SIBO.
Symptoms alone may not be enough to tell if you have SIBO. IBS, celiac disease, food intolerances, and other digestive conditions can cause many of the same problems, and studies have found considerable symptom overlap between people who test positive and negative for bacterial overgrowth.
Beyond Digestive Symptoms: Nutrient Deficiencies, Fatigue, and Brain Fog
While SIBO almost always causes digestive symptoms, its effects can sometimes go beyond the gut. When bacterial overgrowth interferes with the small intestine’s ability to digest food and absorb nutrients, problems such as weight loss and nutritional deficiencies can develop.
Not everyone with SIBO develops problems absorbing nutrients. In more severe cases, malabsorption can contribute to deficiencies in vitamin B12 and the fat-soluble vitamins A, D, and E. These deficiencies can have effects of their own; for example, vitamin B12 deficiency can contribute to fatigue, weakness, or numbness and tingling.
Brain fog is another symptom that sometimes comes up in conversations about SIBO. People describe feeling mentally cloudy, sluggish, or having trouble concentrating. Research has begun to explore connections between digestive disorders, the gut microbiome, and these cognitive symptoms, but there is still no direct evidence that brain fog is specifically caused by SIBO.
SIBO or IBS? Why Symptoms Alone Cannot Tell You
SIBO and IBS can look remarkably similar. Both can cause bloating, abdominal pain or discomfort, gas, diarrhea, and constipation. In fact, people diagnosed with IBS are more likely to test positive for SIBO than those without an IBS diagnosis, although estimates vary depending on how SIBO is diagnosed.
So how do you know which one is causing your symptoms? Symptoms alone usually cannot tell you. There are two ways to test for SIBO, either through breath testing or bowel aspiration. Neither type of test is perfect and your healthcare provider can help you determine which is best suited to you. Although symptoms alone cannot tell you whether you have SIBO, another important clue is your medical history.
Who May Be at Greater Risk for SIBO?
Problems that slow movement through the small intestine, change its normal structure, or interfere with its natural defenses can make bacterial overgrowth more likely.
For example, motility can be affected by conditions such as diabetes which affects the nerves that control digestion, and Crohn’s disease which may cause chronic inflammation. Structural changes can matter too: systemic sclerosis, small-intestinal diverticula, strictures, and certain gastrointestinal surgeries can interfere with the normal movement of intestinal contents, making bacterial overgrowth more likely.
Having one of these risk factors does not mean you have, or will develop, SIBO. But if you have persistent digestive symptoms along with a condition that makes bacterial overgrowth more likely, that information is worth sharing with your healthcare provider. Learn more in our guide to SIBO as a Secondary Condition.
When to Talk to Your Doctor
If bloating, abdominal discomfort, gas, or changes in your bowel habits keep happening, are getting worse, or are interfering with daily life, it is worth talking with a healthcare provider. These symptoms do not necessarily mean you have SIBO, but they may be worth investigating, especially if you also have a condition or medical history that increases your risk.
Before your appointment, pay attention to which symptoms you have, when they happen, how long they have been going on, and whether your bowel habits have changed. Also mention relevant health conditions, gastrointestinal surgeries, and medications. This information can help your healthcare provider decide whether SIBO testing, or evaluation for another possible cause, makes sense. Not sure how to start the conversation with your doctor? Read our guide to Talking to Your Doctor about SIBO.
Key References
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El Halabi, M. H., et al. (2025). Brain fog in gastrointestinal disorders: Small intestinal bacterial overgrowth, gastroparesis, and irritable bowel syndrome. Journal of Clinical Gastroenterology, 59(9), 842–848. https://doi.org/10.1097/MCG.0000000000002094
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